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Mechanical properties and rolling behaviors of nano-grained copper with embedded nano-twin bundles
Abstract
By means of dynamic plastic deformation (DPD) at liquid nitrogen temperature (LNT), bulk nano-grained copper samples with embedded nano-twin bundles were prepared. Subsequent cold rolling (CR) of the LNT-DPD Cu led to a reduction in quantity of nano-twin bundles and a slight grain coarsening, accompanied by a decrease in grain boundary (GB) energy from 0.34 to 0.22 J m À2 . An increasing CR strain leads to a saturation grain size of $110 nm, which is less than half of that in the severely deformed Cu from the coarse-grained form. Decreased strength and enhanced ductility were induced by CR in the LNT-DPD sample. The saturation yield strength in the LNT-DPD Cu during CR was $105 MPa higher than that in conventional severely deformed Cu, which originates from the finer grains as well as the nano-scale twins in the LNT-DPD sample. The enhanced ductility is primarily attributed to CR induced GB relaxation.
... 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]. ...
... 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]. ...
... 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]. Отмечается, что двойники сопряжены с матрицей обычным двойниковым ориентационным соотношением для кубических металлов [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.
... Полагается, что аномальный рост зерен в массивных однофазных материалах возможен при условии, если в материале есть небольшая доля зерен, способных к росту, в то время как миграция подавляющего большинства зеренных границ значительно затрудненав основном силами поверхностного натяжения в стыках с низкомобильными МУГ [183,194]. 128,129]. Размеры микродвойников составляют ~ 0,1 -0,3 мкм, и присутствие их заметной фракции в структуре криогенно-деформированного материала обусловливает несколько меньший средний размер зерен (табл. ...
... Этот результат отражает фракцию микродвойников в данной микроструктуре (рис. 5.2 (б)) и хорошо соотносится с литературными данными, отмечающими активизацию механического двойникования в меди при криогенной деформации [128,129]. ...
... Данный результат противоречит некоторым литературным данным, в которых отмечается формирование НК структуры в ходе криогенной прокатки меди [128,129]. В этой связи следует отметить, что результаты, опубликованные в указанных работах, получены методами ПЭМ и, таким образом, во-первых, базируются на гораздо худшей статистике, чем данные представленные в этой работе, а, во-вторых, в них не проведено разделение на измеряемые зерна и субзерна. ...
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.
... The tiny grains are bounded with each other by atomically distinct interfaces, without any amorphous phase or pores. The full crystallinity of the single FCC [58][59][60][61][62][63][64][65] are included for comparison. (Reprinted with permission from Ref. [24]). ...
... Thermal stability versus grain size in Cu-Ni and Cu-Al alloys. Grain coarsening temperature (T GC ) as a function of initial grain size for (a) Cu-Ni alloys and (b) Cu-Al alloys, including reported data in the literatures.[10,58,[68][69][70] (Reprinted with permission from Ref.[51]). ...
Most metals exist in form of polycrystalline states consisting of crystalline grains and grain boundaries. These structurally defective boundaries make the materials thermodynamically unstable. Upon heating or straining, polycrystalline metals tend to be stabilized by eliminating grain boundaries through grain coarsening or transforming into metastable glassy phases when the grains are very small. Recently, we found a different metastable structure in polycrystalline face-centered-cubic pure metals and alloys as their grains are refined to a few nanometers with cryogenic high-pressure torsion. In this structure, named as “Schwarz crystal”, the grain boundary networks evolved into the 3D periodical minimal surface structures constrained with high density twin-boundaries. It is thermally so stable that grain coarsening is inhibited at temperatures around the melting point, and exhibits a strength close to the theoretical value. Diffusional processes in alloys like precipitation of intermetallic phase, spinodal decomposition, as well as melting are inhibited with the Schwarz crystal structure. This paper briefly reviews the discovery of this novel metastable structure. The precursory process (grain boundary relaxation) in nanograined metals, formation and structure characteristics of the Schwarz crystals, as well as their thermal stability and strength in different metals and alloys will be introduced with experimental and molecular dynamic simulations. Perspectives and future studies on the structure will be discussed.
... It has been reported that the HCP phase (α-Zr) transforms to the body-centered cubic (BCC) phase (β-Zr) at a temperature higher than 863 • C in pure Zr [41,42]. Therefore, the exothermic peak represents the recrystallization process, where stored strain energy was released upon heating the as-deformed samples [43]. The recrystallization temperature of pure Zr is about 0.4 Tm, where Tm is the melting temperature of pure Zr, i.e. 2123 K [41]. ...
... However, it has been reported that the recrystallization temperature is approximately 500 • C of the as-rolled Zr with an equivalent strain of 1.61 [44,45], which is due to high stored strain energy in severely deformed samples [46]. Furthermore, Furthermore, the higher strain leads to lower recrystallization temperature [43]. Herein, the recrystallization temperature observed from the DSC curve is lower than 500 • C. In addition, the onset temperature of the exothermic peak provides some curial information for us to consider the annealing temperature after swaging. ...
... The grain coarsening temperatures (T GC , the onset temperature for apparent grain coarsening) of Cu-Ni and Cu-Al alloys as a function of the average grain size were determined and compared with that of Cu sample [10] (Fig. 6). For Cu alloys with grain size above 70 nm, the T GC decreases with decreasing size, in agreement with the reported trend for nanograined Cu [1,10,[42][43][44][45]. In this grain size range, T GC of all the Cu alloys with different grain sizes are higher than that of Cu, indicating that addition of Al and Ni can improve the thermal stability of Cu in both micron-grains and nanograins by solid solution pinning [5,22,46]. ...
... For those grains of about 80±10 nm, the GB energy is about 0.55±0.05 J/m 2 , similar as the conventional GB energy for coarse-grained Cu [10,42,44,45] and Cu-Al alloys [65]. While the GB energy of the 45±5 nm grains decrease to 0.17±0.01 ...
Deformation induced grain boundary (GB) relaxation with resultant enhancements in thermal stability was observed in various metals with grain sizes below a critical size. In this study, effects of stacking fault energy (SFE) on the GB relaxation and thermal stability are investigated in several Cu-Ni and Cu-Al alloys with gradient nanograined samples prepared using the surface mechanical grinding treatment. For each alloy, thermal stability drops with a decreasing grain size from submicrometers to about 70 nm. However, two distinct grain size dependences of thermal stability were observed below 70 nm. For Cu-10Ni and Cu-5Ni alloys with higher SFEs than Cu, thermal stability increases for smaller grains, similar to that in pure Cu. For Cu-10Al and Cu-5Al with lower SFEs than Cu, as grain sizes decrease the thermal stability elevates firstly and then drops, exhibiting a stability peak at a certain size. The observed thermal stability in the Cu alloys below 70 nm can be attributed to the GB relaxation induced by plastic deformation dominated by partial dislocation activities. The different behaviors of GB relaxation in these Cu alloys demonstrated its obvious dependence on SFE, which determined the governing deformation mechanisms and hence the degree of relaxation of GBs.
... Based on experimental investigations and theoretical modeling, various analyses on the strengthening and toughening mechanisms of nanotwins have been carried out. For instance, Zhang et al. [12] investigated mechanical properties of NG Cu with NT regions and found that the enhanced ductility could be attributed to grain boundary (GB) relaxation of the nanograins enhanced by plastic deformation during the cold rolling. Ghamarian et al. [13] studied the angle of GB misorientation as well as the type and distribution of dislocations in a relatively large region, and found that the existence and interaction of nanotwins, geometrically necessary dislocations (GNDs), and GBs played important roles. ...
... In order to study the mechanical properties of the NG Cu with NT regions, microstructures with a length 300 μm and width 60 μm are considered as tensile specimens. Although image-digitization technique [39] can be used to resolve the morphological and distribution characteristics of real microstructures after the DPD (and the cold rolling) [7,12], they have been simplified to separate the effects of shape and distribution of NT regions [28]. Here, six idealized microstructures are used, as shown in Fig. 1, with the NG Cu in green and the NT Cu in red. ...
Nanograined (NG) metals with nanotwinned (NT) regions can overcome the inferior ductility of NG metals and achieve high strength and modest ductility. Based on the strain gradient plasticity and Johnson-Cook failure criterion, we simulate the dependences of their strength and ductility on volume fraction, twin spacing, as well as shape and distribution of NT regions in NG Cu. It is found that these factors have significant impact on the overall ductility. In particular, the overall ductility abnormally decreases with the increase in the volume fraction of NT regions, which is directly related to the failure modes of this material system. Interface debonding can explain the above abnormal decrease in overall ductility. In addition, with the increase in twin spacing, fracture of NT regions can cause different reversals of overall ductility. We also found that, when the NT regions are of the oblique square type, the overall ductility is significantly lower than when they are of the circular and square types. In most cases, the array arrangement of NT regions is superior to the staggered arrangement for the improvement of the overall ductility. It is believed that these reported results can contribute to a deeper understanding of this novel material system.
... 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. ...
... Исследования показали, что пластическое течение меди в условиях криогенной деформации сопровождается интенсивным механическим двойникованием, а также формированием полос сдвига. Как следствие, было отмечено формирование микроструктуры со средним размером зерен порядка 0.1 мкм [3,4]. Данная работа была направлена на тщательную аттестацию микроструктуры и механических свойств меди, подвергнутой различной степени криогенной прокатки. ...
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.
... 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. ...
... Удельная доля двойников была довольно низкой. Таким образом, предположение о существенной интенсификации механического двойникования при криогенной деформации [4,5] не оправдались. С другой стороны, для структуры было характерно наличие значительной фракции мелких(<1 мкм) равноосных зерен, практически не содержащих МУГ и расположенных по границам крупных сплющенных зерен. ...
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.
... In addition to grain size effects, the extensive deformation-induced planar defects and resultant substructures observed through TEM in this work may contribute to further hardening by creating additional slip barriers. Hardening effects beyond grain size refinement have been reported for both metal and ceramics through the effects of intragranular defects such as nanotwins [54][55][56]. WC is well known to experience plastic deformation in response to stress [57,58]. As observed in this work, deformation of WC has been previously demonstrated to result in the formation of high densities of stacking faults [59]. ...
Industrial application of superhard materials (Vickers hardness, HV > 40 GPa) such as diamond and cubic boron nitride is limited by high costs and complex routes of synthesis. Tungsten carbide (WC) is a common industrial material valued for its hardness, but falls well short of qualification as a superhard material even in its less common but harder binderless form (HV ∼ 26 GPa). Importantly, recent efforts have demonstrated the potential for alternative materials, such as WC, to achieve similar hardness to diamond and cubic boron nitride via microstructural refinement. However, despite recent advances in sintering technology, even the smallest grained binderless WC (< 100 nm) has failed to achieve HV values above 33 GPa. In this work, multiple hardening mechanisms are exploited through a unique sintering approach proving WC as a candidate superhard material. Environmentally Controlled – Pressure Assisted Sintering (EC-PAS) is utilized to produce > 99% dense, binderless nanocrystalline WC ceramics with hardness as high as 39 GPa. The unprecedented WC hardness is attributed to the combined effects of small average crystallite size and, importantly, deformation-induced nanoscale intragranular defects including stacking faults. The demonstration of the superposition of multiple hardening mechanisms provides a new avenue to improve hardness of ceramics beyond traditional Hall-Petch hardening, yielding new classes of superhard materials.
... The low-energy coherent twin boundaries effectively impede dislocation movement, thereby strengthening MMCs while maintaining good plasticity and work hardening [114]. This advantage applies not only to uniform nano twin metals [115,116] but also to strengthening and toughening heterogeneous structures of MMCs [117]. ...
Increasing application of lightweight, high performance metal matrix composites (MMCs) namely high-strength, high-toughness, across diverse industrial sectors, has led to a noticeable quest for the production of new lightweight MMCs (LWMMCs). Architectural design features of the LWMMCs revealed significant effects on the mechanical properties of the composites as the presence of the reinforcements did. Up to now most of the research projects have focused on the effect of different reinforcements and the design of the architectural features have not been considered widely by the researchers to control the mechanical properties. This paper explores the heterogeneous design configuration of LWMMCs to achieve simultaneous strengthening and toughening. However, aluminum matrix composites (AMCs) are highlighted for their weight reduction potential and enhanced performance in aerospace, electronics, and electric vehicles. In addition, it discusses the role of reinforcements and the intrinsic matrix design in determining the mechanical properties of these composites. Then it is revealed that the remarkable mechanical properties such as elevated strength, malleability, and fracture resilience, which are not found in traditional materials are attributed to the intricate interplay features including stress-strain gradient, geometrically necessary dislocations, and distinctive interfacial phenomena. In addition to the heterogeneous deformation-induced hardening effect, this article tries to reveal the strategic importance of energy dissipation as a means to increase J o u r n a l P r e-p r o o f 2 toughness in LWMMCs by controlling the crack propagation and localizing deformation, while addressing the complexities associated with composite fabrication. KEY WORDS metal matrix composite, strengthening mechanisms, toughening mechanisms, energy dissipation, grain boundary engineering, reinforcement engineering, design configuration Graphical abstract: Introduction:
... The grain boundary energy decreases with grain size reduction, resulting in a lower recrystallization force drive. In addition, the generation of twins during SPD also reduces the grain boundary energy [52]. Therefore, nanocrystalline with a smaller size of about 40 nm in this experiment shows obviously better thermal stability below 200 C, and the DRX temperature is relatively higher than that of coarse grains. ...
... The presence of coherent low energy twin boundaries effectively prevents dislocation motion, strengthening MMCs while maintaining an acceptable level of plasticity and strain hardening [109]. This advantage applies not only to uniformly structured nano-twin metals [110,111], but also to the strengthening and toughening of heterogeneous structures in MMCs [112]. ...
In response to the growing demand for high-strength and high-toughness materials in industries such as aerospace and automotive, there is a need for metal matrix composites (MMCs) that can simultaneously increase strength and toughness. The mechanical properties of MMCs depend not only on the content of reinforcing elements, but also on the architecture of the composite (shape, size, and spatial distribution). This paper focuses on the design configurations of MMCs, which include both the configurations resulting from the reinforcements and the inherent heterogeneity of the matrix itself. Such high-performance MMCs exhibit excellent mechanical properties, such as high strength, plasticity, and fracture toughness. These properties, which are not present in conventional homogeneous materials, are mainly due to the synergistic effects resulting from the interactions between the internal components, including stress-strain gradients, geometrically necessary dislocations, and unique interfacial behavior. Among them, aluminum matrix composites (AMCs) are of particular importance due to their potential for weight reduction and performance enhancement in aerospace, electronics, and electric vehicles. However, the challenge lies in the inverse relationship between strength and toughness, which hinders the widespread use and large-scale development of MMCs. Composite material design plays a critical role in simultaneously improving strength and toughness. This review examines the advantages of toughness, toughness mechanisms, toughness distribution properties, and structural parameters in the development of composite structures. The development of synthetic composites with homogeneous structural designs inspired by biological composites such as bone offers insights into achieving exceptional strength and toughness in lightweight structures. In addition, understanding fracture behavior and toughness mechanisms in heterogeneous nanostructures is critical to advancing the field of metal matrix composites. The future development direction of architectural composites and the design of the reinforcement and toughness of metal matrix composites based on energy dissipation theory are also proposed. In conclusion, the design of composite architectures holds enormous potential for the development of composites with excellent strength and toughness to meet the requirements of lightweight structures in various industries.
... The presence of low-energy coherent twin boundaries effectively impedes the movement of dislocations, thereby strengthening MMCs while still maintaining acceptable levels of plasticity and work hardening [97]. This advantage applies not only to uniformly structured nano twin metals [98,99] but also to the strengthening and toughening of heterogeneous structures in MMCs [100]. ...
In response to the growing demand for high-strength and high-toughness materials in industries like aerospace and automobiles, there is a need for metal matrix composites (MMCs) that can simultaneously enhance strength and toughness. This paper focuses on the design configurations of MMCs, which include both the configurations resulting from reinforcements and the inherent heterogeneity of the matrix itself. The mechanical properties of MMCs are influenced by factors such as reinforcement content, shape, size, and spatial distribution within the composite architecture. Among them, Aluminum matrix composites (AMCs) are particularly significant in aerospace, electronics, and electric vehicles due to their potential for weight reduction and enhanced performance. However, the challenge lies in the inverse relationship between strength and toughness, hindering the widespread utilization and large-scale development of MMCs. The design configuration of composites plays a critical role in achieving concurrent improvements in strength and toughness. This review explores the advantages of toughness, toughening mechanisms, reinforcement distribution characteristics, and structural parameters in the design of composite architectures. Drawing inspiration from biological composites like bone, the development of synthetic composites with homogeneous structural designs provides insights into attaining exceptional strength and toughness in lightweight engineering structures. Additionally, understanding fracture behavior and toughening mechanisms in heterogeneous nanostructures is vital for advancing the field of metal matrix composites. Summarily, the design of composite architectures holds tremendous potential for tailoring AMCs with outstanding strength and toughness, addressing the requirements of lightweight engineering structures in various industries.
... Finally, as shown in Figure 5a,b, submicron-sized ultrafine grains and a certain number of annealing twins may affect the mechanical properties of the alloy. [59][60][61][62][63] The geometric phase analysis (GPA) image ( Figure S4, Supporting Information) shows the stress concentration at the nanotwins, which explains the piling up of dislocations at the interface. ...
L12 phase hardening alloys with excellent mechanical properties are of great significance for structural applications. However, low volume fractions of L12 precipitates in conventional alloys (nearly lower than 60%) tend to limit their practical usage, while the strengths of the alloys generally increase with L12 precipitation contents. Herein, a novel high‐entropy alloy (HEA) Ni35Co35Fe10Al8Ti10B2 with ultrahigh concentration L12 precipitates is successfully designed aided by the calculation of phase diagrams (CALPHAD). The volume fraction of L12 precipitates in this HEA is up to 75% and outperforms that of most of traditional superalloys. The novel L12‐strengthened Ni35Co35Fe10Al8Ti10B2 has an ultrahigh tensile yield strength of ≈1.45 GPa, ultimate tensile strength of ≈1.9 GPa, and great ductility of ≈23% at room temperature. The desirable strength–ductility combination is superior to most of conventional superalloys and reported HEAs, mainly due to the presence of ultrahigh concentration L12 precipitates that act as dislocation obstacles and the formation of numerous stacking faults and deformation twining. This work is expected to provide guidance for developing new high‐performance HEAs with an excellent combination of strength and ductility.
... The lattice strain of the samples was determined by using an X-ray diffraction (XRD, DX2700B, Haoyuan Instrument, China) instrument. On this basis, dislocation density was calculated by the following Equation 1 [29][30][31][32]. In XRD, a Cu target (wavelength = 0.15406 nm) was used, and the scanning rate was 2 °/min. ...
The Cu-1.0% Cr-0.1% Zr alloy in a solid solution state was investigated by ageing treatments at different temperatures and holding times. The structure and performance were characterized and tested by using X-ray diffraction (XRD), a transmission electron microscope (TEM), a universal material testing machine, and an eddy conductivity detector. The influence laws of ageing temperature and the holding time on the structures and properties of the Cu-Cr-Zr alloy were analyzed. Results demonstrated that, with the increase in ageing temperature and holding time, the percentage and size of the Cr precipitated phase increased, and the dislocation density decreased. The tensile strength first increased to the peak value and then decreased. The electrical conductivity increased and the amplitude decreased. The tensile strength of the alloy reached the peak (359 ± 2 MPa) after ageing at 450 °C for 60 min, and the electrical conductivity was 91.9 ± 0.7% IACS. In addition, in the ageing precipitation process, the chromium precipitated phase had face-centered cubic structure (FCC) and body-centered cubic structure (BCC) structures, and the FCC Cr phase can be transformed into a BCC Cr phase. FCC Cr, BCC Cr, and Cu3Zr precipitation phases maintained different orientation relationships with the Cu substrate.
... The preparation methods can be divided into two categories according to the formation mechanism of nanotwins. The first category is the SPD method for deformation twins, such as dynamic plastic deformation (DPD) [75][76][77]; the employed high strain rate and/or low temperature during deformation can effectively suppress dislocation motion and promote twin deformation. The other one is the deposition method for growth nanotwins, such as physical vapor deposition (PVD) [78,79] and electrodeposition [59,[80][81][82]. ...
Nanocrystalline metals developed based on fine grain strengthening always have an excellent strength, but are accompanied by a drop in ductility. In the past 20 years, substantial efforts have been dedicated to design new microstructures and develop the corresponding processing technologies in order to solve this problem. In this article, the novel nanostructures designed for simultaneously achieving high strength and high ductility developed in recent years, including bimodal grain size distribution nanostructure, nanotwinned structure, hierarchical nanotwinned structure, gradient nanostructure, and supra-nano-dual-phase nanostructure, are reviewed. Based on a comprehensive understanding of the simultaneously strengthening and toughening mechanisms, the microstructures and corresponding processing techniques are mainly discussed, and the related prospects that may be emphasized in the future are proposed.
... Many nanograins are visible in the microstructure of cryorolled samples due to the occurrence of the dynamic recrystallization mechanism as discussed in our previous publication [38]. It was reported that mechanical nano twins can be formed in the microstructure of pure copper during plastic deformation [39][40][41]. The nano twins are potential sites for the formation of new grains. ...
To obtain a pure copper with extraordinary strength-ductility-conductivity balance, asymmetric cryorolling followed by low-temperature (200 °C) annealing was employed. The microstructural-mechanical-electrical properties of the samples were investigated. The results showed that multiple twinning was formed after the partial annealing of deformed copper. The copper annealed for 60 min exhibited a hardness, yield strength, and tensile strength of 38.1 HB, 192 MPa, and 422 MPa, respectively, while a large total elongation of 37.2% was obtained. Also, the annealed copper had a larger strain hardening rate at ε higher than 0.023 owing to the creation of twin chains. The fracture surface of the 60-min annealed copper was dominated by only ductile dimple features. The post-annealed copper showed an extraordinary electrical conductivity of 99.58%IACS, which is much larger than that for all pure coppers with UTS higher than 400 MPa. It was found that using second phase particles with high electrical conductivity was a better strengthening mechanism than strain hardening and grain boundary strengthening. An extraordinary strength-ductility-conductivity balance was achieved in the current pure copper compared to other pure coppers. This was due to the existence of copper oxide, and the formation of twin chains in the present work.
... Essentially, the refined NG could be envisaged as a result of the interactions between dislocations and twin boundaries at the nanoscale [34,35]. Similar fragmentation processes of NT in materials with various SFEs subjected to plastic deformation at high strain rates and cryogenic temperatures have been frequently observed in SMAT/SMGT Cu [38,44], DPD Cu [45,46], and Cu-Al alloys [47]. ...
Gradient-nanostructured material is an emerging category of material with spatial gradients in microstructural features. The incompatibility between gradient nanostructures (GNS) in the surface layer and coarse-grained (CG) core and their roles in extra strengthening and strain hardening have been well elucidated. Nevertheless, whether similar mechanisms exist within the GNS is not clear yet. Here, interactions between nanostructured layers constituting the GNS in a Ni alloy processed by surface mechanical rolling treatment were investigated by performing unique microtension tests on the whole GNS and three subdivided nanostructured layers at specific depths, respectively. The isolated nanograined layer at the topmost surface shows the highest strength but a brittle nature. With increasing depths, isolated layers exhibit lower strength but enhanced tensile plasticity. The GNS sample’s behavior complied more with the soft isolated layer at the inner side of GNS. Furthermore, an extra strain hardening was found in the GNS sample, leading to a greater uniform elongation (>3%) as compared to all of three constituent nanostructured layers. This extra strain hardening could be ascribed to the effects of the strain gradients arising from the incompatibility associated with the depth-dependent mechanical performance of various nanostructured layers.
... The technique of Ball milling (BM) was found to be helpful and of use in providing fine powders having a grain size in the range of 10-30 nm [92][93][94][95]. By systematically reducing the grain size to nano-crystalline dimensions, the hydrogen-sorption kinetics was observed to gradually accelerate. ...
An ability to achieve useful properties of structural materials is largely dependent on their bulk microstructure. Over the years, the innate ability to achieve noticeable improvements in structural materials has relied upon processing as a viable means and/or alternative, which in turn determines the resulting microstructure and properties or behavior. Sustained research and development efforts in the domains encompassing materials science, materials engineering and manufacturing processes have made possible the arrival of a time period in which specific properties of a material can be obtained by carefully controlling the architecture of its constituents. Nanostructuring of materials to include both metals and their alloy counterparts is a key for obtaining extraordinary properties that made them attractive for the purpose of selection and use in both structural applications and functional applications. In recent years, the production of bulk nanostructured materials (BNMs) by techniques of severe plastic deformation (SPD) has attracted considerable scientific and technological interest since it offers new opportunities for the fabrication of commercial nanostructured metals and alloys that can be chosen for use in a variety of specific applications. Such nanostructured materials must essentially be not only porosity-free and but also contaminant-free, which makes them an ideal choice for studying, observing and documenting their characteristics, spanning microstructure, properties and mechanical behavior. In this paper, we provide a compelling overview of the approaches most widely used for the purpose of achieving grain refinement using the technique of plastic deformation. An outline of the four most commonly used plastic deformation processing techniques is provided. Salient aspects specific to the technique of equal channel angular pressing (ECAP), high-pressure torsion (HPT) accumulative roll bonding (ARB) of bulk nanostructured metals and surface mechanical attrition treatment (SMAT) of nanostructured layers are provided and briefly discussed. A need for the selection of certain metals and alloys for use in specific applications in the domains spanning, medicine and technology is briefly discussed. The emergence and use of computational nanotechnology, which in essence synergizes the rapid developments in computational techniques and material development, are presented and briefly discussed.
... The movement of dislocations was further hindered by the grains at the boundaries during aging, resulting in a strengthening effect. Grain boundary strengthening can be obtained from the Hall-Petch relationship[46,47]: ...
The high performance of copper alloys is widely welcomed due to their high electrical conductivity and excellent mechanical properties. These alloys are mainly used in electrical, electronic and aerospace fields. In the present work, we proposed a new class of Cu-Co-Si-Ti alloys by incorporating the multiple alloying elements, resulting in the multiple strengthening during heat treatment. It can be observed that Ti addition can significantly improve the micro-hardness of the Cu-Co-Si-alloy. Solution strengthening, deformation strengthening and dual-nanoprecipitation strengthening led to the Cu-Co-Si-Ti alloy with excellent tensile strength (617.9 MPa) and high electrical conductivity (41.7% IACS) using the optimal process of cold rolling by 50% and aging at 500 °C for 30 min. Electron backscatter diffraction technology was used to analyze the microstructure and texture evolution during the copper alloys aging process. It was found that the volume fraction of Goss, Brass, copper and S texture had close connections with the mechanical properties. From multiple strengthening mechanisms, the dual nanoprecipitation strengthening contributed the most due to the nanoprecipitation of Co2Si and Cu4Ti.
... 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.
... 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.
Solid solution-strengthened copper alloys have the advantages of a simple composition and manufacturing process, high mechanical and electrical comprehensive performances, and low cost; thus, they are widely used in high-speed rail contact wires, electronic component connectors, and other devices. Overcoming the contradiction between low alloying and high performance is an important challenge in the development of solid solution-strengthened copper alloys. Taking the typical solid solution-strengthened alloy Cu–4Zn–1Sn as the research object, we proposed using the element In to replace Zn and Sn to achieve low alloying in this work. Two new alloys, Cu–1.5Zn–1Sn–0.4In and Cu–1.5Zn–0.9Sn–0.6In, were designed and prepared. The total weight percentage content of alloying elements decreased by 43% and 41%, respectively, while the product of ultimate tensile strength (UTS) and electrical conductivity (EC) of the annealed state increased by 14% and 15%. After cold rolling with a 90% reduction, the UTS of the two new alloys reached 576 and 627 MPa, respectively, the EC was 44.9%IACS and 42.0%IACS, and the product of UTS and EC (UTS × EC) was 97% and 99% higher than that of the annealed state alloy. The dislocations proliferated greatly in cold-rolled alloys, and the strengthening effects of dislocations reached 332 and 356 MPa, respectively, which is the main reason for the considerable improvement in mechanical properties.
A thermo‐mechanical processing route involving severe cold‐rolling deformation of austenite to generate strain‐induced martensite, followed by short‐time annealing when martensite reverts to austenite, was used to introduce the heterogeneous lamellar structures (HLS) into 304 stainless steel (304ss). Firstly, a nanolamellar structure consisting of mostly martensite with retained austenite grains was formed in the severe cold‐rolled 304ss, which results in an ultra‐high strength of ∽1.9 GPa but poor ductility. After short‐time annealing, a HLS, comprised of soft recrystallized austenite lamellae with micro/sub‐micron grains embedded inside a hard matrix of reversed ultrafine/nano austenite grains containing high‐density dislocations, was introduced into the 304ss through phase reversion and partial recrystallization. The resultant HSLed 304ss exhibits a high yield strength in excess of 1.2 GPa with a large uniform elongation of ∽28%. Strong hetero‐deformation induced (HDI) hardening associated with the joint activation of twin‐induced plasticity (TWIP) and transformation‐induced plasticity (TRIP) effect are responsible for the excellent strength‐ductility combination. The thermo‐mechanical processing route used here can be easily scaled up for industrial production, and could be generalized to other materials.
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Heterogeneous laminates are developed to address the trade-off between strength and ductility, but interface cracking is a major concern. In this work, a heterostructured copper-brass laminate with 20 layers was successfully prepared by diffusion welding + cold rolling + annealing (DRA) technique. The obtained copper-brass laminate was composed of brass layers with fine grains and copper layers with coarse grains. During diffusion welding, a transition layer was found between copper and brass layers, with composition, grain size and hardness gradient. With the help of the gradient transition layer (GTL), the bonding strength of the welding interface is so high that cannot be torn apart during peeling test. The GTL plays important role in coordinating plastic deformation and relaxing stress concentration. An excellent combination of strength and ductility was realized in the copper-brass laminates, which can be attributed to the unique GTL and high density of heterogeneous interfaces. Moreover, the GTL with hardness gradient can prevent micro-cracks from initiating at interfaces, and avoid macro-crack propagating along the welding interface or coarse/fine grained heterogeneous interface.
Copper alloys are widely used as lead frames, electric contact wires and pantographs due to their high electrical conductivity and excellent mechanical properties. At present work, a medium electrical conductivity and excellent mechanical properties of Cu-Co-Si-Ti-Ce alloy was obtained by the vacuum melting process with the optimum performance of 225 HV, 702.5 MPa and 40.8% IACS aging at 500 °C for 60 min via the combinations of multiple strengthening. It can be inferred that the high-volume fraction of Goss, Brass, copper and S texture was one of the main reasons for the increase of micro-hardness by comparing the texture content at different conditions. Moreover, it was observed that Co2Si and Co16Ti6Si7 phases exhibited coherent and semi-coherent interface relationships with the copper matrix, respectively, which can relieve the interfacial stress and reduce the interface energy by GPA analysis. Finally, the contributions of solid solution strengthening, work-hardening, grain boundary strengthening and precipitation strengthening were calculated, contributing most to the precipitation strengthening.
Grain refinement always provides a high strength of metals and alloys but this is accompanied by a remarkable deterioration of ductility. Here we report a pure Zr with a heterogeneous structure, characterized by recrystallized coarse grains embedded in the ultrafine-grained matrix, which is produced by rolling at room temperature and subsequent partial recrystallization. The results show that the heterostructured Zr exhibits a superior combination of high strength and improved ductility and higher strain hardening than its coarse-grained counterparts. In terms of the high strength of heterostructured Zr, it is ascribed to the abundant ultrafine grains and significant hetero-deformation induced strengthening. Whereas good ductility is derived from the dislocation hardening of recrystallized coarse grains, the hetero-deformation induced hardening, and the sharp (0002) basal texture. The present study can contribute to understanding the essence of a desirable combination of strength and ductility of heterostructured materials.
In this research, ultrafine-grained Cu-7at. %Al-3at. %Ni alloys with a twin structure and precipitates were prepared by cold rolling+aging+cold rolling+aging (CR+ACA) and multi-directional forging+aging+cold rolling+aging (MDF+ACA). In contrast with un-deformation sample, significant increases in strength were obtained by MDF+ACA and CR+ACA, reaching 803.7 MPa and 722.8 MPa respectively, while maintaining a good elongation of 9.5% and 8.4%. Compared with CR+ACA, this is noted that a synergistic improvment in strength and elongation is achieved after MDF+ACA. The effect of microstructure on strength and elongation was investigated and the contribution of each strengthening mechanism for strength was analysed. The results show that the enhancement in strength is due to the increased dislocation density as well as decreasing grain size formed in the MDF+ACA process. The higher twinning fraction in the sample after MDF+ACA leads to an increase in elongation due to the ability of twins to storage dislocations which can increase the strain hardening capability of the alloy. In addition, the current work describes a refinement mechanism for obtaining ultra-fine grains using these two different deformation processes by phase-field crystal (PFC) method simulation. The results of this study will help in further preparation of copper alloys with synergistic improvement strength and elongation.
High/medium-entropy alloys (HEAs/MEAs) with simple single-phase face-centered-cubic (fcc) structures have exceptional tensile ductility and outstanding toughness, but their room-temperature strengths are moderate. Here we introduced a mixed nanostructure, composed of high density nano-scale twins in the form of bundles and nanograins in shear bands, in a single-phase fcc FeCrCoNi HEA by means of cryo-rolling at liquid nitrogen temperature (LNT), which result in a substantial enhancement of tensile strength. The as-prepared nanostructured FeCrCoNi HEA exhibits an ultra-high ultimate tensile strength of ∼1.8 GPa, an elongation-to-failure of ∼10%. The ultra-high strength originates mainly from the effective blockage of dislocation motion by the ultra-high density of internal twin and grain boundaries. The microstructural evolution and formation mechanism of the mixed nanostructure in FeCrCoNi HEA were investigated in detail by transmission electron microscopic characterization.
The ternary equiatomic CrCoNi medium-entropy alloys (MEAs) generally suffer from a low strength, which can be improved by mechanical nanotwins induced by cryogenic deformation. After adding Si and reducing the content of Ni, the mechanical twinning in the CrCoNiSi MEA is easy to occur at room temperature (RT), resulting in a higher strain hardening rate to delay necking. After 30% tensile deformation, a lot of thinner mechanical nanotwins are introduced into the CrCoNiSi MEA. After annealing at 773 K for 30 min, the mechanical nanotwins in the nanostructured CrCoNiSi MEA are retained. The production of strength and ductility of the nanostructured CrCoNiSi MEA is 42 GPa·%, which is 26% higher than that of the nanostructured CrCoNi MEA. In addition, the phase transformation from FCC phase to HCP phase in the CrCoNiSi MEA during tensile deformation is responsible for a higher strain hardening rate and a better elongation.
Liquid nitrogen temperature (LNT) rolling at a high strain rate was applied to in-situ TiB2/Cu composites fabricated by powder metallurgy. An artificially controlled heterogeneous distribution of TiB2 particles in a Cu matrix leads to bimodal microstructures in the LNT-rolled composites, i.e. reinforcement-rich regions are constituted by in-situ formed TiB2 particles and nano-Cu grains (about 25 nm), while reinforcement-poor regions are mainly composed of heavily deformed ultrafine Cu grains (about 0.5 µm). Compared with TiB2/Cu composites processed by normal rolling, LNT-rolled TiB2/Cu composites with heterogeneous microstructures exhibit good overall performance with high ultimate tensile strength (594 MPa), high elongation (6.3%), high electrical conductivity (82.6% International Annealed Copper Standard) and improved thermal stability, which is favourable for electrical and electronic device applications.
High purity metals are increasingly demanded in modern manufacturing industries, but their processing and applications are limited by a dilemma that purer metals are thermally and mechanically less stable. The reduced stability of pure metals originates from the weakened drag effect of impurity atoms on the mobility of grain boundaries (GBs) that are hard to stabilize without alloying. Following recent studies on stabilizing nanograined metals by tailoring structures of GBs, here we report that structural relaxation of GBs breaks the purity-stability dilemma in pure Cu. Contrary to the conventional impurity effect, thermal stability and hardness of nanograined Cu samples with relaxed GBs increase (rather than decrease) with higher purities. The discovered anomalous impurity effect, owing to suppression of GB relaxation process with impurity atoms, offers an alternative vector to stabilizing purer metals for advanced processing and applications.
After homogenization treatment, uniaxial experiments at room temperature with strain rate of 1 s⁻¹ are carried out on Mg-Gd-Y(-Sn)-Zr alloys, and the maximum strain is controlled to 8%. Plenty of {10–12} tensile twins appear in the deformed microstructure of Mg-Gd-Y-Zr and Mg-Gd-Y-Sn-Zr alloys. The addition of Sn significantly improves the nucleation ability of twins and effectively reduces the lamellar thickness. In Mg-Gd-Y-Zr alloy, the twins are mostly coarse parallel, while in Mg-Gd-Y-Sn-Zr alloy, they are mostly thin cross. A great quantity of closed regions and twin boundaries introduced by high-density thin cross twins are the main reasons for the high flow stress of Mg-Gd-Y-Sn-Zr alloy. In addition, the analysis of twin variant selection behavior, twin interaction and dislocation transmutation reaction in this paper is conducive to consummate the theory of room temperature deformation of magnesium alloys.
The effect of pre-aging on properties of Cu0.24Cr0.20Sn alloy before rolling and aging was studied in details. The results displayed the pre-aging was useful to improve the microhardness and conductivity of Cu0.24Cr0.20Sn alloy before cold rolling and aging, and the effect increased with the extension of pre-aging time. The microhardness and electrical conductivity of Cu0.24Cr0.20Sn alloy by first pre-aging at 400 °C for 2 h, second 85% rolling and then aging at 300 °C for 1 h can reach 189 HV and 85.4 %IACS, respectively. The TEM results indicated the density of precipitates increased with the increase of pre-aging time, and the interaction between precipitates and dislocations was gradually strengthened in the subsequent room-temperature rolling. The increase caused by pre-aging treatment before rolling and aging was mainly due to dislocation density strengthening.
In this work, we prepared equiatomic AlCoCrFeNi high-entropy alloy (HEA)-particle-toughened, Zr-based metallic glass composites by spark plasma sintering. By adding HEA particles as the second phase, the strength and plasticity of the Zr-based metallic glass composites improved concomitantly. After fracture, High-density dislocations and nanocrystals were formed in the HEA particles due to local severe plastic deformation, which consumed massive strain energy to enable the resistance to crack formation. Substantial lattice distortion imparted a remarkable work-hardening capacity to the HEAs and enhanced crack-tip dislocation trapping, and thus led to an extreme refinement of the grain size. Finite-element analyses indicated that the strain hardening behavior of HEA particles reduced the magnitude of strain localization, promoted generation of multiple shear bands, and stabilized shear band propagation. We attribute the enhanced strength-ductility synergy in the current composites to high-density dislocations and nanocrystal formation in the HEA particles, and stable propagation of multiple shear bands.
An in-depth exploration of the microstructure evolution and characterization of the dislocation density in Cu–Ni–Si–Co alloy was performed using X-ray diffraction electron backscattering diffraction, transmission electron microscopy, three-dimensional atom probe technique, and high-energy X-ray diffraction (HEXRD). The results reveal that the tensile strength and electrical conductivity of the alloy are 937.27 MPa and 45.57% IACS, respectively, and that it has good stress relaxation resistance. There are many dislocation tangles, dislocation lines, dislocation cells, and deformation twins in alloys processed by cyclic cryogenic rolling and low-temperature aging. The alloy contains both 300–600 nm coarse particle precipitates and 5–10 nm rod-like nano-precipitated phases composed of (Ni, Co)2Si. The average dislocation density is (35.967 ± 1.513) × 10¹⁴ m⁻², and the twin stacking-fault probability is 14.414 ± 0.333 × 10⁻³ in the alloy aged at 350 °C for 2 h.
The yield strength levels of solid-solution strengthened nickel-based superalloys are low compared to precipitation-strengthened counterparts. Here, Inconel 625 alloy was used as a model, thermo-mechanical treatments were employed to improve the yield strength of the alloys. Compared to the as-solution condition, the yield strength was increased from 291 to 676 MPa with the ductility reduced from 75 to 50% after cold rolling and annealing at 1073 K for 30 min. The strength-ductility synergy originates from (1)(1) grain boundaries and pre-existing annealing twins act as strong barriers to hinder dislocations motion, and (2) the formation of deformation twins during plastic deformation provides an extra work-hardening region to maintain the excellent ductility. The strengthening effect of annealing twins is determined by the thickness rather than the length fraction. Deformation twins tend to form in the fine-grained Inconel 625 alloy (the grain size ranging from 3.1 to 4.7 μm) during plastic deformation. This is the reason for an extra work hardening region in the hardening curve. Our present work can provide a reference for realizing the strength-ductility synergy of solid-solution strengthened nickel-based superalloys.
Laser shock imprinting (LSI) technology has attracted more and more researchers' interest in the fabrication of high-precision three-dimensional (3D) microstructures. These researchers found that LSI technology can improve the depth, accuracy, and mechanical properties of the formed parts. However, they also found that when the formed parts made of LSI are used in high temperature environment, the formed parts are easy to return to their initial state, which greatly limits the application of 3D microstructure and LSI technology. Therefore, it is very important to improve the accuracy and stability of microstructure and realize the rapid manufacturing of formed parts. Therefore, we use temperature-assisted laser shock imprinting (TALSI) technology to solve these problems. In addition, there have been many studies on the strengthening mechanism of hardness, tensile and fatigue properties of LSI technology, but there is still a blank in the research on its high temperature stability and strengthening mechanism. In this study, warm laser shock imprinting (WLSI) experiments were carried out, followed by high-temperature recovery experiments, and the stress and strain distributions were studied by numerical simulation. Then, the surface morphology, mechanical properties of the laser impact samples were tested and characterized by 3D optical profiler and scanning electron microscope (SEM), as a result, the effects of temperature on the plasticity, flow stress, dynamic yield strength and deformation springback of aluminum foil are obtained. In addition, the microstructure evolution of aluminum foil before and after WLSI treatment was characterized by transmission electron microscope (TEM) and electron backscatter diffraction (EBSD) technology, and the deformation mechanism and high temperature stability mechanism of WLSI were obtained. It is of great significance to understand the forming/strengthening mechanism of laser shock technology and the development of LSI technology in the future.
We used molecular dynamics simulations to investigate the relationship between mechanical properties and deformation mechanisms in CoCrFeMnNi alloys having different compositions. According to deformation twinning activity, the deformation mechanisms in the different CoCrFeMnNi compositions under tensile loading can be classified into three major categories: easy-shear (ES), inter-locks (IL) and bulk hexagonal closed packed (hcp) transformation (BH). We found that the compositions with ES were more likely to rupture early because of the short and fragmented twins. The compositions following the IL pattern promoted the movement of interlocking stacking faults in the twin boundaries that could prolong tensile deformation. Moreover, BH could further increase ductility because the hcp transformation could absorb and dissipate the stacking faults. Experimental observations of immobile stacking-fault networks that impede dislocation motion and further provide preferred sites for the formation of the hcp phase were commonly found in the ES, IL and BH patterns. The calculated intrinsic stacking fault energies of CoCrFeMnNi and CoCrFeNi were consistent with the experimental measurements. The combination of high unstable stacking fault energy with the formation of the IL pattern results in high strength and high ductility in the CoCrFeMnNi HEA system.
The following article is based on the MRS Medal talk presented by William L. Johnson at the 1998 MRS Fall Meeting on December 2, 1998. The MRS Medal is awarded for a specific outstanding recent discovery or advancement that has a major impact on the progress of a materials-related field. Johnson received the honor for his development of bulk metallic glass-forming alloys, the fundamental understanding of the thermodynamics and kinetics that control glass formation and crystallization of glass-forming liquids, and the application of these materials in engineering.
The development of bulk glass-forming metallic alloys has led to interesting advances in the science of liquid metals. This article begins with brief remarks about the history and background of the field, then follows with a discussion of multicomponent glass-forming alloys and deep eutectics, the chemical constitution of these new alloys, and how they differ from metallic glasses of a decade ago or earlier. Recent studies of deeply undercooled liquid alloys and the insights made possible by their exceptional stability with respect to crystallization will then be discussed. Advances in this area will be illustrated by several examples. The article then describes some of the physical and specific mechanical properties of bulk metallic glasses (BMGs), and concludes with some interesting potential applications.
The first liquid-metal alloy vitrified by cooling from the molten state to the glass transition was Au-Si, as reported by Duwez at Caltech in 1960. Duwez made this discovery as a result of developing rapid quenching techniques for chilling metallic liquids at very high rates of 10 ⁵ –10 ⁶ K/s.
A bulk nanograined Cu sample embedded with nanoscale twins is produced by means of dynamic plastic deformation at cryogenic temperatures. It exhibits a tensile yield strength of 610 MPa and an electrical conductivity of 95% IACS at room temperature. The unique combination of a high strength and a high conductivity is primarily attributed to the presence of a considerable amount of nanoscale twins which strengthen the material significantly while having a negligible influence on electrical conductivity.
Copper sheet samples composed of nanometer scale lamellar twins was produced by electrodeposition. The coherent lamellar twin boundaries were within 20° of being parallel to the sheet plane in more than 60% of the grains. The electrodeposited sample was cold rolled to 30 and 85% reductions in thickness and the structural evolution during cold rolling was examined by transmission electron microscopy (TEM) and high resolution TEM (HRTEM). Extensive activity of partial dislocations along twin boundaries and of perfect dislocations within twins (in particular in coarse twins >100nm) were identified. Moreover, it was found that shear banding occurred, which locally destroyed the lamellar twin structure. A dislocation structure developed within the shear bands, and such a structure evolved with strain and gradually replaced the lamellar twin structure. After 85% deformation, a large volume fraction of the lamellar twin structure was replaced by a lamellar dislocation structure characteristic of high strain rolling where the lamellar dislocation boundaries are almost parallel to the rolling plane. It was also found that the structural scales are coarser in the lamellar dislocation structure than in the initial lamellar twin structure.
The achievement of both high strength and high electrical conductivity in bulk materials is challenging in the development of multifunctional materials, because the majority of the strengthening methods reduce the electrical conductivity of the materials significantly. At room temperature, dislocations have little scattering effect on conduction electrons. Thus, a high density of dislocations can strengthen conductors without significantly increasing the resistivity. However, at room temperature (RT), which is defined as 295 ± 2 K in this paper, deformation can only introduce a limited number of dislocations in pure metals due to dislocation annihilation, i.e. recovery. This limitation is expanded by a well-controlled liquid nitrogen temperature (LNT), which is defined as 77 ± 0.5 K in this paper, deformation process that permits accumulation of both nanotwins and a high density of dislocations accompanied by significantly less recovery than that in RT-deformed samples. The dislocations are organized into refined dislocation cells, with thicker cell boundaries in LNT-deformed samples than those deformed at RT. LNT deformation stores more energy in the material than RT deformation. LNT deformation produces bulk pure Cu with a yield strength about 1.5 times that of RT-deformed Cu. The RT resistivity increase is less than 5% compared with that of annealed Cu.
It is well known that plastic deformation induced by conventional forming methods such as rolling, drawing or extrusion can significantly increase the strength of metals. However, this increase is usually accompanied by a loss of ductility. For example, with increasing plastic deformation, the yield strength of Cu and Al monotonically increases while their elongation to failure (ductility) decreases. The same trend is also true for other metals and alloys. We report an extraordinary combination of high strength and high ductility produced in metals subject to severe plastic deformation (SPD). We believe that this unusual mechanical behavior is caused by the unique nanostructures generated by SPD processing. The combination of ultrafine grain size and high-density dislocations appears to enable deformation by new mechanisms. This work demonstrates the possibility of tailoring the microstructures of metals and alloys by SPD to obtain both high strength and high ductility. Materials with such desirable mechanical properties are very attractive for advanced structural applications. (17 refs.)
A nanostructured surface layer was formed on an Inconel 600 plate by subjecting it to surface mechanical attrition treatment at room temperature. Transmission electron microscopy and high-resolution transmission electron microscopy of the treated surface layer were carried out to reveal the underlying grain refinement mechanism. Experimental observations showed that the strain-induced nanocrystallization in the current sample occurred via formation of mechanical microtwins and subsequent interaction of the microtwins with dislocations in the surface layer. The development of high-density dislocation arrays inside the twin-matrix lamellae provides precursors for grain boundaries that subdivide the nanometer-thick lamellae into equiaxed, nanometer-sized grains with random orientations.
Deformation-induced grain growth has been reported in nanocrystalline (nc) materials under indentation and severe cyclic loading, but not under any other deformation mode. This raises an issue on critical conditions for grain growth in nc materials. This study investigates deformation-induced grain growth in electrodeposited nc Ni during high-pressure torsion (HPT). Our results indicate that high stress and severe plastic deformation are required for inducing grain growth, and the upper limit of grain size is determined by the deformation mode and parameters. Also, texture evolution suggests that grain-boundary-mediated mechanisms played a significant role in accommodating HPT strain.
The influence of the microstructure and the misorientation relationship between grains on the mechanical properties is investigated in specimens of ultrafine-grained copper processed by equal channel angular extrusion (ECAE) route BC in 1, 2, 4, 8, 12 and 16 passes is described. XRD peak broadening analyses showed a decrease in the lattice strain at roughly constant domain size as the number of passes increased. Analysis by TEM showed that the microstructure evolves from lamellar boundaries and elongated cells towards a more equiaxed homogeneous microstructure. On the microscale, observed by TEM, the degree of misorientation among subgrains/cells increases and the width of boundaries decreases while the cell/subgrain size remains approximately constant as the number of passes increases. Yield stress and ultimate tensile stress reach a maximum after four passes. From 4 to 16 passes the strength of the material decreases and the uniform elongation increases. It is suggested that the increase in the ductility (and decrease in strength) are associated with the operation of recovery mechanisms which decrease the boundary volume and the total dislocation density causing an increase in the mean free path of dislocations. The application of two single parameter models describing the constitutive behavior of the mechanical properties helps in the understanding of storage and annihilation of dislocations during deformation and the microstructural observations.
The nanocrystalline (nc) Cu samples with different microstrains but the same grain size were obtained by annealing a magnetron-sputtered nc Cu specimen. Quantitative X-ray diffraction (XRD) measurements show that with an increment of the microstrain from 0.14 to 0.24% the thermal expansion coefficient (TEC) of crystalline lattice increases by about 12%, the static displacement of atom from the equilibrium position (BS) increases from 0.47±0.09 to and Debye characteristic temperature (D) decreases from 307.1±3.1 to 279.2±2.8 K. The microstrain effect on thermal properties in the nc Cu might be attributed to the change in density of grain boundary defects/dislocations. The present investigation demonstrates that the thermal properties of nc materials are determined by not only the grain size but also the microstructure of grain boundaries.
Nanocrystalline metals--with grain sizes of less than 100 nm--have strengths exceeding those of coarse-grained and even alloyed metals, and are thus expected to have many applications. For example, pure nanocrystalline Cu (refs 1-7) has a yield strength in excess of 400 MPa, which is six times higher than that of coarse-grained Cu. But nanocrystalline materials often exhibit low tensile ductility at room temperature, which limits their practical utility. The elongation to failure is typically less than a few per cent; the regime of uniform deformation is even smaller. Here we describe a thermomechanical treatment of Cu that results in a bimodal grain size distribution, with micrometre-sized grains embedded inside a matrix of nanocrystalline and ultrafine (<300 nm) grains. The matrix grains impart high strength, as expected from an extrapolation of the Hall-Petch relationship. Meanwhile, the inhomogeneous microstructure induces strain hardening mechanisms that stabilize the tensile deformation, leading to a high tensile ductility--65% elongation to failure, and 30% uniform elongation. We expect that these results will have implications in the development of tough nanostructured metals for forming operations and high-performance structural applications including microelectromechanical and biomedical systems.
Methods used to strengthen metals generally also cause a pronounced decrease in electrical conductivity, so that a tradeoff must be made between conductivity and mechanical strength. We synthesized pure copper samples with a high density of nanoscale growth twins. They showed a tensile strength about 10 times higher than that of conventional coarse-grained copper, while retaining an electrical conductivity comparable to that of pure copper. The ultrahigh strength originates from the effective blockage of dislocation motion by numerous coherent twin boundaries that possess an extremely low electrical resistivity, which is not the case for other types of grain boundaries.
While some superior properties of nanostructured materials (with structural scales below 100 nm) have attracted numerous interests of material scientists, technique development for synthesizing nanostructured metals and alloys in 3-dimensional (3D) bulk forms is still challenging despite of extensive investigations over decades. Here we report a novel synthesis technique for bulk nanostructured metals based on plastic deformation at high Zener-Hollomon parameters (high strain rates or low temperatures), i.e., dynamic plastic deformation (DPD). The basic concept behind this approach will be addressed together with a few examples to demonstrate the capability and characteristics of this method. Perspectives and future developments of this technique will be highlighted.
The effects of deformation on the structure of copper and its low stacking-fault energy alloys are reviewed and it is shown that these may be classified according to the method of examination used. In rolled low stacking-fault energy alloys the deformation sequence involves the formation of stacking faults, mechanical twins, and shear bands that are at first restricted to individual grains but subsequently extend from one surface to the opposite surface. In copper the deformation sequence involves the formation of equiaxed cells of dislocations, microbands, clustering of microbands, and shear-band formation.
Ultrafine-grained (UFG) copper produced by severe plastic deformation
demonstrates an unusual combination of high strength and rather high
ductility at room temperature. To obtain the UFG microstructure, the
equal channel angular extrusion technique was used in the current work.
The microstructure, crystallographic texture and grain boundary
distributions were studied in asreceived and cold-rolled samples. The
microstructures and textures generated by rolling of the UFG sample were
compared with those formed in identically rolled conventional
coarse-grained copper. Rolling to 83% resulted in drastic changes of all
microstructural parameters owing to the transformation of the
microstructure with initially equiaxed grains into a banded
microstructure with mainly elongated and subdivided grains having
orientations typical of a rolling texture. Additional rolling led to the
strengthening of the main texture components. The emergence of cube
oriented {001} 100 grains was distinguished in the rolled UFG samples.
During storage at ambient temperature these grains grew at the expense
of deformed adjacent grains, thus transforming the rolled UFG
microstructure into a partially recrystallized state.
Atomic-scale computer simulations have previously identified a deformation mechanism, which becomes important in nanocrystalline metals with grain sizes below 10—15 nm. Instead of proceeding through dislocation activity in the grains, the deformation occurs by slip events in the grain boundaries, leading to a reverse Hall-Petch effect, i.e. a decrease in hardness with decreasing grain size. In this paper, the consequences of this shift in deformation mode are investigated for systems subjected to large strains in a cyclic deformation pattern. In most coarse-grained metals, severe plastic deformation leads to grain refinement. Indeed, severe plastic deformation is often used to generate nanocrystalline metals with grain sizes down to hundred nanometres. The simulations indicate that these processes are suppressed in sufficiently small grains, and instead the sliding events in the grain boundaries dramatically enhance diffusion processes, and lead to grain coarsening as the deformation proceeds.
Two basic equations are derived for deducing the dislocation density in powdered materials from the particle size and strain breadth measured from the Debye-Schemer spectrum. In the particle size estimate, it is assumed that the material has a block structure similar to that found in microbeam studies and that the dislocations lie along the block surfaces. The number of dislocations along each face, n, is not known. In the strain broadening estimate the x-ray line broadening from a dislocation array is calculated in terms of the broadening due to an isolated dislocation and a strain energy factor F, which allows for the effect of dislocation arrangement. Both methods involve an unknown quantity but by equating the two results it is possible in most cases to get both a narrow bracket for the dislocation density and considerable information on the dislocation arrangement.
Pure copper powder was employed to study the effects of ball milling on the development of the structure and properties of ductile metals. The results indicate that larger spheres with diameters of about 2–2.5 mm are created after 20 h of ball milling. The formation of such spheres is mainly due to sphere-to-flake or sphere-to-sphere welding. This welding is not complete, leaving large pores and curved voids in the spheres. The average grain size of such spheres in 10–100 nm. The increase in lattice strain is about 0.2%. The microhardness increases from 45 MPa (unmilled) to 220 MPa (milled for 20 h). High-resolution transmission electron microscopy (HRTEM) investigations show the following: (a) the deformation of hall-milled copper proceeds by [112](111) twinning or high-order twinning: (b) the [112](111) twins are thickned by passage of (a/6)[112] twinning partial dislocations; (c) subgrains tend to form in the twins. In addition to twinning, dislocation slip plays an important role in the deformation process; the mobility of 60° dislocations and their pile-up in the crystals can lead to the formation of subgrains. Crystal refinement leads to an increase in the number of grain boundaries; both low-angle and high-angle grain boundaries with local strain and a high density of dislocations are observed. The estimated mean dislocation density is more than 1014 m−2, which is hardly even reached in plastically deformed metals. The different kinds of structural defects which exists in the grain boundaries and within the crystals may result in increased strength and microhardness, increased free energy and changes in other properties of ball-milled materials.
This paper deals with the mechanical behavior of Cu and solid–solution Cu–Al alloys that were shock-deformed to 10 and 35 GPa. All the shock-deformed materials showed shock-strengthening that was greater at higher shock pressure and decreased with decreasing stacking fault energy (SFE) at both shock pressures. In the literature, shock-strengthening has been qualitatively ascribed to a greater dislocation density and the formation of deformation twins without addressing the question as to why shock-strengthening is lower in low SFE materials. This question is addressed in the present work by quantifying the twin contribution to the total post-shock strength. The twin contribution was found to increase with decreasing SFE suggesting that the contribution of dislocations concurrently decreases. The stored energy of as-shock-deformed materials was measured and found to decrease with decreasing SFE implying a lower net stored dislocation density in the lower SFE alloys. It is suggested that a lower net stored dislocation density in low SFE alloys results in the observed lower shock strengthening.
Two essentially different metals, fcc copper and hexagonal titanium, were deformed by equal channel angular pressing (ECAP) up to eight passes. The microstructure developed as a result of severe plastic deformation (SPD) was studied by X-ray peak profile analysis. The formation of submicron grain sized structures was studied as a function of the number of ECAP passes. Thermal stability of the microstructure in both copper and titanium was examined by differential scanning calorimetry (DSC). During the isothermal heat-treatment of copper a bi-modal microstructure was formed, as manifested in a special shape of the peak profiles. In titanium, a considerable fraction of dislocations gets annihilated at temperatures well below the exothermic peak in the DSC curve.
If the dark matter particles decay or annihilate, the resulting product particles could ionize the baryonic gas during the cosmic dark age. The ionization history of the Universe thus provides a powerful constraint on the decay and annihilation of dark matter. Recent CMB observations can be used to constrain the property of the dark matter. Here I review recent progresses on this constraint of dark matter property.
Work hardening behaviors of electrodeposited ultrafine-grained Cu with nanoscale growth twins are investigated by means of uniaxial continuous tensile and loading–unloading tests. A high density of nanoscale twins leads to a significant enhancement in flow strength and work hardening rate, while the work hardening coefficient (n) does not show any obvious variation with the twin density. These effects were analyzed in terms of the post-deformation microstructures.
The influence of grain size or cell size on the mechanical behavior of metals is well known, although debate continues about details of defect control mechanisms. In the past, difficulty in producing bulk samples with grain sizes smaller than about 1 {mu}m limited mechanical behavior studies in finer-grained materials. Expressions relating strength to grain size, determined from studies of coarse-grained materials, suggest that reducing grain size into the sub-micrometer range results in increased mechanical strength at low homologous temperatures. At high temperatures, diffusional creep effects may lead to increased ductility. This paper reports the major results to date of an ongoing study of the mechanical behavior of nanocrystalline metals produced by the inert-gas condensation method; some results have been reported elsewhere. Studies of the tensile strength, low-temperature creep and Vickers microhardness of Cu, Pd and Ag reported here are complemented in this broader study by processing studies, x-ray grain-size and strain analyses, and high resolution microscopy studies of nanostructure and microstructure. The work provides a basis for predicting low-temperature mechanical behavior of ultrafine-grained metals, subject to some significant constraints imposed by the processing conditions. 16 refs., 2 figs., 3 tabs.
The microstructure of ultrafine-grained fcc metals (Al, Al–Mg alloys, Cu and Ni) produced by applying severe plastic deforma-tion (SPD) techniques is studied by X-ray diffraction line profile analysis. It is found that Mg addition promotes efficiently the reduc-tion of the crystallite size and the increase of the dislocation density in Al during SPD process. In Al–Mg alloys the crystallite size reaches its minimum value at lower strain than the dislocation density saturates. The results also show that the yield strength cor-relates well with that calculated from the dislocation density using the Taylor equation.
The stress–strain relationships of high-purity aluminum and copper were investigated over a wide range of strain by combining data obtained in the conventional tensile and compression testing of annealed samples with data obtained after processing by equal-channel angular pressing (ECAP) to high imposed strains. The true stress–true strain curves were analyzed mathematically taking the absolute strain as zero for the annealed condition and equal to the strain imposed through ECAP for the as-pressed samples. It is shown that, over the entire range of strain inherent in these experiments, the macroscopic stress–strain behavior of Al and Cu may be represented by an exponential power-law constitutive relationship which reduces to the conventional Hollomon power-law re-lationship at low strains and to the Voce exponential relationship at high strains. It is demonstrated that the results obtained from tensile or compressive testing to low strains at room temperature are sufficient, when used with the new constitutive relationship, to provide detailed information on the nature of the stress–strain behavior to high strains. The validity of the new relationship is supported by theoretical considerations which incorporate the major micro-mechanisms of plastic deformation.
Shear band development in consolidated nanocrystalline and ultrafine-grained Fe has been monitored as a function of overall strain from the onset of plastic deformation. The deformation mechanisms of the grains inside the shear bands, the origin of the inhomogeneous deformation, and the propensity for shear localization in nanostructures are explained based on microstructural information acquired using transmission electron microscopy. © 2002 American Institute of Physics.
An electro-deposited Cu sample with a high density of nano-scale growth twins shows an ultrahigh tensile strength (∼1GPa) with a considerable plastic strain (>13%). Both the strength and the ductility increase with a decreasing twin lamellae thickness. The yield strength values follow the empirical Hall–Petch relationship for conventional polycrystalline Cu.
The effect of dilution on the microstructure, hardness and wear properties of two nickel base NiCr hardfacing alloys deposited using the gas tungsten arc welding (GTAW) process has been studied. Dilution from the base metal altered the microstructure, volume fraction and type of precipitates in the deposit, all of which varied with the distance from the deposit/substrate interface. The microstructural variation in the deposit was accompanied by corresponding variation in the deposit hardness. A pin on disc wear test, carried out using pins with varying thickness of deposit, showed that the wear resistance of the deposit increased with increasing thickness of the deposit, indicating that the wear resistance decreases with increasing dilution from the base metal. The present study brings out the effect of dilution from the substrate material on the properties of NiCr hardface deposits and the need to ensure a minimum thickness of GTAW deposits of these hardfacing alloys for obtaining the desired wear resistance.
In this study a series of a segmented copolyester, poly(4,4′-dioxy-2,2′-dimethyl-azoxybenzene dodecanedioyl) (PMABD)-co-polyoxypropylene 400 (POP), was prepared. The chain length of PMABD studied (n) was varied from 7.8-18.2, and that of POP was unchanged. The intrinsic viscosity of the segmented copolyesters was 1.04-1.30, and the number average molecular weight obtained was 2.53 × 104−3.49 × 104 g/mol. The mesophase texture and thermal properties of the segmented copolyesters were measured as functions of n. It was found that the insert of flexible POP between those liquid crystalline domains of PMABD did affect thermotropic properties of PMABD. As the n value was 9.0 and 7.8 (or 7.4 and 8.6% by weight POP) the texture appeared as cholesteric-like oily streaks. The effect could not be attained by simply copolymerizing a mesogenic moiety with a pair of spacers of different lengths. The fluidity and domain structure of the flexible dodecanedioyl-POP-dodcanedioyl segments are taken into account for the obtained results. © 1995 John Wiley & Sons, Inc.
Flaw-free bulk Cu specimens with a high density of nano-scale mechanical twins were produced by means of dynamic plastic deformation at liquid nitrogen temperature. The nano-twinned Cu exhibits a tensile yield strength of 600 MPa and an elonga-tion-to-failure of 11%. The significant strengthening originates from the effective blockage of glide dislocations by numerous twin boundaries and dislocations.
Simultaneous increase of the ductility and strength of bulk ultra-fine-grained (UFG) Cu is achieved by introducing large amounts of deformation twins and high-angle grain boundaries via cryodrawing and cryorolling (red plots and image). Bulk UFG materials usually have high strength but disappointingly low ductility. Most previous attempts to enhance the ductility of single-phased UFG materials sacrificed their yield strength. This work provides a new approach for increasing ductility without sacrificing strength.
The interaction between screw dislocations and coherent twin boundaries has been studied by means of molecular dynamics simulations for Al, Cu and Ni. Depending oil the material and the applied strain, a screw dislocation approaching the coherent twin boundary from one side may either propagate into the adjacent twin grain by cutting through the boundary or it may dissociate within the boundary plane. Which one of these two interaction modes applies seems to depend oil the material dependent energy barrier for the nucleation of Shockley partial dislocations. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Final yield strengths of metals plastically deformed in one‐dimensional strain by shock compression are calculated from the theory of shock propagation and work hardening. An energy analysis of the shock‐relief cycle employs a Mie‐Grüneisen‐type equation of state and appropriate Hugoniot compressibility curve to determine the thermodynamic states of the material and the plastic work done. The residual elastic strain energy stored in the metal was estimated assuming that a portion of the plastic work remains in the form of lattice defects. Results for copper and nickel are found to be in reasonable agreement with experimental data.
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.
The elastic and tensile behavior of high-density, high-purity nanocrystalline Cu and Pd was determined. Samples with grain sizes of 10–110 nm and densities of greater than 98% of theoretical were produced by inert-gas condensation and warm compaction. Small decrements from coarse-grained values observed in the Young's modulus are caused primarily by the slight amount of porosity in the samples. The yield strength of nanocrystalline Cu and Pd was 10–15 times that of the annealed, coarse-grained metal. Total elongations of 1–4% were observed in samples with grain sizes less than 50 nm, while a sample with a grain size of 110 nm exhibited > 8% elongation, perhaps signifying a change in deformation mechanism with grain size. Hardness measurements followed the predictions of the Hall-Petch relationship for the coarse-grained copper down to ≈ 15 nm, and then plateaued. Hardness values (divided by 3) were 2–3 times greater than the tensile yield strengths. Processing flaws may cause premature tensile failure and lower yield strengths. The size and distribution of processing flaws was determined by small-angle neutron scattering. Tensile strength increased with decreasing porosity, and may be significantly affected by a few large processing flaws.
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.
We present a comprehensive computational analysis of the deformation of ultrafine crystalline pure Cu with nanoscale growth twins. This physically motivated model benefits from our experimental studies of the effects of the density of coherent nanotwins on the plastic deformation characteristics of Cu, and from post-deformation transmission electron microscopy investigations of dislocation structures in the twinned metal. The analysis accounts for high plastic anisotropy and rate sensitivity anisotropy by treating the twin boundary as an internal interface and allowing special slip geometry arrangements that involve soft and hard modes of deformation. This model correctly predicts the experimentally observed trends of the effects of twin density on flow strength, rate sensitivity of plastic flow and ductility, in addition to matching many of the quantitative details of plastic deformation reasonably well. The computational simulations also provide critical mechanistic insights into why the metal with nanoscale twins can provide the same level of yield strength, hardness and strain rate sensitivity as a nanostructured counterpart without twins (but of grain size comparable to the twin spacing of the twinned Cu). The analysis also offers some useful understanding of why the nanotwinned Cu with high strength does not lead to diminished ductility with structural refinement involving twins, whereas nanostructured Cu normally causes the ductility to be compromised at the expense of strength upon grain refinement.
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.
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 microstructural evolution and formation mechanism of nanostructures in bulk pure Cu samples induced by dynamic plastic deformation (DPD) at high strain rates and cryogenic temperatures were investigated using transmission electron microscopic characterization. Three different mechanisms were identified for the plastic deformation and microstructural refinement, including dislocation manipulation and rearrangement, deformation twinning forming nanoscale twin/matrix (T/M) lamellae in bundles, and shear banding in the T/M lamellae. An increasing tendency of deformation twinning and shear banding was observed at higher strains. For strain ε = 2.1, a mixed nanostructure is formed in the DPD Cu bulk sample with nanoscale T/M lamellae making up about 33% of the volume and nano-sized grains making up about 67%. The nanograins can be classified into three types in terms of their origin: (i) nanograins derived from fragmentation of nanoscale T/M lamellae with an average transverse size of about 47 nm; (ii) nanograins in shear bands with an average transverse size of about 75 nm; and (iii) nanograins derived from dislocation cells with an average transverse size of about 121 nm. The high density of deformation twins induced by high strain rates and cryogenic temperatures in DPD, distinct from that in conventional severe plastic deformation, plays a crucial role in formation of the nano-sized grains.
Microstructural evolution and grain refinement in pure Cu subjected to surface mechanical attrition treatment (SMAT) were investigated by means of systematic transmission electron microscope observations. Two different mechanisms for plastic strain-induced grain refinement in Cu were identified, corresponding to different levels of strain rate. In the subsurface layer of the SMAT Cu samples with low strain rates, grains are refined via formation of dislocation cells (DCs), transformation of DC walls into sub-boundaries with small misorientations, and evolution of sub-boundaries into highly misoriented grain boundaries. The minimum size of refined grains via this process is about 100 nm. In the top surface layer (thickness <25 μm) with a high strain rate, the grain refinement includes: (i) formation of high-density, nanometer-thick twins dividing the original coarse grains into twin–matrix (T–M) lamellae; (ii) development of dislocation walls that further subdivide the T–M lamellae into equiaxed nano-sized blocks; (iii) evolution of these preferentially oriented blocks into randomly oriented nanosized grains. The minimum size of such refined grains is about 10 nm. The present study demonstrates the critical role of strain rate on the plastic strain-induced grain refinement processes and on the minimum grain size obtainable via plastic deformation.
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.
The presence of grain coarsening is studied in bulk nanocrystalline Cu samples synthesized by inert gas condensation and in electrodeposited (ED) Ni under uniaxial compression. Grain coarsening is clearly evidenced in Cu, but is less pronounced than coarsening under an indenter. No coarsening is observed in ED-Ni.
The motion of planar, symmetrical <112>- and <111>-tilt boundaries under the influence of a mechanical shear stress was investigated and compared with experiments on curved, symmetrical <112>- and <111>-tilt boundaries with the same angles of misorientation under the influence of a constant capillary force. It was found that the motion of planar low angle grain boundaries as well as planar high angle boundaries can be induced by the imposed external stress and that the dependence of the activation enthalpies for grain boundary motion on the misorientation angle is different for planar and curved <111>-grain boundaries. The different observed activation enthalpies are attributed to different migration mechanisms for grain boundary motion. It seems that the activation of a migration mechanism depends on the misorientation angle and the driving force and that grain boundaries have the possibility to move by different migration mechanisms. Irrespective of whether grain boundaries are planar or curved there was a distinct transition between low angle and high angle grain boundaries at the same misorientation angle.
The evolution of microstructure and the mechanical response of copper subjected to severe plastic deformation using equal channel angular pressing (ECAP) was investigated. Samples were subjected to ECAP under three different processing routes: BC, A and C. The microstructural refinement was dependent on processing with route BC being the most effective. The mechanical response is modeled by an equation containing two dislocation evolution terms: one for the cells/subgrain interiors and one for the cells/subgrain walls. The deformation structure evolves from elongated dislocation cells to subgrains to equiaxed grains with diameters of ∼200–500 nm. The misorientation between adjacent regions, measured by electron backscatter diffraction, gradually increases. The mechanical response is well represented by a Voce equation with a saturation stress of 450 MPa. Interestingly, the microstructures produced through adiabatic shear localization during high strain rate deformation and ECAP are very similar, leading to the same grain size. It is shown that both processes have very close Zener–Hollomon parameters (ln Z ∼ 25). Calculations show that grain boundaries with size of 200 nm can rotate by ∼30° during ECAP, thereby generating and retaining a steady-state equiaxed structure. This is confirmed by a grain-boundary mobility calculation which shows that their velocity is 40 nm/s for a 200 nm grain size at 350 K, which is typical of an ECAP process. This can lead to the grain-boundary movement necessary to retain an equiaxed structure.