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The effect of cryogenic temperature and change in deformation mode on the limiting grain size in a severely deformed dilute aluminium alloy
Abstract
With the aim of investigating the factors that limit the production of true nanograined materials by cryogenic severe deformation, the grain structures formed in an Al–0.1%Mg alloy have been studied in plane strain compression at temperatures down to 77 K, following prior severe plastic deformation (SPD) by equal channel angular extrusion. Changing the deformation mode alone had little effect on increasing the rate of grain refinement. At the minimum cryogenic temperature (77 K) the samples still contained ∼30% low angle boundaries and a nanoscale high-angle boundary (HAB) spacing was only obtained in one dimension. At high strains a steady-state minimum HAB spacing was approached, irrespective of the temperature, where the rate of grain refinement stagnated. It is shown that the minimum grain size achievable in SPD is limited by a balance between the rate of compression of the HAB spacing and dynamic grain coarsening. At low temperatures this is controlled by abnormally high boundary migration rates, which are difficult to explain with existing theories of grain boundary mobility.
... Этот результат свидетельствует о том, что эволюция зеренной структуры на этой стадии деформации уже не определяется только исключительно геометрическим фактором, а какие-то иные механизмы также заметно влияют на формирование структуры. Подобный эффект обнаружен в криогенно-деформированном алюминии [102], однако на настоящий момент он пока не получил должного объяснения. ...
... «Криос» по-размером зерен НК-диапазона[126,127]. Тем не менее, микроструктура после криогенной деформации отличается существенно меньшим размером зерен, чем структура после «обычной» холодной деформации[102]. В связи с этим 500 K, и это может привести к рекристаллизации внутри полос[128,133].деформации[153]. ...
... Как следует из рисунка 5.7, криогенно-деформированный материал характеризуется сравнительно более сильной текстурой, хотя этот эффект и уменьшается с накоплением деформации от осадки (e≈1,0) к сдвигу под высоким давлением (e≈8,4). Подобный результат получен и на чистом алюминии[102], где он интерпретирован как следствие исчезновения полос сдвига/сброса, размывающих текстуру, с понижением температуры деформации. Следует отметить, впрочем, что этот эффект может быть также связан и с затруднением поперечного скольжения дислокаций, так как уменьшение числа действующих систем скольжения, или активности некоторых из них, может способствовать интенсификации процесса формирования текстуры.Сопоставление эталонов текстуры с экспериментальными данными на рисунке 5.7 (а)-(в) показывает, что пластическое течение меди при криогенной осадке ближе к схеме Закса, а при комнатной температурек схеме Тейлора.Это косвенно указывает на затруднение поперечного скольжения дислокаций в условиях криогенной деформации. ...
The monograph is devoted to the study of the features of the structure formation process and the possibility of obtaining an extremely fine-grained structure in an fcc metal subjected to plastic deformation at a cryogenic temperature, using as an example pure copper. Attention is paid to the study of the formation of the structure and the main mechanisms of cryogenic plastic deformation by rolling methods, shell precipitation, shell precipitation followed by rolling, shell precipitation followed by “abc” deformation, as well as shear under high pressure; the study of the effect of prolonged exposure at room temperature on the stability of the structure of cryogenically-deformed technically pure copper; assessment of the effectiveness of the use of cryogenic deformation for grinding the structural components in technically pure copper as compared with similar deformation at room temperature. The book may be useful to specialists dealing with the problems of solid state physics, nanomaterials and nanotechnologies.
... Annealing twins are present in the microstructure of the as-received material as seen in 1. The rate of grain refinement slows down, when the imposed strain during CGP is increased, as noted in other SPD methods [113][114][115][116]. ...
... A considerable refinement was not recorded as seen in 1. The rate of grain refinement decreases with the increase in imposed strain during CGP as indicated in other SPD methods [113][114][115][116] 2. Grain refinement effectiveness of CGP is lower relative to other SPD techniques, as it imposes a simple deformation field where slip during groove pressing is cancelled out by reverse slip during straightening [60]. ...
... According to the data taken from the microscopical image shown in Figure 5 1. The rate of grain refinement decelerates with the increase in imposed strain during CGP as occured in other SPD methods [113][114][115][116]. ...
... Более сложные структурные изменения при криодеформации протекают в легированном алюминии. В работе [14] исследовался сплав Al-Mg 0,13 масс.%. Образцы подвергались двум видам ИПД: равноканальному угловому сжатию (РКУС) при 298 К (уровень деформации 10) и последующему плоскому сжатию непосредственно в жидком азоте (772 К) с деформацией 2,8. ...
... Для примера: при деформации сплава при комнатной температуре среднее расстояние между высокоугловыми граница-Рис. 4. Дифракционные картины обратно рассеянных электронов: левыйпосле РКУС; правый -после последующей прокатки при 77 К [14]. ...
... Рис. 5. Зависимость расстояния между большеугловыми границами от степени истинной деформации при различных температурах [14]. МИКРОСТРУКТУРА И МЕХАНИЧЕСКИЕ СВОЙСТВА МЕТАЛЛОВ И СПЛАВОВ 309 ми составляет 3 мкм. ...
The goal of a given review is to consider the main directions to modify the structure and mechanical properties in the bulk and on the surface of metals and alloys via the severe plastic deformation at the liquid-nitrogen temperature. Investigations of the effect of such a type of the cryodeformation on the processes of nanostructuring as well as the characteristics of the metallicmaterials’ strength and ductility with different crystal lattices and the stacking fault energies are resulted. The possibility to achieve an optimal combination of both strength and ductility by the cryodeformation and subsequent annealing is shown.
... It is worth to mention that when long slip bands interact with a parallel group of dense dislocation walls and microbands, localized shear deformation could cause slip bands to evolve (offset) from a straight line and become S-shaped bands (S-bands) as shown in Fig. 7(b). The S-band formation is temperature sensitive, that it could be suppressed or even avoided at very low processing temperatures close to the liquid nitrogen temperature [142]. ...
... Despite of complicated deformation mechanisms operating in the SPD-induced grain refinement processes, the microstructure of each processed material will eventually reach an steady state at which further deformation does not change the overall microstructure, i.e., the average grain size remains unchanged with further straining. The minimum average grain size achievable depends on intrinsic material properties such as SFE [106,180,405] and external SPD conditions including processing temperature [142,170], the SPD technique [127,406], applied pressure [86,146,407], and strain rate [105,408]. ...
... This can be understood by examining the temperature effect on the two competing processesgrain refinement and grain growth. There are numerous literatures showing that grain refinement can be enhanced by lowering the temperature [142,206,417]. For high SFE materials in which full dislocation activities governs the grain refinement, by lowering the homologous temperature the dislocation density can be increased to enhance grain refinement [142,418,419]. ...
Bulk nanostructured (ns)/ultrafine-grained (UFG) metallic materials possess very high strength, making them attractive for high strength, lightweight and energy efficient applications. The most effective approach to produce bulk ns/UFG metallic materials is severe plastic deformation (SPD). In the last 30 years, significant research efforts have been made to explore SPD processing of materials, SPD-induced microstructural evolutions, and the resulting mechanical properties. There have been a few comprehensive reviews focusing mainly on SPD processing and the mechanical properties of the resulting materials. Yet no such a review on SPD-induced microstructural evolutions is available. This paper aims to provide a comprehensive review on important microstructural evolutions and major microstructural features induced by SPD processing in single-phase metallic materials with face-centered cubic structures, body-centered cubic structures, and hexagonal close-packed structures, as well as in multi-phase alloys. The corresponding deformation mechanisms and structural evolutions during SPD processing are discussed, including dislocation slip, deformation twinning, phase transformation, grain refinement, grain growth, and the evolution of dislocation density. A brief review on the mechanical properties of SPD processed materials is also provided to correlate the structure with mechanical properties of SPD-processed materials, which is important for guiding structural design for optimum mechanical properties of materials.
... SEVERE plastic deformation (SPD) has been considered an attractive technique for producing ultrafine grained (UFG) and nanocrystalline (NC) materials, [1][2][3][4][5][6] frequently with a saturated or steady-state grain size at a high level of strain for given deformation paths. [7][8][9] Typically, the size range of grains in the steady state, especially in pure metals, is of the order of several hundreds of nanometers and, thus, lies outside the commonly defined NC range (< 100 nm). ...
... This approach, known as cryogenic deformation, has recently been employed to produce nanostructured (NS) materials. [2,4,[13][14][15][16] For example, finer grains have been obtained in Al [2] and Ni [14,17] by cryogenic deformation than their counterparts obtained at room temperature (RT). Another effective way to introduce further grain refinement is by changing deformation modes, as reported in Cu, [18] Ni, [19] Al, [2] and interstitial-free (IF) steel, [20] where the samples were subjected to equal-channel angular pressing (ECAP) followed by subsequent cold rolling (CR) or high-pressure torsion (HPT). ...
... This approach, known as cryogenic deformation, has recently been employed to produce nanostructured (NS) materials. [2,4,[13][14][15][16] For example, finer grains have been obtained in Al [2] and Ni [14,17] by cryogenic deformation than their counterparts obtained at room temperature (RT). Another effective way to introduce further grain refinement is by changing deformation modes, as reported in Cu, [18] Ni, [19] Al, [2] and interstitial-free (IF) steel, [20] where the samples were subjected to equal-channel angular pressing (ECAP) followed by subsequent cold rolling (CR) or high-pressure torsion (HPT). ...
A bulk nanolaminated (NL) structure with distinctive fractions of low- and high-angle grain boundaries (fLAGBs and fHAGBs) is produced in pure nickel, through a two-step process of primary grain refinement by equal-channel angular pressing (ECAP), followed by a secondary geometrical refinement via liquid nitrogen rolling (LNR). The lamellar boundary spacings of 2N and 4N nickel are refined to ~ 40 and ~ 70 nm, respectively, and the yield strength of the NL structure in 2N nickel reaches ~ 1.5 GPa. The impacts of the deformation path, material purity, grain boundary (GB) misorientation, and energy on the microstructure, refinement ability, mechanical strength, and thermal stability are investigated to understand the inherent governing mechanisms. GB migration is the main restoration mechanism limiting the refinement of an NL structure in 4N nickel, while in 2N nickel, shear banding occurs and mediates one-fifth of the total true normal rolling strain at the mesoscale, restricting further refinement. Three typical structures [ultrafine grained (UFG), NL with low fLAGBs, and NL with high fLAGBs] obtained through three different combinations of ECAP and LNR were studied by isochronal annealing for 1 hour at temperatures ranging from 433 K to 973 K (160 °C to 700 °C). Higher thermal stability in the NL structure with high fLAGBs is shown by a 50 K (50 °C) delay in the initiation temperature of recrystallization. Based on calculations and analyses of the stored energies of deformed structures from strain distribution, as characterized by kernel average misorientation (KAM), and from GB misorientations, higher thermal stability is attributed to high fLAGBs in this type of NL structure. This is confirmed by a slower change in the microstructure, as revealed by characterizing its annealing kinetics using KAM maps.
... Because both GB and TJ migration counteract grain elongation, the generation of bulk nanostructured materials with enhanced aspect ratios necessitates the development of strategies that subdue them to a great extent. Furthermore, deformation modes or temperatures where shear band formation may occur should also be avoided, as such localized shear can fragment lamellae as well [16,33]. Strain path changes have been suggested as a means to overcome the problem of structural recovery and to suppress these processes, leading to a reinvigoration of boundary generation, offering the possibility for further grain refinement [33]. ...
... Furthermore, deformation modes or temperatures where shear band formation may occur should also be avoided, as such localized shear can fragment lamellae as well [16,33]. Strain path changes have been suggested as a means to overcome the problem of structural recovery and to suppress these processes, leading to a reinvigoration of boundary generation, offering the possibility for further grain refinement [33]. However, results on structures already deformed to saturation have shown that a strain path change does not necessarily result in further refinement. ...
... Of course, this will only hold true, for homogenous deformation, where the generated structure results from a balance between refinement and the restoration processes and will not be fulfilled for deformation modes, alloys or temperatures where intense shear banding is present. Such localized shear can again lead to fragmentation of the lamellae and thus deviation from the above description [16,33]. ...
It has been successfully proven that severe cold deformation can be used to generate bulk nanostructured materials. However, there is a limit to grain refinement, as recovery processes, together with grain boundary and triple junction motion, occur to continuously fragment and remove grains. Although believed to be mechanically driven at low temperatures, it is clear that such movement can be thermally assisted, as reflected in decreased grain aspect ratios at elevated deformation temperatures. Interestingly, for tantalum, a different behaviour can be found. Grain aspect ratios after high pressure torsion increase steadily up to deformation temperatures of 673 K before dropping again. Moreover, extremely large aspect ratios of 10 can be generated. The reasons for this behaviour are discussed and it is suggested from results based on nickel that this phenomenon can be adapted to any metal. This offers the possibility to selectively process materials with extreme grain aspect ratios, to achieve structures which are thought to be the key to the design of materials that combine both, exceptional strength and good damage tolerance.
... Причины этого явления не вполне ясны. Иногда это связывается с наступлением определенного равновесия между деформационным уменьшением зерен и их 2 термоактивированным ростом [4]. Таким образом, добиться гарантированного формирования реальных НК-структур посредством ИПД в большинстве случаев пока не удается, и нужно искать новые пути для достижения НК-диапазона размера зерен. ...
... В работах, выполненных преимущественно на высокопластичной меди, была выявлена следующая картина эволюции микроструктуры в ходе криогенной деформации [4][5][6]. При относительно небольших степенях деформации отмечено интенсивное двойникование [5,6], хотя считается, что медь не склонна к механическому двойникованию. Криогенные двойники, как правило, являются очень тонкими (~ 50 нм) и часто объединены в целые колонии, которые эффективно фрагментируют исходные зерна [5,6]. ...
Исследована возможность существенного измельчения зерен в технически чистой меди путем криогенной осадки. Установлено, что эволюция структуры в целом определялась сплющиванием исходных зерен в ходе деформации. Анализ текстурных данных и спектра разориентировок показал, что основным механизмом пластического течения являлось обычное {111}<110> дислокационное скольжение при несущественном вкладе механического двойникования.
... В последнее время был проведен ряд попыток использования криогенной прокатки для измельчения микроструктуры [2][3][4], причем большая часть этих работ была выполнена на высокопластичной меди. ...
... Данный эффект, скорее всего, связан с небольшими отклонениями реальных плоскости и направления прокатки от идеальных.Текстура латуни обычно формируется при прокатке ГЦК металлов на умеренные величины обжатия. Например, образование этой текстуры было отмечено в ходе относительно небольшой криогенной деформации алюминия[2]. Таким образом, исходя из текстурных данных, можно предположить, что ни криогенные условия деформации, ни исходная мелкозернистая структура не привели к изменению фундаментальных механизмов пластического течения в условиях криогенной прокатки.3.3. ...
Проведена тщательная аттестация микроструктуры и механических свойств меди, подвергнутой различной степени криогенной прокатки. Показано, что эволюция зеренной структуры, в основном, определялась геометрическим эффектом деформации. На основе анализа текстурных данных был сделан вывод, что криогенные условия деформации не привели к фундаментальному изменению характера пластического течения, и основным механизмом деформации было дислокационное {111}<110> скольжение. Установлено, что криогенная прокатка приводит к существенному увеличению прочности и некоторому снижению пластичности.
... One of the promising but poorly studied ways of processing the ultrafine-grained (UFG) structure in metals and alloys is the so-called cryogenic deformation, which is carried out at temperatures below 120 K (-153°С) [1][2][3][4][5][6][7][8]. It is considered that low deformation temperatures, suppressing dynamic recovery, promote the accumulation of internal stresses due to the extremely high densities of crystal lattice defects and thus intensify grain refinement under straining and subsequent annealing. ...
... However, there are a number of investigations that have shown that deformation even to high strains (with e > 1) at cryogenic temperatures does not guarantee the UFG structure processing. For example, severe plastic deformation (SPD) of pure aluminum via cryorolling did not result in the formation of the expected nanocrystalline structure and even did not ensure significant grain refinement [1,7]. The studies of copper have also shown that a decrease in the deformation temperature, suppressing doublecross slip of lattice dislocations, complicates the formation of strain-induced boundaries and thus restricts grain fragmentation (grain subdivision) and the formation of new fine grains via dynamic recrystallization. ...
The influence of the energy of electro-pulse processing (EPP) on the structure and hardness of pure nickel deformed at room and liquid nitrogen temperatures is studied. The metal was subjected to isothermal rolling with a strain of 90% and subsequent EPP with integral current densities Kj in the range from 0.06 to 0.19×10 ⁵ A ² s/mm ⁴ corresponding to the calculated temperature range of 130-925°C. It was found that decreasing the rolling temperature of Ni resulted in a more uniform and less misoriented nanocellular structure and a 15-20 HV higher hardness. Subsequent EPP decreased the hardness due to recovery, discontinuous recrystallization with the formation of annealed twins and growth of new grains. It is revealed that a more uniform fine-grained structure with a smaller grain size is formed in the cryorolled metal in a narrower interval and at less EPP energies.
... One main mechanism studied during cryogenic rolling is geometrical effects of strain. It has been observed that a heavily elongated grain structure form in both aluminium [6] and copper [9] aligned in rolling direction without any significant grain fragmentation or subdivision. Furthermore, the grain structure includes a high fraction of low angle grain boundary inside the individual grains. ...
... In 20% Re , a -fiber texture including Goss orientation (011) [100] with intensity of 1.9 times and Brass orientation (011) [2][3][4][5][6][7][8][9][10][11] with intensity of 1.5 times, in addition to a higher intensity (3.2 times) orientation of (112) [1 -10] have been developed ( Fig. 6 (a)). All the main texture components have been weakened in compared to initial material and instead some transient texture has been evolved ( Fig. 7 ). ...
Recently, advanced oxide dispersion strengthened (ODS) copper alloys have been developed using mechanical alloying process as a fusion material. In this study, to develop a superior ODS copper alloy containing 0.5wt% Y2O3, the effect of cryogenic rolling on microstructure development and tensile properties was studied using high resolution EBSD, TEM and tensile tests. During cryogenic deformation of ODS copper, grain structure remains in submicron size scale as a combinatorial result of geometrically effects, nanotwin bundle deformation, interaction of dislocations with fine oxide particles and some diffusional processes including static recovery and recrystallization. Clear microstructural characterizations confirmed nucleation of fine new oriented recrystallized grains mainly on the HABs of 80%cryogenic rolled ODS copper. Quantitative analyses indicated grain boundary migration at room temperature following cryogenic deformation originated from high driving force induced by grain boundary bulging and high mobility induced by vacancies. The tensile properties of cryogenic deformed samples showed superior tensile strength than room temperature deformation leading to UTS: 624 MPa, elt: 5.5%, while saturation of strength between 60%-80% reduction, approved occurrence of softening by diffusional processes.
... Consequently, as more GNBs are accommodated, the width of lamellar structures become smaller and smaller, eventually forming a nanolaminated structure. Nanolaminated structure with mainly HAGBs is commonly observed in severely cold-rolled aluminum and its alloys with high SFE [44][45][46]. At low and medium strains, the boundaries of cell blocks tend to be inclined to the rolling plane, while at large strains, these boundaries become almost parallel to the rolling plane, consistent with our observations (Fig. 10). ...
... Один из перспективных и сравнительно малоисследованных методов деформационного структурирования металлов и сплавов с целью придания им ультрамелкозернистого (УМЗ) и нанокристаллического строения (размер кристаллитов -зерен и субзерен, менее 1 и 0.1 мкм, соответственно) базируется на криогенных условиях обработки, т. е. деформации при температурах ниже 120 К (−153°С). В основе такого подхода лежит общеизвестное представление о том, что чем ниже температура деформирования, тем сильнее в обрабатываемом материале подавляются процессы динамического возврата, приводя к большему накоплению дефектов кристаллического строения и повышению внутренних напряжений [1][2][3][4][5]. При этом подразумевается то, что измельчение зерен и субзерен достигается активизацией как динамических (в процессе деформации, в особенности большой пластической деформации), так и постдеформационных, статических процессов. ...
The influence of the energy of electric pulse treatment (EPT) in the range of integral current densities (Kj) from 0.06×10(5) to 0.29×10(5) A2s/mm4 on the structure and hardness of coarse-grained high-purity Al, isothermally rolled up to a total strain of 90% at a liquid nitrogen temperature, was studied. It was found that EPT with an energy up to Кj=0.104×10(5) A2s/mm4 practically did not affect the microhardness obtained in the cryorolled aluminum (45–50 HV). An increase in the EPT energy to Кj=0.121×10(5) A2s/mm4 led to a rapid drop of the hardness to 30 HV followed by its gradual stabilization near 25 HV at higher Кj values. It was established that an enhanced microhardness of rolled Al resulted from formation of a well developed cellular structure with a crystallite size of about 2 μm, containing less than 10% of ultrafine grains of about 4 μm in size. The minor hardness changes after EPT with Кj up to 0.104×10(5) A2s/mm4 were related to occurrence of recovery and continuous recrystallization, resulted in improving the deformation structure without noticeable changes in its type and the crystallite sizes. Therewith the softening caused by a partial decrease in the scalar dislocation density and the microdeformation of the crystal lattice was compensated by increasing the high angle boundaries fraction. At EPT with Кj=0.121×10(5) A2s/mm4, the deformation structure was severely replaced by the fine-grain recrystallized one with the grain size of 19 μm, and resulted in the loss of the strenthening effect, caused by rolling. With further increase in the EPT energy, an extensive grain growth was observed, leading to the formation of a non-uniform structure grains and to appropriate material softening, owing to the grain coarsening. It was concluded that the restoration processes that took place during EPT were similar in nature to those
that occur during furnace annealing of heavily deformed materials. Therewith, the short time of the thermal exposure on the deformed metal during EPT was compensated by high applied energies.
... Разработка и получение металлов и сплавов с размером зерен в десятые и сотые доли микрометра (субмикро-и нанокристаллов) с заданными физико-химическими свойствами является важной проблемой современного материаловедения [1]. В последнее время был проведен ряд попыток использования криогенной деформации для измельчения размера зерен [2][3][4], причем большая часть этих работ была выполнена на высокопластичной меди. Представляется актуальным подробное изучение микроструктуры после криогенной деформации, а также механизмов ее формирования. ...
The development and production of metals and alloys with grain sizes of tenths and hundredths of a micrometer (submicro- and nanocrystals) with desired physicochemical properties is an important problem of modern materials science [1]. Recently, a number of attempts have been made to use cryogenic deformation to grind grain size [2–4], and most of this work was performed on highly plastic copper. It seems relevant to a detailed study of the microstructure after cryogenic deformation, as well as the mechanisms of its formation. This work was aimed at a thorough certification of the microstructure of copper subjected to varying degrees of low-temperature deformation. For the certification of the microstructure, a relatively new method of automatic analysis of backscattered electron diffraction patterns (EBSD) was used.
... Получение субмикро-и нанокристаллических материалов с заданными свойствами -важная задача современного материаловедения [1]. Одним из перспективных направлений в этой области является деформация при криогенной температуре [2][3][4][5]. Однако эффективность этого подхода пока до конца не ясна и поэтому актуальной задачей является изучение микроструктуры после криогенной деформации, а также механизмов ее формирования. ...
The production of submicrocrystalline and nanocrystalline materials with desired properties is an important task of modern materials science [1]. One of the promising directions in this area is deformation at a cryogenic temperature [2-5]. However, the effectiveness of this approach is not yet completely clear, and therefore the urgent task is to study the microstructure after cryogenic deformation, as well as the mechanisms of its formation.
... Large deformation at cryogenic temperatures is sometimes considered as a promising and costeffective method for producing bulk ultrafine-grain materials [1][2][3][4][5][6][7][8][9]. This approach is believed to be particularly effective for materials prone to mechanical twinning and/or shear banding. ...
The static-annealing behavior of cryogenically-rolled Cu-30Zn brass over a wide range of temperature (100-900 °C) was established. Between 300 and 400 °C, microstructure and texture evolution were dominated by discontinuous recrystallization. At temperatures of 500 °C and higher, annealing was interpreted in terms of normal grain growth. The recrystallized microstructure developed at 400 °C was ultrafine with a mean grain size of 0.8 μm, fraction of high-angle boundaries of 90 pct., and a weak crystallographic texture.
... According with [65], more aggressive cutting parameters, higher cutting speed in particular, cause an increasing in grain refinement effect in Ti6Al4V machining. Besides, lower cutting temperatures produce a more restrained grain refinement, resulting in a descending trend in grain size with more effective coolant employment, culminating with the usage of cryogenic fluids, that limits the grain growing effects and consequently shows the highest grain refinement phenomenon on the machined surface [66]. The grain size reduction is almost negligible respect to the bulk material under dry machining at the lowest cutting speeds. ...
The metallurgical phenomena taking place during machining processes affect the thermo-mechanical properties of the severely deformed materials, influencing, consequently, the process behavior. The microstructural modifications are difficult to be evaluated when the material is subjected to high speed deformations that are typical of material removal processes. Therefore, the microstructure-based numerical simulations can represent a useful tool able to properly predict their mechanics. Hard turning experiments were conducted on Ti6Al4V alloy, involving different process parameters and lubri-cooling conditions. The worked samples surfaces were assessed in terms of resulting microstructural changes and microhardness. The obtained results (cutting forces, temperature, and surface metallurgical modifications) were considered to develop and validate a physics-based model able to describe the microstructural phenomena occurring under large deformation processes, taking into account the influence of the physical phenomena that accommodate the material plastic strengthening and their resulting effects on the process variables. The dislocations reciprocal influence and their interaction with the material lattice were considered to understand the material viscoplastic flow. Moreover, also the recrystallization phenomena influencing the grain size related strengthening were considered to formulate the model. Then, the developed material model was implemented via user sub-routine in a commercial finite element (FE) software. The FE model was used to in-depth analyze the inner evolution of the processed material and to predict the variables of industrial interest. A good agreement was shown between the experimentally measured variables and the numerically predicted results. Moreover, the model was employed to investigate additional machining conditions via finite element analysis (FEA), demonstrating a huge capability to improve the manufacturing process performances, leading to a deeper knowledge of microstructural evolution and the material machinability under various process conditions.
... Some studies have shown that the low stacking fault energy (SFE) [23,25,26]，high strain rate [27,28] and deformation at cryogenic temperatures [29,30] are favorable for the formation of deformed twins. In addition, both the low SFE and the cryogenic temperature can promote grain refinement and inhibit dynamic recovery [31,32]. The low SFE also helps to form high-density stacking faults. ...
This article aims to investigate the mechanical properties of the gradient structured (GS) Cu–Al–Zn alloys. To prepare the GS samples, three kinds of Cu–Al–Zn alloys with different stacking fault energies (SFEs) were processed by surface mechanical attrition treatment (SMAT) at cryogenic temperature. The results show that the Cu-5.5%Al-4.5%Zn alloy having the lowest SFE processed by SMAT for 5 min exhibits an optimized combination of strength and ductility. For both the initial annealed sample and the SMAT samples processed for the same processing time, uniform elongation (UE) and ultimate tensile strength (UTS) increase with decreasing SFE, whereas the yield strength (YS) is not sensitive to SFE. In the case of the same SFE, as the SMAT processing time increases, the YS and UTS of the GS alloy increase and the UE decrease. The optimized SMAT processing time can sufficiently refine the grains in the GS layer, which can increase the strength and maintain good ductility. Also, sufficient grain refinement promotes the better accumulation of geometrically necessary dislocations (GNDs), thus forming rather effective hetero-deformation induced (HDI) stress strengthening and HDI hardening to enhance strength while maintaining good ductility. The combination of low SFE and optimized SMAT processing time is beneficial to obtain an optimized combination of strength and ductility. Additionally, since the near-surface layer of the GS material is plastically deformed earlier than the sub-surface layer, the accumulation of GNDs at the interface between the two layers also contributes to HDI strengthening and HDI hardening to improve strength and ductility.
... Processing time in the former or the number of passes in the latter control the fraction of the UFG phase but not its minimum grain size. This is always at a minimum saturated level determined by material purity and homologous temperature of processing [8,[83][84][85]. Sonic-wave and high-energy milling have also been attempted for SUS304L steel but obtained properties are inferior to MM and JM processed counterparts so far. ...
Recent efforts in engineering metals with high structural efficiency have resulted in developing a new category of artificial materials with heterogeneous microstructures architected across multiple scales. In this critical assessment, a relatively new concept of heterogeneous bimodal harmonic-structure (bHS) materials is introduced and analysed. It is shown that the bHS concept is applicable to a large variety of metallic materials and is most efficient in scenarios where changes in the chemical composition of materials are restricted for some reason. Basic principles, weaknesses and advantages along with present development status and perspectives are discussed. The overview of critical performance characteristics of various bHS materials is provided, and interesting directions for future research, development and applications are proposed.
... Having attracted significant scientific attention not only due to intrinsic, inimitable mechanical properties and physical of different materials with nanostructure, but also by SPDprocessed materials advantages compared with other NC materials, the bulk nanostructured materials were fabricated using SPD processes in recent years. SPD techniques particularly lead to cope with a lot of problems regarding compacted samples residual porosity, ball milling impurities and practical usages of the applied materials [29,[36][37][38][39][40][41][42][43][44][45][46][47][48][49][50]. On account of structural developments, SPD producing materials seems to be a really complex process as below: (i) activities of dislocation and/or twins formation in addition to the interaction between dislocations and twin boundaries result in grain refinement, (ii) when the initial grain sizes have little sizes than those of the smallest ones of a SPD process, there is a growth in the grain, (iii) phase transformations like the room temperature and the TiNi partial amorphization α → γ transformation in Fe-C composite, and (iv) redistribution of elements [51][52][53][54][55][56][57][58][59][60]. ...
Grain size effect on corrosion general trends was clarified by Ralston and Birbilis ́ ́ s review by sorting them with three different corrosion status (active, passive, and active/passive) having diverse potentiodynamic polarization curves. The active type made them realize that finer grain size showed an acceleration in corrosion currents whereas the passive type had a totally opposite trend. A mixed type of the prior two types being lower in the passive zone and the active zone of higher was indicated within the active/passive type. There is a big problem whether these attitudes retain in submicron size-or nano-scale grain size. Ultrafine-grained (UFG) materials have drawn remarkable attention through twenty years ago. Several severe plastic deformation (SPD) processes have been applied in order to make UFG materials in which the accumulative roll bonding (ARB) procedure is the most effective one among them. Structure of UFG divulges an advancement in mechanical properties along with distinct corrosion behavior, but not always resulting in better corrosion resistance. Diverse relevant work is reviewed in this paper in order to date the studied passive behavior of UFG aluminum, copper, and titanium, having produced utilizing ARB procedure. In this study, the grain size effect using ARB procedure on passive behavior of aluminum, copper, and titanium has been evaluated in related passive environments using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods.
... Therefore, the dislocations can provide heterogeneous nucleation sites for the GP zones that easily grow and transform to the β′′ phase. Dislocations also improve the strength of Al-Mg-Si alloy according to the Peirls-Nabarro equation [14]: ...
... Minimum grain size (MGS) or steady-state grain size (SSGS) obtainable by cryogenic deformation is commonly studied in metals with a refined structure, and it occurs when a balance between dislocation generation and annihilation results in an equilibrium state. Recent studies show that the MGS is dependent on atomic bond energy and related parameters, such as specific heat capacity, activation energy for self-diffusion and homologous temperature, indicating that MGS is reasonably independent of the SFE ( Ref 12,13). In contrast, Parvin and Kazeminezhad suggest that there is a correlation between SFE and MGS ( Ref 18). ...
The present study concerns the cryogenic processing of metals with simultaneous analysis of x-ray diffraction in a synchrotron ring. The mechanical properties improvement related to cryogenic processing of metals is attributed to the partial suppression of dynamic recovery. Thus, commercially pure metals with different stacking fault energies (silver, copper and aluminum) were deformed by uniaxial tensile tests and characterized by in situ x-ray diffraction, at room (293 K) and cryogenic (77 K) temperatures. The cryogenic processing allows a simultaneous improvement in ductility and strength for silver and copper and an improvement in strength for aluminum. This difference in mechanical properties was investigated by means of variations in crystallite size, microstrain and also the amount and size of dimples on the fracture surface. The microstructural refinement at cryogenic temperatures shows a tendency related to the stacking fault energies.
... LN2 allows for strain energy to be stored primarily as elastic energy in the rapidly cooled surface layer of the burnished workpiece. The applied energy is therefore stored primarily in the form of dislocations and elastic strain, both of which drive grain refinement via dynamic recrystallization (DRX) and grain subdivision mechanisms [22] Similar to other SPD operations, the minimum burnishing-induced grain size is thought to be closely related to the temperaturecompensated strain-rate, as is defined by the Zener-Holloman parameter (Z) [23]. Where microstructural changes from DRX are defined through a temperature compensated strain rate as is shown below in equation 1. ...
Finishing processes such as shot peening, polishing and burnishing have a major influence on the functional performance of manufactured components. In this study, cryogenic burnishing is investigated as a low-force technique for imparting nanostructured grains and hardness increase in the processed surface layers of Ti-6Al-4V alloy. Using a multi-pass approach, deflection of the workpiece can be significantly reduced, allowing for processing of thin walled aerospace components such as turbine blades. Looking forward, the ability to design and manufacture nanostructured components at industrial scales is envisioned to enable the creation of new products with engineered surface layers for improved functional performance.
... For instance, the prevalence of the Copper orientation has been found in commercial 2519 aluminum alloy (studied in the present work) which has been rolled at ambient conditions [28]. On the other hand, it has been reported recently that a lowering of the rolling temperature to cryogenic range may promote a formation of the Brass-type texture {110} < 112 > [11,34,40], i.e. a component belonging to the α-fiber. This phenomenon is sometimes explained by a suppression of thermalassisted cross-slip [34]. ...
In this work, microstructural evolution of a solutionized Al-Cu-Mg alloy during rolling at liquid-nitrogen temperature was studied and concomitant strengthening effect was elucidated. It was found that a prior solid-solution treatment as well as a lowering of deformation temperature to the cryogenic range essentially suppressed dislocation mobility. This promoted an abrupt increase of dislocation density but considerably retarded microstructural processes, particularly texture evolution and development of deformation-induced boundaries, thus suppressing grain refinement. Hence, the strengthening effect of the cryogenic rolling was mainly contributed by the work hardening mechanism.
... This is well understood on the basis of the balance between the work hardening and the dynamic recovery phenomena in severely deformed aluminum alloys (Sakai et al., 2014). This balance is disturbed at cryogenic temperature by reduction of dynamic recovery rate leading to promotion of work hardening (Prangnell and Huang, 2008b). Consequently, the cryogenic flow curves of all specimens, even those processed by higher ARB cycles, are characterized by a remarkable hardening. ...
... In Ref. [7], one finds also that the Al-4Mg alloy develops mainly Brass and S components following the same MDF process. This may be attributed to Mg in solid solution which reduces dynamic recovery [36]. The second explanation of texture development during MDF of Al-Cu-Mg alloys should be discussed by considering the large intermetallic particles and the initial texture component before deformation. ...
... Conversely, the negative or cryogenic [12,[15][16][17][18] temperatures applied at the SPD (S 2 PD) processes can essentially slow down the DRX or even almost completely hindered it. The outcome is the formation of ultrafine and/or nanoscale grain structures even in the materials of relatively low melting temperature (high DRX ability), such as copper [11,12], aluminium [19] and magnesium [20] alloys. ...
Ultrasonic impact treatment of stainless steel AISI 321 was carried out at room temperature (in the argon environment - argon-UIT) and at cryogenic temperature (in liquid nitrogen - cryo-UIT) in the constrained conditions with application of the same mechanical energy to the treated specimens. The time dependencies of the surface hardness HV after the cryo-UIT and argon-UIT processes were of sigmoidal and parabolic shapes, respectively. The microstructural evolution of the surface layers of the deformed specimens was studied by X-ray diffraction (XRD), optical microscopy (OM), transmission electron microscopy (TEM) and selective area electron diffraction (SAED) analyses. The volume fractions of the deformation-induced α' (V α') and ε (V ε) martensites were estimated using XRD approach and by magnetic and density measurements. Compared to the room temperature UIT in the argon environment (argon-UIT), the liquid nitrogen UIT generates a higher density of the deformation twins and stacking faults. In addition, a higher V α' (∼53%) and V ε (∼3.5%) were observed after cryo-UIT in deeper surface layers (∼200μm). The argon-UIT process leads to the formation of either rectangular twin blocks or dislocation cells, which size ranged 200-500nm. Conversely, after cryo-UIT (e ≈0.95), a nanoscale grain structure of heterogeneous nature (α' and γ phases) was formed in the outmost surface layer simultaneously with the areas filled with networks of deformation twins and stacking faults. The minimum grain size of α'-martensite D α' and austenite D γ was respectively 25 and 45nm, and twin thickness/spacing was of 60-120nm. Both types of the microstructure contribute to the material strength and result in higher hardness of the cryo-UIT processed specimens (∼5-5.66GPa) compared to that of the argon-UIT processed ones (∼4.3GPa). With the increase in Zener-Hollomon parameter lnZ, the grain/twin/spacing size is decreased while the V α' and surface microhardness HV are increased.
... SFE is the balance between repulsive force between the two partial dislocations and the attractive force due to surface tension between the stacking faults [12]. For high SFE materials, the stacking fault width is extremely narrow, which facilitates cross- slip of the dislocation between the planes, leading to faster DRV even at low homologous temperatures [13]. For low and medium SFE materials, DRV phenomenon is rather difficult owing to greater stacking fault gaps [14,15]. ...
Present work aims to investigate the unique microstructural development during severe plastic deformation (SPD) of aluminum and its alloys under cryogenic temperature and during the post cryo-SPD annealing treatment. The influence of alloy chemistry and stacking fault energy (SFE) on recovery, recrystallization and grain growth phenomenon has been examined by various characterization techniques. Commercially pure Al alloy, non-heat treatable Al-4.5%Mg alloy and heat treatable Al-4.5%Cu alloy exhibiting varied SFE have been selected for this investigation. First, a significant variation in the cryo-SPDed microstructure has been observed between pure aluminum and its alloys. It has been attributed to the ‘polyslip’ deformation mechanism prevalent only in pure aluminum due to its very high SFE. Second, due to this inherent difference in their cryo-SPDed microstructures and influence of alloying elements, both the Al alloys show a substantial difference in their microstructural response during annealing in comparison with pure Al. For pure aluminum, grain rotation and coalescence is the dominating restoration mechanism. However, for aluminum alloys, rapid vacancy-assisted recovery, polygonization and recrystallization of equiaxed, high-angled, sub-micrometric grains have been observed. The grain growth mechanism of these recrystallized grains is heavily influenced by solute drag and particle drag effects. For non-heat treatable Al-4.5%Mg alloy, abnormal grain growth has been observed. In heat treatable Al-4.5%Cu alloy, nano-particles have precipitated in the grain boundaries, thereby retarding the grain growth process and inducing high microstructural thermal stability.
In this study, 2219 Al alloy forgings were forged at a high temperature of 510 °C and a low temperature of 460 °C. The effect of forging temperature on the microstructure, texture and mechanical properties of the 2219 Al alloy forgings was analyzed through x-ray diffraction, tensile properties tests, scanning electron microscopy, electron back scattering diffraction and transmission electron microscopy. The results showed that, when the forging temperature was increased from 460 to 510 °C, the average ultimate tensile strength (UTS), yield strength (YS), and elongation (EI) in three directions of the 2219 forgings were increased to 460.3 ± 0.6 MPa, 347.3 ± 0.4 MPa, and 9.7 ± 0.3%, where were increases of 5.4%, 3.3%, and 26%, respectively. After 510 °C-formed forgings, the recrystallization fraction and the content of substructure in the matrix increase obviously, which is beneficial to improve the forgings anisotropy (IFA). The IFA in UTS, YS and EI of the 2219 forgings decreased by 54.3, 18.7, and 7.3, respectively. These differences were attributed to a diminished coarse second phase, which increasing the oversaturation of the alloy in forgings formed at 510 °C. Thus, it can be concluded that high-temperature-formed 2219 Al alloy forgings have improved mechanical properties and weakened anisotropy.
The pursuit of high quality, high efficiency, and low investment in ceramic particles reinforced Ni-based composite coating has prompted continuous advancement in surface modification technology. Herein, the WC/Ni60 coatings were prepared by a coaxial alternating magnetic field (AMF) coupled plasma arc cladding technology. The effects of AMF on the distribution of reinforcing particles and the improvement of coating quality were investigated. The results demonstrated that the AMF significantly suppressed the sedimentation behavior of WC particles within the coating, and the WC particles were dispersed and uniformly distributed under the AMF intensity of 20 mT. With the increase of the AMF intensity (10 mT–30 mT), the semi-quantitative evaluation of the combustion loss rate indicated a rise in the dissolution degree of WC particles from 10.1% to 33.4%, leading to a gradual increase in the content of precipitated carbides (i.e., M6C, M7C3) with varied morphological characteristics. AMF-induced magnetic stirring facilitated nucleation, contributing to remarkable grain refinement and an elevation in the proportion of high-angle grain boundaries. Mechanical testing witnessed that the AMF-assisted samples are superior in microhardness and wear resistance performance compared with the counterpart without AMF. In particular, the coating with AMF intensity of 20 mT exhibited the most uniform distribution of microhardness.
The effect of thermomechanical treatment, based on combination of isothermal cryorolling at liquid nitrogen temperature to strains ranging from 30 to 90 % and subsequent single electric pulsing with an integral current density Kj up to 1.6 ×104 A2 s / mm4, on the structure and hardness of an initially coarse-grained pure nickel was studied. It is shown that the considered treatment is quite a powerful method to control the Ni structure and strength through grain and grain boundary design. The right chose of conditions of cryogenic straining and electro-heat action give ability to high efficient production of abnormally and partially work-hardened sheets with light tuning of crystallite size and fractions of recrystallized grains and high angle boundaries, in particular, the fine-grained sheet with a grain size of about 3 μm and 50 % fraction of the twin boundaries.
Research on the deformation behavior of ultrafine-grained (UFG) pure aluminum (Al) sheets was conducted at a temperature of −196 °C to provide a new strategy for forming complex components with UFG sheets. In this work, a UFG pure Al sheet with an average grain size of 0.9 μm was prepared by cold rolling (CR) and recovery annealing. Then, the mechanical behaviors of the UFG pure Al sheet were evaluated by uniaxial tensile tests and bulging tests. The fracture morphology, microstructure evolution and surface topography were observed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM). The UFG pure Al exhibits considerable strain hardening and superior ductility at −196 °C, significantly different from the obvious strain softening behavior that occurs at room temperature (RT). As the temperature is decreased from RT to −196 °C, the uniform elongation (UEL) is improved 24.2 times from 1.2% to 30.2%, and the total elongation (TEL) is improved 3.3 times from 11.8% to 51.2%. The limiting dome height (LDH) and the ultimate bulging rate, K, of the UFG pure Al sheet at −196 °C are 1.6 times and 2.9 times greater than those at RT, respectively. Microstructure observations show that many dislocation substructures such as dislocation pile-ups, dislocation cells and dislocation networks are formed in UFG pure Al deformed at −196 °C, that would otherwise be absent in its counterpart deformed at RT. This unusual phenomenon is attributed to the remarkable transition of the dominant deformation mechanism of UFG pure Al from significant dynamic recovery accompanied by grain boundary sliding (GBS) to dislocation multiplication and accumulation with decreasing temperature.
Purpose
This paper aims to study the electrochemical corrosion performance of ultrafine-grained (UFG) Cu bulk in 0.5 M NaCl solution.
Design/methodology/approach
UFG Cu bulk were prepared by impacting at −196°C and following heat treatment. The electrochemical corrosion behaviors of coarse-grained (CG), impacted and subsequently annealed at 190°C Cu bulks were studied.
Findings
All the bulks displayed typical active-passive-transpassive behaviors (dual passive films without stable passive regions). The resistance to corrosion of impacted Cu bulk was notably superior to that of CG Cu bulk, and subsequently annealing further improved its corrosion resistance.
Social implications
Except for mechanical properties, corrosion performance has been considered to be one of the most important aspects in bulk UFG metallic materials research for the prospective engineering applications.
Originality/value
Cryogenic impacting could effectively reduce grain size of CG Cu bulk to UFG scale and induce high density dislocation. Subsequent annealing resulted in a further decrease of grain size even to nanoscale, as well as nanometer twins. The grain refinement, high density dislocation and annealing twins effectively enhance the passivation capability, resulting in an increase in the corrosion resistance.
Microstructure evolution during “abc” deformation of commercial-purity titanium was investigated. Continuous dynamic recrystallization (CDRX) played a dominant role in the microstructural changes, although mechanical twinning also contributed to some extent. It was found that CDRX developed preferentially at the original grain boundaries, thus resulting in a necklace-type microstructure. This effect was attributed to the comparatively-high local stresses in these areas, which promoted pyramidal <c+a> slip and thus enhanced evolution of deformation-induced boundaries. It was also established that changes in strain path resulted in a noticeable re-organization of the pre-existing dislocation structure due to a Bauschinger Effect and thus promoted essential material softening. This phenomenon retarded the evolution of dislocation boundaries and thus tended to lead to stagnation in microstructure refinement.
In this paper, high-strength ultrafine-grained (UFG) pure aluminum (Al) was successfully achieved via a two-step processing route including room-temperature ECAP process for continuously 16 passes and post liquid nitrogen cryo-rolling for 50% reduction. The two-step processing route was observed to produce ultrafine grains with an average grain size of 0.98 μm and has obvious advantages for the preparation of high-strength pure Al in terms of the strength-ductility synergistic effect, which endows the pure Al excellent UTS value of 180 MPa, good Ef of 17.7% and a very high increase ratio (about 84%) of strength. The high strength was mainly attributed to the outstanding fine-grain strengthening provided by the combination of large-pass ECAP and cryo-rolling. In addition, the better uniform elongation of ECAP-rolled pure Al than ECAPed one was primarily dictated by the enhanced strain hardening capacity and the elongated grain shape.
In this study, a novel set of constrained groove processing routes was put forward especially to underline the variation of strain paths. Namely, 304 stainless steel sheets were processed by five distinctly specified constrained groove pressing routes. The enhancement of mechanical properties along with anisotropic behavior after each pass were compared for each route. Significant elevation in the hardness and tensile strength was observed after the initial pass while the successive passes demonstrated relatively less increase. Notable improvements in tensile strengths were observed for route E and C along two perpendicular directions up to 30% and 40%, respectively. Route A displayed the highest anisotropy in relation with its processing scheme. Microstructural evolution during processing was examined indicating slight refinement irrespective of the strain path.
The cyclic deep cryogenic treatment was proposed to improve both the hardness and corrosion resistance of the high strength 7075 aluminum alloy. The effect of different CDCT times on the exfoliation corrosion and intergranular corrosion of the alloys were observed by scanning electron microscope. The corrosion behaviors of the alloys were monitored by electrochemical techniques. The hardness of the alloy was measured by Vickers hardness tester. Furthermore, the microstructures of the alloys were examined by transmission electron microscope. The results show that the corrosion resistance and hardness are strongly affected by the precipitate state. The discontinuous grain boundary precipitates and the wide precipitate free zones will enhance the corrosion resistance. The fine precipitates distributed evenly in the matrix can increase the hardness. After the CDCT, the corrosion resistance is remarkably improved without sacrificing the hardness. The best combination of the hardness and corrosion resistance is exhibited for the alloy treated with the CDCT twice.
High strength and ductility are vital material properties required in alloys used for structural applications at cryogenic temperatures. However, the simultaneous provision of these characteristics is challenging because the two properties are mutually contrasting from the perspective of dislocation motion. Herein, we prepare binary Al-xMg alloys supersaturated with Mg using a combined technique of twin-roll strip casting and subsequent thermo-mechanical treatment; this produces Al alloys with both high strength and ductility at cryogenic temperatures. Experiments show that gradually adding Mg to Al significantly improves cryogenic tensile properties of the alloys. At the cryogenic temperature of the liquid N2 (−196 °C), the Al-5Mg and Al-7Mg alloys exhibit the ultimate tensile strength (UTS) of 370 and 450 MPa with the corresponding ductility of 93% and 87%, respectively. Comparing Al-xMg alloys with differing Mg contents elucidates avenues of simultaneously improving these properties for utilization at cryogenic temperature. This study provides a simple and robust approach for producing low-cost Al-xMg alloys with excellent cryogenic properties for structural applications.
Grain refinement of materials by severe plastic deformation, investigation and understanding of their properties and phenomena has been a subject of intensive research over the last three decades. Along with the invention and development of these processes it has been recognized, that grain refinement is not indefinite but stagnates for single phase materials. Accordingly, the minimum grain sizes achievable are in the range between 50 and 500 nm. Motivated to find ways to overcome these limitations, effort has been made to understand the reasons behind. Various processes were suggested to cause saturation of grain fragmentation. While all of these assumptions and models were not based on direct observations, recently in-situ approaches allowed significant progress in understanding microstructural evolution and the principle restoration processes during severe straining. It is the aim of this work to recap important earlier findings, reconsider proposed models and to present our current understanding of the processes limiting grain refinement upon severe straining. Further it will be discussed, that similar processes generally occur during deformation of such fine scaled materials. They do not only govern saturation of grain refinement during severe deformation but more important also the mechanical response and performance of these materials. This motivates an in-depth understanding and therefore open questions as well as discrepancies between various experiments will be deliberately highlighted to stimulate further research and thus progress within this area.
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A TiC nanoparticle modified non-weldable nickel-based superalloy Inconel 738LC has been fabricated via laser powder bed fusion. The formation of refined grains with a larger high angle grain boundary length density and zigzag grain boundaries inhibits the initiation and propagation of hot cracks, leading to a 97% reduction in cracking and improved mechanical properties.
Three FCC commercially pure metals (aluminum, copper and silver) were deformed by uniaxial tensile tests and were characterized by in situ X-ray diffraction, using a synchrotron source, at room (293K) and cryogenic (77K) temperatures. The partial suppression of dynamic recovery due to cryogenic processing allows an improvement in mechanical properties, such as ductility and strength. This suppression results in an increase in the internal defects density of metals during the strain, promoting microstructural refining and increase of microstrain. The microstructural refinement is manifested by dimples evolution on the fracture surface and reduction of average crystallite size. All metals present higher mechanical strength at cryogenic temperature, nevertheless the ductility only was increased in copper and silver. This behavior is attributed to lower stacking fault energy of these metals in comparison with aluminum.
Keywords: Cryogenic deformation, stacking fault energy, microstrain, dynamic recovery, twinning.
Grain refinement of materials by severe plastic deformation, investigation and understanding of their properties and phenomena has been a subject of intensive research over the last three decades. Along with the invention and development of these processes it has been recognized, that grain refinement is not indefinite but stagnates for single phase materials. Accordingly, the minimum grain sizes achievable are in the range between 50 and 500 nm. Motivated to find ways to overcome these limitations, effort has been made to understand the reasons behind. Various processes were suggested to cause saturation of grain fragmentation. While all of these assumptions and models were not based on direct observations, recently in-situ approaches allowed significant progress in understanding microstructural evolution and the principle restoration processes during severe straining. It is the aim of this work to recap important earlier findings, reconsider proposed models and to present our current understanding of the processes limiting grain refinement upon severe straining. Further it will be discussed, that similar processes generally occur during deformation of such fine scaled materials. They do not only govern saturation of grain refinement during severe deformation but more important also the mechanical response and performance of these materials. This motivates an in-depth understanding and therefore open questions as well as discrepancies between various experiments will be deliberately highlighted to stimulate further research and thus progress within this area.
After a short recall of various techniques that use shots to induce surface severe plastic deformation and a brief survey of the advantages of having a gradient structure for mechanical properties, this manuscript presents recent developments taking advantages of the “reactivity” of these modified surfaces in the fields of corrosion, “duplex” surface treatments as well as potential applications for an easier activation of H-storage materials. The importance of controlling the processing parameters (including temperature) to get the optimum gradient structure depending on the desired applications as well as the necessary requirements for a high quality microstructure and chemical characterizations are also highlighted.
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We present results of atomistic computer simulations of spontaneous and stress-induced grain boundary (GB) migration in copper. Several symmetrical tilt GBs have been studied using the embedded-atom method and molecular dynamics. The GBs are observed to spontaneously migrate in a random manner. This spontaneous GB motion is always accompanied by relative translations of the grains parallel to the GB plane. Furthermore, external shear stresses applied parallel to the GB and normal to the tilt axis induce GB migration. Strong coupling is observed between the normal GB velocity vn and the grain translation rate v||. The mechanism of GB motion is established to be local lattice rotation within the GB core that does not involve any GB diffusion or sliding. The coupling constant between vn and v|| predicted within a simple geometric model accurately matches the molecular dynamics observations.
In discussing hardening characteristics in terms of crystalline lattice defects, in most cases the properties and kinetics of dislocations and their arrangement have been considered. However, during plastic deformation also vacancies and/or vacancy type defects are produced in very high densities which are typically close to those of vacancies in thermal equilibrium at the melting point. The effect of high vacancy concentrations on the hardening characteristics is two-fold: (i) direct effects by impeding the movement of dislocations (ii) indirect one by inducing climbing and annihilation of edge dislocations leading to softening or even absolute decreases in strength. This paper presents first measurements of deformation induced vacancies in SPD materials which have been achieved by combined evaluation of resistometry, calorimetry and X-ray diffraction. The density of vacancies during and after SPD deformation is found to be markedly higher than in cases of conventional deformation and/or coarse grained material which may be partly attributed to the particular conditions of SPD namely the enhanced hydrostatic pressure as well as the changes in deformation path. It is suggested to make this high vacancy concentration responsible for both dynamic and static recovery and/or recrystallisation processes recently found during and after SPD, being potential reasons for enhanced ductility and superplasticity which only occur with nanomaterials originating from SPD. Recent publications show that in alloys, SPD induced vacancies can also enable the existence of phases which do not appear in the equilibrium diagram.
Research on suspension optimization of a 2-DOF vehicle-seat model using a stochastic optimal control technique. A 2-DOF math and dynamics model of vehicle-seat suspension system is built. Using linear stochastic optimal control theory, according to the stochastic input statistics characteristics of the road, in the condition of not all of the variables are known, using Kalman-Bucy filter to evaluate unknown variables, a optimal control regular is found out. The suspension system of the earth moving machinery was optimized with respect to ride comfort and working space of the seat suspension. Improvement of seat dynamics is regarded as the easiest solution to solve the ride comfort problem of the earth moving machinery. The simulation study demonstrated that the vehicle-seat active control system is more effective than passive system in improving the ride comfort.
This paper attempts to study the microstructures of 1Cr18Ni9Ti stainless steel subjected to compression deformation at liquid-nitrogen temperature and room temperature and subsequently annealing. The microstructure of 1Cr18Ni9Ti stainless steel subjected to compression deformation of 60% reduction at liquid-nitrogen temperature composes of two phases, deformation-induced martensite and austenite, with morphology of nanoscale laths ranging 20–80nm in width. For the sample of room-temperature compression deformation in 60% reduction, the microstructure composes of mainly austenitic deformation twin with the average thickness of about 200nm and a few amount of deformation-induced martensite and stacking faults. Nearly nanograined microstructures can be obtained in the two deformed samples by annealing at 580°C for 30min. Deformation at liquid-nitrogen temperature for the steel can have a positive effect on developing high-angle boundaries.
The effect of coarse and fine second-phase particles on the formation of ultra-fine grained (UFG) structures have been compared during severe deformation by equal-channel angular extrusion using routes A (no rotation) and BC (+90° rotation). The presence of coarse particles has been found to increase the rate of grain refinement with route A and the homogeneity of the submicron grain structure formed, but appears less effective using route BC. In contrast, the presence of fine dispersoids inhibits the development of new high-angle grain boundaries and the formation of an UFG structure with both routes. Retardation is far more pronounced with rotation of the sample and the dispersiod-containing alloy processed by route BC contained mainly subgrains. The mechanisms operating in each case are discussed.
The annealing behavior of a 5083 Al alloy deformed at cryogenic temperature was investigated, focusing on the evolution of microstructures and mechanical properties. The optimization of the cryogenic rolling and subsequent annealing conditions resulted in a mixture of equiaxed grains of less than 200 nm and elongated subgrains, exhibiting a good combination of uniform elongation and high strength.
Commercial purity aluminium has been cold-rolled to reductions from 40 to 99% or true thickness strains (εt) from 0.5 to 5.0. Applying transmission electron microscopy techniques the microstructural evolution has been followed and structural parameters such as spacing between dislocation boundaries and misorientation angles across these boundaries have been measured and analyzed. This analysis shows that the microstructural evolution does not saturate at large strains. In parallel, the flow stress and strain hardening behavior has been determined by uniaxial tensile testing of rolled speciments. These stress–strain curves do not saturate in the strain range examined, even when changes in the Taylor M-factor are considered. This agrees with the evolution in the boundary spacing and misorientation angle, which are considered to be the strength determining parameters based on the operation of two mechanisms, dislocation strengthening and grain boundary strengthening. Following this description, the individual strength contributions are calculated and their addition leads to flow stress values and strain hardening behaviour in good agreement with those determined experimentally.
The effect of fine dispersoids on the mechanisms and rate of grain refinement has been investigated during the severe deformation of a model aluminium alloy. A binary Al–0.2Sc alloy, containing coherent Al3Sc dispersoids, of ∼20 nm in diameter and ∼100 nm spacing, has been deformed by equal channel angular extrusion to an effective strain of ten. The resulting deformation structures were quantitatively analysed using high-resolution electron backscattered diffraction orientation mapping, and the results have been compared to those obtained from a single-phase Al–0.13Mg alloy, deformed under identical conditions. The presence of fine, non-shearable, dispersoids has been found to homogenise slip, retard the formation of a cellular substructure and inhibit the formation of microshear bands during deformation. These factors combine to reduce the rate of high-angle grain boundary generation at low to medium strains and, hence, retard the formation of a submicron grain structure to higher strains during severe deformation.
Methods of deforming metals to large strains are reviewed and the process of equal channel angular extrusion is analysed i detail. The development of microstructure during large strain deformation is discussed, and it is concluded that the mai criterion for the formation of a sub–micron grain structure is the generation of a sufficiently large fraction (> 0.7) o high–angle grain boundary during the deformation process. For aluminium alloys, it is found that a low–temperature annea is required to convert the deformed microstructure into an equi–axed grain structure. The material, microstructural and processin factors that influence the formation of such fine–grain microstructures are discussed, and the stability of these microstructure at elevated temperatures is considered.
For any physical or chemical process - like detrapping of electrons, annealing of point defects or desorption - obeying a rate equation, very often the process under investigation is enhanced by heating the system at a constant rate. Various procedures have been given to calculate the value of the activation energy of the underlying process. Nevertheless in most publications on recovery concentration curves and intensity peaks the result is given only in the form of diagrams without any determination of the system parameters. An easy estimate for the activation energy can be made using the half-width delta equals T//2 - T//1 and the temperature of the peak maxima.
Structural evolutions in an Armco iron subjected to severe plastic deformation by torsion under high pressure are analyzed with conventional and high resolution electron microscopes. The substructure observed at low strains appears to shrink with increasing deformation and transforms at very high strains into grain boundaries. The resulting grain size decreases down to a constant submicrometric value. Meanwhile, the material strength, as revealed by micro hardness measurements, levels out. Dislocation densities and internal stress levels are used to discuss the structural transformations. Hydrostatic pressure and deformation temperature are believed to modify the steady-state stress level and structural size by impeding the recovery processes involving diffusion.
Nanocrystalline iron with grain sizes of 13–30 nm has been prepared by mechanical attrition using several different types of mills with different milling intensity. The grain size and the strain of the powders as well as the milling time for achieving the ultimate grain size strongly depend on the milling intensity and the milling temperature. The results obtained under different experimental conditions are discussed in the framework of previously proposed models for the evolution of nanocrystalline grain sizes during mechanical deformation.
Pure copper with ultrafine grain sizes and nanoscale subgrain (dislocation) structures was prepared by using severe plastic deformation through cold rolling at subambient temperatures, with or without subsequent recovery annealing. We report coexisting high strength and tensile ductility (large elongation to failure and ductile fracture). Factors leading to the simultaneous strengthening and toughening with increasing cold deformation and microstructural refinement are discussed. © 2002 American Institute of Physics.
High resolution electron backscattered diffraction analysis has been used to compare the evolution of the deformed state during severe deformation of two model materials: an Al-0.13 wt-%Mg alloy and an interstitial free steel. The alloys were deformed by equal channel angular extrusion up to a total effective strain of 10, at 20 and 500 degreesC, respectively. At strains of <2, new high angle boundaries were formed from primary deformation bands. At strains of 2-5 (corresponding to high conventional strains) the new high angle grain boundaries, associated with the deformation bands, rotated towards the billet axis creating a lamella structure. Narrow bands of fine grains were also formed in unstable crystal orientations. With increasing strain the separation of the lamella high angle boundaries reduced until at very high strains their spacing was equal to the subgrain width and the long ribbon grains broke up into shorter segments, After an effective strain of 10 the microstructures for both materials consisted of submicron grains with an aspect ratio of 3, In general it was observed that the interstitial free steel refined more rapidly with strain than the aluminium alloy. MST14767.
Plane strain compression tests at 5 s−1 and at temperatures of 270–480°C have been carried out on an Al–1Mg–1Mn alloy containing a bimodal distribution of intermetallic particles and after a prior heat treatment to coarsen all particles to greater than 1 μm in size. During the heat treatment, recrystallisation of the initially hot worked material only proceeded with coarsening of the fine particles. During subsequent hot deformation, thin foil electron microscopy revealed that identical subgrain structures were developed in the two materials by dynamic recovery at temperatures below 450°C. At higher temperatures, the initially recrystallised material showed localised particle stimulated dynamic recrystallisation. The subsequent static recrystallisation rate was more than 103 times faster in the material free from small particles.MST/751
Equal channel angular extrusion has been used to deform an Al–3 wt-%Mg alloy to an effective strain of 10, resulting in a 0.2 µm grain size. In the as deformed condition the yield strength was increased to ∼500 MPa. During annealing the grain structure coarsened uniformly and the yield stress was found to follow the Hall–Petch relationship, even in the submicron range. There was an abrupt transition in elongation at a grain size of ∼0.5 µm. Samples with smaller grain sizes showed no uniform elongation and limited ductility. For slightly greater grain sizes there was only a relatively small reduction in elongation, compared to a coarse grained material, while the yield stress was still increased by a factor of over four. Reducing the grain size to the submicron range led to far higher Lüders strains than are normally observed in Al–Mg alloys.
Single crystals of {100}〈100〉 and {110}〈100〉 orientations of a high-purity Al–0.05% Si single-phase aluminum alloy have been deformed under plane strain compression at elevated temperatures. The deformed crystals of the {100}〈100〉 orientation contained bands of subgrains of a range of size and misorientation, whereas the {110}〈100〉 crystals gave very uniform microstructures. The specimens were annealed at temperatures between 300 and 450 °C and measurements of the subgrain growth have been made using high-resolution electron backscattered diffraction (EBSD) in a field emission gun scanning electron microscope (FEGSEM). Detailed analysis of deformed and annealed crystals revealed a strong correlation between subgrain growth and misorientation, and the analysis of the data enabled the mobility of low-angle boundaries and activation energies in the misorientation range of 2–20° to be determined.
Pure nickel was selected for a detailed investigation of the experimental parameters influencing grain refinement and microstructural evolution during processing by high-pressure torsion (HPT). Samples were examined after HPT using microhardness measurements, transmission electron microscopy and orientation imaging microscopy. Processing by HPT produces a grain size of ~170 nm in pure Ni, and homogeneous and equiaxed microstructures are attained through-out the samples when they are subjected to at least ~5 whole revolutions under applied pressures of at least ~6 GPa. For these conditions, the distributions of grain boundary misorientations are similar in the center and at the periphery of the samples. A simple model is proposed to explain the development of a homogeneous microstructure in HPT.
This paper presents the performance characteristics of an air-suspended high-Q MEM inductor that is characterized based on an open-short deembedding scheme and used for accurate and reliable evaluation of the parasitic effects experienced from feed lines as well as contact pads. The validity and effectiveness of the de-embedding method are proven with a fixed-length MEM transmission line along with various feed-line topologies. The measured inductances of the 20-μm elevated transmission line provide consistent data, as compared to the simpler open de-embedding method. For more realistic and complicated MEM inductors with 1.5 and 2.5 turns, the characterized inductance values agree very well with the calculated data, based on the Greenhouse algorithm. The measured and calculated inductances are within the 5% range. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 43: 355–358, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20468
High-purity nickel (99. 99 pct) with a grain size of 80 to 100 µm was deformed by cold-rolling from 37 to 98 pct reductions
(von Mises effective strains ofεvm = 0. 5 to 4. 5). The deformation microstructures and texture at five strain levels were observed and characterized using
transmission electron microscopy (TEM) and neutron diffraction. The microstructures evolved within a framework common to medium
and high stacking fault energy fee polycrystals. This framework consists of structural subdivision by higher angle boundaries
(geometrically necessary boundaries) at one volume scale and at a smaller volume scale by lower angle cell boundaries (incidental
boundaries) for all strain levels. We have characterized the dislocation boundaries, including dense dislocation walls (DDWs),
microbands (MBs), and lamellar boundaries (LBs) in terms of crystallographic and macroscopic orientations, morphology, and
frequency of occurrence. The microstructural evolution is discussed with special emphasis on factors that contribute to the
transition from structures characteristic of small and medium strain microstructures to those characteristic of large strain
microstructures.
The effect of grain-boundary dislocation transformations on diffusion in nanocrystalline solids is discussed. A theoretical
model describing the enhancement of diffusion processes associated with the climb of grain-boundary dislocations in nanocrystalline
solids is developed.
The shearing associated with equal-channel angular (ECA) pressing was examined using optical microscopy. Samples of pure A1
with a large grain size were subjected to ECA pressing to different strains and then examined on three orthogonal planes.
Samples were pressed without any rotation or with rotations of either 90 or 180 deg between each consecutive pressing. The
experimental observations are compared with models which predict the shearing characteristics associated with ECA pressing
under different conditions. It is demonstrated that there is good agreement, in terms of both the grain elongation and the
shearing within individual grains, between the experimental results and the predictions of the models.
There have been long-standing concerns about the stability of the internal structure of nanocrystalline metals. In this letter we examine grain growth in nanocrystalline Cu under the microhardness indenter, examining the influence of temperature of indentation and sample purity. Surprisingly, it is found that grain coarsening is even faster at cryogenic temperatures than at room temperature. Sample purity is seen to play an important role in determining the rate of grain growth. Fast grain coarsening can affect the outcome of mechanical tests, especially if they involve large stresses and high-purity samples.
Molecular-dynamics simulations are used to elucidate the coupling between grain growth and grain-boundary diffusion creep in a polycrystal consisting of 25 grains with an average grain size of about 15 nm and a columnar grain shape. Consistent with our earlier simulations of grain-boundary diffusion creep, albeit in the absence of grain growth, we find that initially, i.e. prior to the onset of significant grain growth, the deformation proceeds via the mechanism of Coble creep. Also, consistent with our earlier grain-growth simulations in the absence of stress, two growth mechanisms are observed during the deformation: growth due to curvature-driven GB migration and growth resulting from grain rotation-induced grain coalescence. The comparison of the grain growth observed in the presence of the applied stress with that solely in response to temperature as the driving force enables us to identify the mechanisms by which external stress affects grain growth. In particular, we find that both GB migration and grain rotation are accelerated by the deformation.
In situ nanoindentation within a transmission electron microscope is used to investigate the deformation mechanisms in ultrafine-grained Al films. Deformation-induced grain growth resulting from grain boundary migration, grain rotation and grain coalescence is commonly observed as the indentation proceeds. In situ studies of nanograined films suggest that the same mechanisms are operative, though the difficulty of imaging nanosized grains makes the evidence less clear. The results suggest that grain growth and coalescence are important modes of response in the deformation of ultrafine- and nanograined materials.
Results are presented showing how the specific area of grain boundaries changes during plastic straining for an initial structure comprised of random equi-axed grains. These calculations for homogeneous strain are also compared with heterogeneous deformation derived from crystal plasticity finite element modelling. Other deviations from ideal behaviour are also discussed.
The mechanical behavior of a cryomilled Al–10Ti–2Cu (wt.%) alloy has been studied by performing uniaxial tension tests at temperatures ranging from room temperature to 525°C. Elastic–nearly perfectly plastic stress–strain behavior is observed at all temperatures. Tension–compression asymmetry of the room temperature yield stress is also observed. These characteristics are in agreement with those recently reported in the literature for single-phase NC materials. The flow stress (700 MPa at room temperature) decreases dramatically with increasing temperature. Testing of material following thermal exposures suggests that microstructural coarsening alone cannot account for the decrease in strength with increasing temperature. From a coarsening standpoint, this material appears to be very thermally stable. The ductility is influenced by several factors. Low levels of internal porosity along with the presence of fine oxide and carbide dispersoids contribute to lower ductility. The absence of work hardening exhibited by the Al–10Ti–2Cu also leads to reduced strain to failure. The features observed on fracture surfaces suggest that fracture occurs by the nucleation and growth of voids at particle–matrix interfaces. Evidence of fracture along prior powder particle boundaries is present as well. The microstructure consists primarily of regions containing grains measuring in the range 30–70 nm. Large grained regions consisting of nominally pure Al ranging in size from 300 to 500 nm are also present. No evidence of dislocation activity within either the fine or large grained regions can be found in the as extruded material. Specimens deformed at room temperature and 93°C reveal evidence of dislocation activity within the large grain regions. Dislocation configurations suggest an Orowan bypass mechanism. No dislocations are found within the 30–70 nm size grains following tensile deformation.
The drag on a grain boundary produced by an impurity atmosphere is examined in detail, and is found to depend on the velocity of the grain boundary relative to the diffusivity of the impurity and its interaction with the grain boundary. At high velocities the faster diffusing impurities have the greater drag; whereas at low velocities the reverse is true. With increasing impurity concentration or with decreasing temperature a boundary may experience a transition due to a changing interaction with its impurity atmosphere. The nature of the transition depends on driving force and may give rise to a large apparent activation energy, as well as jerky boundary motion due to the existence of a range of conditions where two boundary velocities are possible. Special orientation effects that may result in textures are expected to occur more easily at high velocities.ZusammenfassungDie Behinderung der Korngrenzenbewegung durch Verunreinigungen wird im einzelnen untersucht; sie hängt demnach vom Verhältnis der Korngrenzengeschwindigkeit zur Diffusionsgeschwindigkeit der Verunreinigungen und deren Wechselwirkung mit der Korngrenze ab. Bei hohen Geschwindigkeiten bremsen die leichter diffundierenden Verunreinigungen stärker, bei kleinen Geschwindigkeiten ist es umgekehrt. Mit zunehmender Konzentration der Verunreinigungen oder mit abnehmender Temperatur kann eine Korngrenzenumwandlung eintreten, bedingt durch eine veränderte Wechselwirkung mit den Verunreinigungen. Die Art der Umwandlung hängt von der treibenden Kraft ab und bewirkt eine groβe, scheinbare Aktivierungsenergie, sowie eine ruckweise Korngrenzenbewegung, deren Ursache ein Bereich ist, in dem zwei Korngrenzengeschwindigkeiten möglich sind. Spezielle Orientierungseffekte, die zu Texturbildung füren können, sollten bei hohen Geschwindigkeiten leichter eintreten.
The deformation processes occurring in an Al1 wt% Mg alloy of commercial purity have been studied using plane strain compression (PSC). As part of a matrix of tests covering a range of temperatures and strain rates experienced in industrial hot rolling, the combination T = 305°C and has been studied in detail. This is near the top of the possible range for the Zener-Hollomon parameter, Z, in hot rolling. Experimental measurements of cell size, misorientation between cells and dislocation density were obtained as a function of strain. The qualitative interpretation of the microstructure was enhanced by the judicious use of both optical and transmission electron microscopy.The observations in this paper illustrate the complexity of the evolution of microstructure with strain, and also how important microstructural features (such as the lamellae of dislocation cells) can be overlooked. In spite of the complexity, there is a striking degree of self-similarity in the microstructure over a wide range of strain. The frequency distribution of cell size, or misorientation across different categories of boundary, essentially change very little with respect to their mean values.
In nanostructured metals with grain sizes of the order of 100 nm, dislocation mechanisms remain dominant in controlling plastic deformation. These materials, similar to their coarse-grained counterparts that have been subjected to heavy cold work, can no longer go through the several strain hardening stages of normal metals and are hence susceptible to plastic instabilities such as necking in tension. For processing and applications, it is obviously important and often necessary to control such inhomogeneous plastic deformation. Here we demonstrate three strategies to achieve relatively large stable tensile deformation in nanostructured metals, using the pure Cu processed by equal channel angular pressing as a model. The first approach uses an in situ formed composite-like microstructure, such as a bimodal grain size distribution, to impart strain hardening to the material and attain large uniform tensile strains while maintaining the majority of the strengthening brought forth by nanostructuring. In the second route, deformation is conducted at low temperatures, such as 77 K. The material regains the ability to work harden due to suppressed dynamic recovery. Uniform elongation is achieved as a result, together with an elevated strength at the cryogenic temperature. The third method takes advantage of the elevated strain rate sensitivity of the flow stress of the nanostructured Cu, especially at slow strain rates. Using the stabilizing effects of strain rate hardening on tensile deformation, nearly uniform strains can be acquired in absence of strain hardening. We also discuss the deformation mechanisms involved in these approaches to assess their applicability to nanocrystalline metals with grain sizes well below 100 nm, where normal dislocation activities become severely suppressed.
Severe deformation processing is an emerging technique, for the production of submicron grain structures in metallic alloys, that involves plastic deformation to ultra-high strains (typically εvm>7). Severe deformation processing has several advantages over other techniques for producing materials with ultra-fine grain structures, in that it is relatively inexpensive, can be used to process conventional alloys, and can potentially be scaled up to produce large quantities of material. In this paper, the mechanisms by which grain refinement takes place during severe deformation and the effect of the material and process variables on the microstructural evolution in Al-alloys are discussed.
The effect of second-phase particles on the rate of grain refinement during severe deformation processing has been investigated, by comparing the microstructure evolution in an AA8079 aluminium alloy, containing 2.5 vol.% of ~2 μm particles, with that in a high purity, single-phase, Al-0.13% Mg alloy, deformed identically by ECAE to an effective strain of ten. The materials were analysed by high-resolution EBSD orientation mapping, which revealed that grain refinement occurred at a dramatically higher rate in the particle-containing alloy. A submicron grain structure could be achieved by an effective strain of only five in the particle-containing alloy, compared to ten in the single-phase material. The mechanisms that contribute to this acceleration of the grain refinement process are discussed.
Plane strain forming in commercial processes commonly includes elevated temperature multi-step reductions. An understanding of microstructural development during this process is critical because it dictates the properties of the material subsequent to hot forming. Earing behavior, formability, and mechanical response of the final product are all dependent upon the processing parameters which dictate microstructural evolution. The present work focuses upon the development of hot deformation textures in aluminum during this type of processing. Commercial purity aluminum specimens exhibiting two different starting textures were deformed in channel die compression experiments to simulate the plane strain deformation conditions imposed by the rolling mill. Initial structures consisted of a randomly textured material and a preferentially cube orientated texture to investigate the effects of starting texture on hot deformation processing and the resulting microstructures. Rate and temperature dependence of texture evolution was experimentally and theoretically investigated in conjunction with this. The relative stability of cube orientations within polycrystals deformed in plane strain is demonstrated for certain deformation conditions. Finally, experimental observations of the evolution of orientation flow from the alpha to the beta fiber are discussed.
The microstructure evolution of the intermetallic compound Ni3Al during severe deformation by torsion under high quasi-hydrostatic pressure, which eventually results in the formation of a disordered nanocrystalline structure with a high level of internal stresses, was investigated as a function of the amount of shear strain. At the microstructural level, the crystals were first fragmented by the propagation of twins, then nanosized equiaxed crystallites with high misorientations were formed. At the macroscopic level, there is evidence that the cold-worked structure first formed at the sample surface and then propagated into the whole volume.
The microstructures developed during deformation to large rolling strains in single and two-phase aluminium alloys with a wide range of grain sizes has been investigated, and the major parameters of the microstructure determined by high resolution electron backscatter diffraction (EBSD). It is found that the behaviour of initially fine-grained (<5 μm) alloys is significantly different from that of the large-grained (>50 μm) alloys. In the finer-grained alloys no significant grain fragmentation occurs, and at larger strains, when the spacing of high angle boundaries approaches the crystallite size, a considerable amount of the high angle boundary is removed by a process of dynamic recovery.
Influence of cryo-rolling reduction and annealing of commercially pure (CP) Al is evaluated in four aspects: microstructure, mechanical properties, electrical conductivity and general corrosion. It is shown that by selecting optimal cryo-rolling reduction and subsequent annealing condition result in ultrafine grains in CP Al with good combination of high strength and ductility. Electrical conductivity of the cryo-rolled samples decreased due to increased number of the electron scattering centers (lattice defects and grain boundary area). However, optimization of cryo-rolling and annealing treatment could restore the conductivity coupled with high strength in CP Al. Corrosion behaviour of cryo-rolled CP Al improved after annealing treatment. High dissolution rate and low thermal stability of the ultrafine grain structure could override the anticipated advantage of uniform corrosion in ultrafine grain CP Al.
Fully dense nanocrystalline tungsten (nc-W) with extremely high strength (∼3.0 GPa under quasi-static compression and ∼4.0 GPa under dynamic compression) has been obtained by high-pressure torsion (HPT) at low temperature (500 °C). The nanocrystalline microstructure is revealed by transmission electron microscopy (TEM). The grain boundaries (GBs) are mostly of the large-angle type. High-resolution TEM (lattice images) suggests that the GBs are clean and well defined (atomically sharp). GBs are non-equilibrium and of a high-energy nature. Edge dislocations are present within the grains. The authors hypothesize that these edge dislocations, combined with a depleted impurity concentrations along pre-existing GBs, contribute to enhance the ductility of nc-W. Under dynamic compression, the specimens exhibit localized shearing followed by cracking and subsequent failure, similar to their ultrafine-grain (UFG) counterparts processed by equal-channel angular pressing plus cold rolling, and to many other body-centered cubic metals with UFG/nanocrystalline microstructures. The shear band width in the HPT-processed nc-W is much smaller (shear band width <5 μm) than that observed in the UFG counterparts (shear band width ∼40 μm).
Molecular dynamics simulation is used to study the tensile mechanical properties of face-centred cubic Ni nanocrystalline materials with mean grain size of 12 nm. Three samples are considered: one as-prepared, another annealed at 800 K, and one in which additional structural disorder has been introduced to the grain boundary region. From the room-temperature deformation properties, a reduction in plastic strain is observed when grain boundaries and triple junction regions approach more equilibrium conditions. It is also observed that similar atomic activity within the grain boundary region exists under both applied stress and high-temperature conditions, indicating a close relationship between atomic-scale relaxation and inter-grain deformation mechanisms within the nanocrystalline system.
We investigated the motion of planar symmetrical and asymmetrical tilt boundaries in high-purity aluminium with <112>- and <111>-tilt axes under the influence of an external mechanical stress field. It was found that the motion of low-angle grain boundaries as well as high-angle grain boundaries can be induced by the imposed external stress. The observed activation enthalpies allow conclusions on the migration mechanism of the grain boundary motion. The motion of planar low- and high-angle grain boundaries under the influence of a mechanical stress field can be attributed to the movement of the grain boundary dislocations which comprise the structure of the boundary. A sharp transition between low-angle grain boundaries and high-angle grain boundaries was observed at 13.6°, which was apparent from a step of the activation enthalpy for the grain boundary motion. For the investigated boundaries the transition angle was independent of tilt axis, impurity content and tilt boundary plane.
Four principal factors contribute to grain-boundary strengthening: (a) the grain boundaries act as barriers to plastic flow; (b) the grain boundaries act as dislocation sources; (c) elastic anisotropy causes additional stresses in grain-boundary surroundings; (d) multislip is activated in the grain-boundary regions, whereas grain interiors are initially dominated by single slip, if properly oriented. As a result, the regions adjoining grain boundaries harden at a rate much higher than grain interiors. A phenomenological constitutive equation predicting the effect of grain size on the yield stress of metals is discussed and extended to the nanocrystalline regime. At large grain sizes, it has the Hall–Petch form, and in the nanocrystalline domain the slope gradually decreases until it asymptotically approaches the flow stress of the grain boundaries. The material is envisaged as a composite, comprised of the grain interior, with flow stress σfG, and grain boundary work-hardened layer, with flow stress σfGB. The predictions of this model are compared with experimental measurements over the mono, micro, and nanocrystalline domains. Computational predictions are made of plastic flow as a function of grain size incorporating differences of dislocation accumulation rate in grain-boundary regions and grain interiors. The material is modeled as a monocrystalline core surrounded by a mantle (grain-boundary region) with a high work hardening rate response. This is the first computational plasticity calculation that accounts for grain size effects in a physically-based manner. A discussion of statistically stored and geometrically necessary dislocations in the framework of strain-gradient plasticity is introduced to describe these effects. Grain-boundary sliding in the nanocrystalline regime is predicted from calculations using the Raj–Ashby model and incorporated into the computations; it is shown to predispose the material to shear localization.
Unique mechanical properties have been measured in submicrometer freestanding nanocrystalline Al films, where discontinuous grain growth results in a fundamental change in the way in which the material deforms. In contrast to the limited ductility normally associated with nanocrystalline metals, these nanocrystalline films exhibit extended tensile ductility. In situ X-ray diffraction and postmortem transmission electron microscopy point to the importance of stress-assisted room temperature grain growth in transforming the underlying processes that govern the mechanical response of the films: nanoscale deformation mechanisms give way to microscale plasticity.
The mechanisms of deformation and damage evolution in electrodeposited, fully dense, nanocrystalline Ni with an average grain size of ~30 nm and a narrow grain size distribution were investigated by recourse to (i) tensile tests performed in situ in the transmission electron microscope and (ii) microscopic observations made at high resolution following ex situ deformation induced by compression, rolling and nanoindentation. Particular attention was also devoted to the characterization of the structure in grain interiors and in the vicinity of grain boundaries at Angstrom-level resolution in the as-deposited material and following compression, and to the real-time video-imaging of the evolution of dislocation activity and damage during deformation; these images are presented in this paper and in the web sites provided as supplementary material to this paper. These observations clearly reveal that dislocation-mediated plasticity plays a dominant role in the deformation of nanocrystalline Ni examined in this study. Fracture surface examination confirms dimpled rupture with the scale of the dimples being several times larger than the grain size. Dislocation emission at grain boundaries together with intragranular slip and unaccommodated grain boundary sliding facilitate the nucleation of voids at boundaries and triple junctions. Individual monocrystal ligaments, formed by the growth/linking of these voids, undergo extensive local plasticity to the extent that many of them neck down to a chisel point. These voids as well as those that may have existed prior to deformation can act as nucleation sites for dimples leading to fracture that does not occur preferentially along grain boundaries. The transmission electron microscopy observations of in situ and ex situ deformed specimens are synthesized to formulate a mechanistic framework that provides new insights into the mechanisms of flow and fracture in nanostructured metals.
Mechanical alloying process using ball-milling techniques, has received much attention as a powerful tool for fabrication of several advanced materials, including amorphous, quasicrystals, nanocrystalline and composite materials, etc. This research is focused on production of Al–Al2O3 composite materials by mechanical alloying method and on investigation of its microstructure. For this purpose a horizontal ball mill was designed and manufactured. Aluminum and alumina powders, with specified size and weight percent, were added to the mill. The mixed powders were milled at different times. The milled powders were pressed and sintered under argon gas control. Microstructure of produced composite was investigated by scanning electron microscope. The results show that increasing milling time causes to make fine alumina powders as well as uniform distribution within aluminum, also in steady-state stage increasing milling time has not significant effect on their size distribution within aluminum. The results of atomic analysis of initial and milled powders at different times show that at the beginning of milling, the powders will tend to absorb iron and gradually their susceptibility decrease until steady-state condition is prevailed. The result of infrared spectroscopy does not show any evidence of compounds except alumina.
Nanocrystalline metals and alloys, with average and range of grain sizes typically smaller than 100 nm, have been the subject of considerable research in recent years. Such interest has been spurred by progress in the processing of materials and by advances in computational materials science. It has also been kindled by the recognition that these materials possess some appealing mechanical properties, such as high strength, increased resistance to tribological and environmentally-assisted damage, increasing strength and/or ductility with increasing strain rate, and potential for enhanced superplastic deformation at lower temperatures and faster strain rates. From a scientific standpoint, advances in nanomechanical probes capable of measuring forces and displacements at resolutions of fractions of a picoNewton and nanometer, respectively, and developments in structural characterization have provided unprecedented opportunities to probe the mechanisms underlying mechanical response. In this paper, we present an overview of the mechanical properties of nanocrystalline metals and alloys with the objective of assessing recent advances in the experimental and computational studies of deformation, damage evolution, fracture and fatigue, and highlighting opportunities for further research.
Recent results of the development of the severe plastic deformation methods to fabricate bulk nanostructured materials as well as results of their thorough structural characterization and investigations of their unusual deformation behavior and novel mechanical properties are presented. The structural model of nanomaterials processed by severe plastic deformation methods is developed on the basis of the obtained results.
Pure copper is deformed by high pressure torsion and the resulting microstructure is studied. Small structural elements are formed. Their size decreases with increasing strain and reach a steady-state. The misorientation between neighbouring structural elements increases with strain and finally reaches a nearly random distribution. The steady-state size decreases with increasing pressure and decreasing temperature. The shape of the elements suggests the continuous formation of new elements during steady-state deformation. This would be a process similar to dynamic recrystallisation.
Scanning electron microscopy and high resolution electron backscatter diffraction (EBSD) have been used to study substructural development during cold rolling of a single-phase Al–0.1 Mg alloy, the use of EBSD enabling more detailed quantitative measurements to be made than are possible with the transmission electron microscope (TEM). At low strains, bands of elongated cells, aligned at approximately 35° to rolling direction are formed. As the applied strain was increased, intersecting thinner and more widely spaced bands form within many grains, flow becomes localised within these new bands and they develop into microshear bands, which shear the original elongated cell structures. The changes in the scale of the microstructural features, the development of misorientations of the various types of low angle boundary and the alignment of the features to the rolling plane have been measured as a function of strain. The results are compared with previous TEM investigations of deformed aluminium, and a qualitative model of the microstructural evolution is proposed.