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Morphology evolution and orientation characteristics of precipitate of ultra-high strength titanium alloy
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The control of the post-forging cooling rate has been a key issue in the industrial production process of titanium alloys. We investigated texture evolution and variant selection (VS) during β → α transformation through high-temperature compression experiments followed by quantitative control of varying cooling rates. Results show that post-forging cooling rates affect β grains, α variants, and α / β textures. The α precipitation inhibits motions of β static recrystallization (βSRX) grain boundaries and thus leads to grain refining from 0.1°C/s to 0.05°C/s. Further analysis reveals that lamellae grain boundary widmanstatten α (αWGB) keeps growing rapidly within β-grain in an interface instability manner at 0.1-0.05°C/s. Most of α-phase with 50°-60°/<-12-10> is preferentially precipitated at β-medium angle GBs between 30°-45° and strictly follows BOR with the side of of adjacent β-grain with the same or similar {110} or {111}. Moreover, the texture type transforms gradually from RGoss {110} <1-10> to Brass {110} <1-12> from 25°C/s to 1°C/s. βSRX grains exhibit (102) [-201] texture, while the corresponding α has textures of <0001>//Z and <11-20>//Y from 1°C/s to 0.05°C/s. Our findings lay a profound theoretical foundation in microstructure evolution of near-β titanium alloy for industrial production.
Diffusional-displacive transformations are generally associated with mechanical properties of materials such as high strength. Understanding structure evolution of these transformations is of great interests from both physics and application perspectives. By combining atomic resolution electron microscopy, energy dispersive spectroscopy and first principles calculations, a continuous β→αʺ→α transformation process has been quantitatively characterized at the atomic level in a metastable β-Ti alloy during aging treatment. The transformation is revealed to develop by a novel mechanism involving continuous structural and compositional changes towards the equilibrium assisted by compositional fluctuation in the β matrix. Moreover, the product phase induces a precipitate-matrix lattice mismatch, thus produces a coherency strain field surrounding the precipitates. The coherent strain field contributes significantly to the increasing hardness of the alloy after aging. These results have great potential for tailoring thermomechanical treatment routes and improving mechanical properties of materials.
Abstract In titanium alloys, variant selection (VS) of grain boundary α (GBα) at prior β grain boundaries (GBs) during α precipitation has a significant influence on the subsequent transformation pathway and transformation texture development of the α phase, and thus on the final mechanical properties. In this paper, we assess systematically the applicability of all current empirical rules for VS of GBα at prior β GBs using experimental characterization of GB misorientation, boundary plane inclination, and orientation relationships between the GBα and adjacent β grains in Ti-5553. We find that all these VS rules may be violated for a given β grain boundary. Based on the experimental observations, the frequencies of each of these empirical rules being either violated or followed have been assessed, and from this assessment, we analyze theoretically whether the arguments underlying each of these rules are physically sound and why rules are violated, and then discuss when a sound prediction may be made using these empirical VS rules.
Through three-dimensional phase-field simulations, dislocations are found to exert significant influences on variant selection and subsequent development of transformation texture during α precipitation in α/β titanium alloys. It is found that, for the dislocation configurations considered in the current study, the elastic interaction between α precipitates and dislocations dominates variant selection during the nucleation stage, whereas the habit plane orientations of α precipitates relative to the dislocation lines play an important role in variant selection during the growth stage. In general, edge dislocations exhibit a much more prominent effect than screw dislocations. Various morphological patterns of α precipitates formed by heterogeneous nucleation around dislocations of different configurations are revealed, including clusters of multiple variants and a special “tent” structure that appears as a pyramid with each of its faces composed of a particular α variant. Two types of frequently observed clusters of α variants are found to share either a common <1 1 1>β axis or a common {1 1 0}β plane. The primary tent structure is able to induce the nucleation of secondary α through autocatalysis, reducing the degree of variant selection. The effect of undercooling on variant selection is also investigated in the context of competition between the chemical driving force for α precipitation and the elastic interaction between dislocation and precipitate.
During β -processing of α /β and β titanium alloys, variant selection (VS) of grain boundary α (GBα) is one of the key factors in determining the final transformation texture and mechanical properties. It has been observed frequently that GBα prefers its 〈0 0 0 1〉α pole to be parallel to a common 〈1 1 0〉β pole of the two adjacent β grains and results in a micro-textured region across the grain boundary (GB) and, as a consequence, slip transmission may take place more easily across that GB. In order to investigate how such a special prior β GB contributes to VS of GBα, we develop a crystallographic model based on the Burgers orientation relationship (BOR) between GBα and one of the two β grains. The model predicts all possible special β grain boundaries at which GBα is able to maintain BOR with both β grains. A new measure for VS of GBα, (βΔJβBOR)(βΔJβBOR), i.e., a measure of the deviation of the actual OR between the GBα and the non-Burgers grain from the BOR, is proposed. For the particular alloy chosen for experimental observations, Ti-5553, it is found that when the misorientation angle θmθm of (βΔJβBOR)(βΔJβBOR) (instead of the closeness between two closet {110}β poles between two β grains widely used in literature) is less than 15°15°, misorientation between the two β grains dominates the VS of GBα and, in particular, the variant with minimum θmθm is always selected for GBα. A possible effect due to grain boundary plane inclination on VS is also discussed.
In the present study, the role of β grain coarsening on α variant selection has been investigated in the near-titanium alloy Ti–21S (Ti–15Mo–3Nb–3Al–0.21Si). The material was first thermomechanically processed in a fully β stabilised condition in order to obtain a fine β grain size before undertaking controlled β grain-coarsening heat treatments. Two different cooling regimes ensured that either all β was retained at room temperature or significant α formation was achieved during cooling with predominant nucleation from β grain boundaries. Detailed electron backscatter diffraction (EBSD) characterisation was carried out on the β quenched and slowly cooled samples in order to compare the predicted α texture based on the β texture measurements assuming no variant selection with the measured α textures. A strong correlation was found between β coarsening and level of variant selection. It was also found that the grain coarsening is driven by the predominant growth of low energy grain boundaries, which strengthen specific β texture components that are part of the 〈1 1 1〉∥ND γ fibre. Finally, it was possible to demonstrate that the strengthened β texture components promote β grain pairs with a common 〈110〉, which is known to enhance variant selection when α nucleates from β grain boundaries.
Beta-Ti alloys contain sufficient concentrations of beta stabilizing alloy additions to permit retention of the metastable beta phase after cooling to room temperature. Decomposition of the metastable beta phase results in the formation of several possible phases, at least two of which are metastable. Concurrently, equilibrium alpha phase often forms first by heterogeneous nucleation at the alpha grain boundaries with an accompanying precipitate free zone observed adjacent to the grain boundary alpha. The grain boundary regions are softer than the precipitation hardened matrix. As a consequence, fracture follows the prior beta grain boundaries, especially in high-strength conditions. This fracture mode results in low tensile ductility and/or fracture toughness. This article will describe methods of minimizing or eliminating grain boundary alpha formation by using metastable transition precipitates to nucleate alpha more rapidly. The effects on fracture behavior also will be described.
This paper focused on the interface relationships between primary α (αp) and the precipitation secondary α (αs) phase at different cooling rates from 965℃. The rapid cooling rate favors the precipitation of the secondary αs. And the low cooling rate is conducive to the epitaxial growth of the primary αp. One key finding is that once the grain boundary (αGB) nucleated on the αp could grow forward without maintaining the Burgers orientation relationship (BOR) with adjacent β grains. What’s more, HADDF and Fourier-filtered HRTEM images elucidated that two adjacent αp particles share a common plane but not the same atomic arrangement direction in crystal spatial. Three precipitation types of the αs were discussed through EBSD and 3D spatial crystalline orientation. Specifically, the αs shares common {11 2¯ 0} plane with the αp grains respectively in Type-I and Type-II, but displaying different c axis angle (60o in Type-I and 80° in Type-II). For the Type-III, the secondary αs shares common {10 1¯ 0} plane with the primary αp, and the angle of c axis between them is 60 o. Moreover, the primary αp do not maintain the BOR with the pre-existing β, except for Type-I.
The grain boundary α (αGB) in titanium alloys is believed to significantly impact the subsequent β → α transformation, the evolution of α microtexture and the final mechanical properties. The αGB with remained β phase has been generally observed in Ti-6Al-2Sn-4Zr-6Mo and other α + β titanium alloys. In this study, the growth behavior and variant selection of αGB formed by β processing were investigated by the interrupted furnace cooling experiment. In the early growth period of the αGB, discontinuous chevrons αGB particles grew, which was significantly associated with Mo element distribution. Besides, the double-Burgers orientation relationship (2-BOR) variant selection criterion according to the θ2-BOR (misorientation of the close-oriented αGB variant pair of parent β1 and β2) deviation is refined in this study. At β grain boundary with θ2-BOR deviation below 8º, the αGB orientation is 100% predicted. In this case, the αGB grains are always close to the closest theoretical variant pair of both sides β grains. When θ2-BOR is over 14º, the criterion is not predictive.
The transformed α grains of selective laser melted Ti-6Al-4V display a strong (0001) fiber texture along the building direction, whereas during SLM printing, the parent high temperature β grains have preferentially grown in the <001> parallel to the building direction. In the current work, we have studied the phenomenon of variant selection occuring during the cubic to hexagonal phase transformation by investigating in detail the distribution of misorientation pairs and intervariant boundaries observed in orientation contrast maps acquired by EBSD. It was found that local variant selection within a single parent grain governed the martensitic transformation as it was observed that the experimentally assessed distribution of intervariant axis/angle pairs significantly differed from the one that can be expected if the Burgers orientation relationship applies with equal probability of the 12 possible variants. The variant selection could be explained by considering the degree of self-accommodation, which is corroborated by the characteristic morphologies formed by three- variant clusters as observed in the orientation contrast maps. Irrespective of the misorientation, the grain boundary characterization distribution revealed a strong anisotropic distribution of the internal interfaces with a tendency of alignment of these interfaces with the {hki0} prismatic planes. The dominant [10¯553¯]/ 63.26° intervariant boundaries displayed primarily twist and 180-tilt (3¯210) boundary planes.
As a crucial branch for titanium industry, high-strength titanium alloys (HS-TAs, with UTS ≥ 1100 MPa) are indispensable structural materials for advanced engineering applications such as aerospace and marine fields. Along with the expansion of HS-TAs’ market, achieving satisfying synergies of high strength, high ductility (elongation ≥ 6%) and high toughness (KIC ≥ 50 MPa⋅m1/2) has been identified as the uppermost technical bottleneck for their research and development. To overcome the challenge, two primary strategies have been initiated by the titanium community, developing novel alloys and innovating processing technologies. For the former, a dozen of newly-developed alloys were reported to exhibit excellent strength-ductility-toughness combinations, including Ti-5553, BT22, TC21 and Ti-1300, for which the ideal mechanical performances were based on specific microstructures realized by low impurity rate (e.g. oxygen content ≤ 0.15 wt%), complicated processing and complex heat treatment. For the latter, several innovatory forging and heat treatment technologies were originated for the mature alloys to optimize their balanced property by extraordinary microstructural characteristics. In this review, we provide a comprehensive overview over the research status, processing and heat treatment technologies, phase transformation, processing-microstructure-property correlation and strengthening-toughening mechanism of HS-TAs for aerospace engineering applications manufactured via melting-forging process. Finally, the prospects and recommendations for further investigation and development are proposed based on this review.
Morphological variations of grain boundary (GB) allotriomorphs and widmanstatten sideplates (WS) of alpha precipitate in a beta titanium alloy, have been systematically investigated by combining three-dimensional (3D) phase-field simulations, 3D experimental characterization and crystallographic orientation analysis. The inclination angle, theta, between the habit plane of the GBalpha and the hosting GB plane is found to dictate the morphologies of GB precipitates. For the first time, three distinct regimes of alpha morphology at a beta GB, separated by two critical angles (theta_c^1, theta_c^2 ) are observed: theta<=theta_c^1, GBalpha alone decorates the GB as a continuous layer; theta_c^1<theta<=theta_c^2, alpha precipitate appears as a mixture of GBalpha and a WS emanating from it; theta>theta_c^1, WS alone grows directly from bare beta GBs. The dramatic morphological variation owes its origin to the dynamic interplay between the spreading along GBP and the intragranular growth of the GB precipitate, and its dependence on theta.
Variant selection, a common but complex phenomenon in Ti alloys, is not only governed by microstructural characteristics that affect the nucleation process of variants, such as grain orientation, grain boundaries, and residual α phase, but also remarkably influenced by some kinetic factors, such as cooling rate and residual stress, especially for the additive manufactured Ti alloys. To determine the influence of cooling rates on variant selection of laser solid formed (LSF) Ti-6Al-4 V alloys, the selection of α variant in different zones of LSFed samples (possessing different cooling rates) but belonging to one single β grain was systematically studied. With the electron backscatter diffraction (EBSD) data, it was revealed that, though all 12 kinds of α variants appeared under different cooling rates, the area percentage of some variants remarkably deviated from the corresponding theoretical values at different cooling rates. To characterize the change of variant selection quantitatively, the length fraction of α/α boundary distinguished by types of angle/axis was further analyzed statistically. The results show that the length fraction of α/α boundary of type IV (63.26°/[−1055–3]) is larger than that of other types when the cooling rate is high (bottom zone) due to the high residual stress, while at low cooling rates (middle zone), the α/α boundary of type II (60°/[11–20]) dominates, which could be attributed to the self-accommodation mechanism during the β → α phase transition. The insights gained about the influence of cooling rate on the selection of α variant are helpful for understanding the microstructural evolution in LSFed Ti alloys.
Morphology of secondary grain boundary α phase (αGB) has great influence on grain boundary Widmanstätten α phase (αWGB) cluster and further fracture toughness and ductility of the titanium alloys. In this paper, the bifurcation behavior and morphology of secondary αGB in TC18 titanium alloy were investigated through high and low temperature two-step heat treat experiment (910℃/30 min→800℃/45 min/WQ) combined with a three-dimensional reconstruction experiment. It was found that αGB showed flat, equiaxed, zig-zag morphologies, and αGB bifurcated to reduce the energy of the system and maintained the stability of structure by interfacial instability nucleation. There exist two types of bifurcation behaviors, bi-direction bifurcation and tri-direction one. The significant difference between them is nucleation positions, β/β grain boundary or the triple junction (TJ) of β grain boundary. The bifurcation behavior of α phase at TJ is related to its original morphology. Bi-direction bifurcation occurs mostly in the flat αGB, but is not observed in the zig-zag αGB. The present results provide new information on αGB of TC18 titanium alloy.
The crystallographic arrangements of the α-phase variants in α-β titanium alloys remains largely unidentified due to the crystallographic complexity involved while being essential to understand the α-β microstructural intricacy. To improve the current understanding, specimens of two columnar-grained α-β Ti alloys (Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo) and two equiaxed-grained α-β Ti alloys (Ti-6Al-4V and Ti-4Al-2V) were fabricated by laser metal powder deposition (LMD). Electron backscatter diffraction (EBSD) analyses were applied to more than 10⁵ α-phase variants in each alloy. The results revealed that the Type 4 α/α variant boundary ([10¯553¯]/63.26∘) is prevalent in the two columnar-grained α-β alloys while the Type 2 α/α variant boundary ([112¯0]/60∘) is common in the two equiaxed-grained α-β alloys. Further EBSD characterisation indicates that α-variant selection tends to be more prevalent in equiaxed prior-β grains, featured by the Category I triple-α-variant clusters, which mostly terminate on dense {101¯1} planes with lower boundary energy. Conversely, columnar prior-β grains show significant Category II triple-α-variant clusters, which mostly terminate on less dense {41¯3¯0} planes with higher boundary energy. Self-accommodation to compensate for the β→α transformation strain is assumed to be the major underlying mechanism. The implications of these findings for understanding the tensile strengths are discussed in conjunction with the Schmid factor of α-variants calculated in columnar- and equiaxed-grained Ti-6Al-4V.
Effect of cooling rates, i.e., air cooling and furnace cooling, after solution in α+β phase-field on variant selection, coarsening behavior of α phase and microstructure evolution were investigated in α+β TC21 alloy. The textures of primary α (αp) and lamellar α (αL) in β phase transformation microstructure (βt) were analysed separately, and the orientation relationship among αp, αL and the parent β phase were studied. In addition, the influence of the microstructure characteristics on the tensile properties was investigated. The results showed that all parent β grains, despite their different orientations, produced 12 ideal αL variants with the same texture components and interweave to form a basketweave βt structure under the air-cooling condition. The αp without Burgers orientation relationship (BOR) with β phase exhibited obviously texture component without overlapping the αL texture component. The volume fraction of αp in the furnace-cooled sample (about 50%) was higher than that of the air-cooled sample (about 12%), while the size of it slightly increased with decreasing the cooling rate. In each β grain, the thick αL in the same orientation formed an α colony. A typical 3 variant colonies which were related to each other were observed. Consequently, the αL spatial orientation distribution showed more heterogeneity. Moreover, the BOR between αp and β and the same orientation of some αL and the surrounding αp grains resulting in the overlapping of αp texture component and αL texture component. At last, the relationship between microstructure and tensile properties was analysed.
Evolution of grain boundary α (GB α) in a α + β titanium alloy had been examined by morphology and crystallographic orientation analysis. The results indicate that GB α only retains a Burgers orientation relationship (BOR) with one of the adjacent β grains in most of the prior β/β boundaries. The colony α (single crystallographic variant of parallel α plates) tends to evolve in the adjacent β grain that maintains BOR with GB α, whereas the dendritic α (small protuberances of GB α) tends to grow in the other adjacent β grain that does not maintain BOR with GB α. Additionally, two α colonies developing into two adjacent β grains from GB α keep the same crystallographic orientation as that of GB α when GB α maintain a near BOR with both adjacent β grains in a special prior β/β boundary (60°/〈1 1 0〉). These observations are understood from the perspective of minimum interfacial energy and strain energy.
Based on the statistical analysis on the orientation relationship (OR) among adjacent primary α (αp) grain, β phase and secondary α (αs) phase in a bimodal-structure near-α titanium alloy, the effect of special αp on the variant selection of αs was investigated. The results showed that only a small part of αp grains held the Burgers OR (BOR) with the neighbouring β phase, and the αs tended to preferentially nucleate on the BOR-αp grain and shared a common orientation with the BOR-αp grain. Meanwhile, the driving force for the variant selection was discussed base on the competition between the αp-β and αp-αs interfacial energies, which indicated that the variant selection resulted from the minimal αp-αs interfacial energy.
The paper studies the effect of oxidation on the plastic deformation and fracture of near β titanium alloy Ti–5Al–5V–5Mo–1Cr–1Fe produced by radial shear rolling. It is shown that the aging of the rolled titanium alloy in ambient air at 550 °C for 3 h leads to the formation of an oxide (alpha-case) layer 1.5–2 μm thick with a simultaneous decrease in the strength and ductility of oxidized specimens under tension at room temperature. The study of the gauge section surface and fracture behavior revealed that the reason for ductility reduction in alloy specimens with an oxide layer in tension is the development of surface microcracks, whose density and depth increase with strain. The growth of microcracks to a depth exceeding the alpha-case layer thickness causes brittle fracture. The strength decrease of alloy specimens with an alpha-case layer is caused by a decrease in the microhardness of the near-surface layer due to reduced V and Mo concentrations, which influences the solid-solution hardening of titanium alloys, as well as by a change in the β-phase decomposition kinetics due to oxygen diffusion.
The mechanisms of static globularization of Ti-17 alloy during heat treatment are investigated at 820 °C and 840 °C. Static globularization fraction increases with the increasing time and temperature during heat treatment. The quantitative analysis indicates that the process of static globularization can be divided into the two distinct stages. The first is a fast globularization stage and occurs at the beginning of heat treatment, and the second is a slow globularization stage and includes microstructure change during prolonged heat treatment. The first stage consist of the globularization of alpha lamellae via boundary splitting, whereas the second stage is controlled by microstructure coarsening including termination migration and Ostwald ripening. Boundary splitting is a process in which the distortion energy is rapidly released, and this process is finished via shearing surfaces caused by shear deformation and substructure generated by recovery. The duration is about 0.5–1 h. Termination migration and Ostwald ripening are the diffusion process of elements, and they are influenced by temperature and geometrical shapes of microstructure. The driving power comes from the energy gradient among the different positions in the same alpha particle or the different alpha particles. Termination migration leads to the separation and coarsening of the lamellar alpha and Ostwald ripening causes the decreasing of amount of particles and microstructure coarsening. Boundary splitting, termination migration and Ostwald ripening jointly promote static globularization process of Ti-17 alloy.
In the present work, we investigated the texture evolution as well as the role of the β texture intensity on the α-variant selection and microstructure morphology during the α→β→α phase transformation in Ti60 alloy. Different microstructures and textures were obtained through forging the Ti60 bars into diameters of 45 mm denoted as D45 and 30 mm as D30. Subsequently, small samples with the same size cut from both bars were heat-treated above the β transus followed by furnace cooling and air cooling. We found that the β texture intensity, cooling rate, and variant selection affect both the texture intensity and microstructure morphology of the transformed α phase. The high temperature β phase exhibits stronger (110) parallel to axial direction fiber texture in the β annealed D30 bar than that for the D45 bar. A basketweave microstructure was found in the β annealed D45 bar after air cooling whereas a coarse α colonies populated by fine α lamellae formed in the D30 bar. For furnace cooled samples, the α texture is stronger than that in the air cooled sample in D45 bar whereas the intensities of the textures are very similar to air cooled sample in the D30 bar. The influence of the β texture intensity and variant selection on the α texture evolution were studied by comparing the four experimental results and simulation data. The effect of α variant number in a β grain on α texture intensity, base on variant selection, and the effects of β texture intensity on microstructure morphology are discussed.
The elastic interactions among α precipitates and their effects on variant selection are investigated. The stress field around a semi-coherent α lath is first calculated using phase field microelasticity theory and then the precipitation process of β→α+β in Ti-6Al-4V is simulated using a 3-D phase field model. The orientations of secondary α variants induced by a primary (pre-existing) α variant obtained from the phase field simulations are consistent with those of variants within commonly observed α clusters in experiments. The formation of α/α orientation relationship of the [112¯0]α/60°-type, [10‾553]α/63.26°-type and [0001]α/10.53°-type frequently observed in experiments could be a direct consequence of autocatalysis during nucleation and growth of α variants. Moreover, the competition among α variants nucleated simultaneously around a primary α lath for limited number of preferred nucleation sites also results in the selection of variants possessing specific crystallographic orientation with respect to the primary α precipitates.
The effects of cooling rate following the β or α/β heat treatment on microstructure and phase transformation are investigated for BT25y alloy. For this purpose, BT25y alloy is soaked at 1000 °C, 980 °C, 960 °C, 940 °C or 920 °C for 10 min, and then control cooled at rate of either 0.15 °C/s, 1.5 °C/s, 15 °C/s, 45 °C/s, 90 °C/s or 150 °C/s to room temperature. Microstructure observations indicate that the microstructure of BT25y alloy is significantly influenced by the cooling rate. When material is cooled from the β phase field at the lower rate, the αGB and αWGB phases are precipitated, and this transformation process can be divided into the four stages: (a) the formation of αGB, (b) the connection of adjacent αGB, (c) the precipitation of αWGB, (d) the growth of αWGB. However, the increasing of the cooling rate will greatly restrain the precipitations of αGB and αWGB phases. In this case, the acicular martensite α′ is precipitated inside β grain. The primary equiaxed-α is retained when material is cooled down from the α/β phase field. The content and size of equiaxed-α decrease with the increasing of solution temperature, but is independent on the cooling rate. At the lower cooling rate, the lamellar α is precipitated and its thickness increases with the increasing of solution temperature. But the increasing cooling rate will weaken the precipitation capacity of the lamellar α. Instead, the martensite α′ phase is precipitated and gradually takes the place of the lamellar α with the increase of cooling rate. In conclusion, whether material cools down from β single phase field or α/β two-phase field, a phase transformation law is summarized for BT25y alloy. The lamellar α is the only precipitated phase when the cooling rate ≤15 °C/s. The precipitated phase consists of the lamellar α and martensite α′ phase when the cooling rate is 15 °C/s ∼90 °C/s. The only martensite α′ phase is precipitated when the cooling rate ≥90 °C/s.
Morphology and variant selection (VS) of grain boundary (GB) allotriomorphs and Widmanstätten side-plates of α phase in an α/β titanium alloy, Ti-6Al-4V (wt%), are investigated using a three-dimensional phase field model. The structures of low-angle GBs (misorientation θm ≤ 10°) are modeled as discrete dislocation networks using Frank-Bilby theory. It is shown that α allotriomorphs and side-plates compete with each other during precipitation and the final morphology and selected α variants exhibit a strong correlation with the GB dislocation structures. While the side-plate morphology is more preferred by a symmetrical tilt GB with θm ∼ 10°, it can also be induced by a pure twist GB with θm ≤ 5°. Quantitative analysis indicates that precipitate morphology and VS are determined by the interplay among (i) elastic interaction between a nucleating α precipitate and the GB dislocation networks, (ii) growth anisotropy determined by the relative inclination of the habit plane with respect to the GB dislocations, (iii) density of nucleation sites for the same variant and coalescence during growth, and (iv) spatial confinement from simultaneously nucleated neighboring α variants of dissimilar types.
The intergranular lattice strains in the near-β titanium alloy Ti-β21S have been measured in the longitudinal and transverse directions during tensile loading using neutron diffraction. The obtained results have been modelled and explained using a two-phase elasto-plastic self-consistent model. We first identified the best set of the main model parameters (single crystal elastic constants, critical resolved shear stresses) and investigated the influence of these parameters on the behaviour of both the polycrystal and the diffracting volume. Good agreement between the experimental results and those predicted by the model was found for the macroscopic stress-strain dependencies and lattice strain developments of Ti-β21S.
The influence of transformation temperature on microtexture development associated with α precipitation at β/β grain boundaries (GB) in the near-β Ti17 alloy was studied using electron backscatter diffraction and considering isothermal treatments. For the alloy studied and the temperature range considered, decreasing the transformation temperature decreased the local microtexture strength within each prior β grain because of a larger number of αWGB colonies (standing for α Widmanstätten GB) formed per β grain, each colony increasing by one the number of α orientations inside each prior β grain. This larger number of αWGB colonies was a consequence of faster formation along β/β GB of their precursors, the allotriomorphic αGB grains (standing for α-GB) at lower transformation temperatures, as evidenced by detailed examination of the first stages of αGB formation. αGB crystallographic orientations frequently followed a variant selection (VS) criterion based on the alignment of (0 1 1)β//(0 0 0 1)αGB//(0 1 1)β. From a statistically relevant number of observations, VS was found to be more frequent at a lower transformation duration and a lower temperature, but the effect was not significant enough to influence the final α microtexture, considered at the scale of one prior β grain. αGB grains that followed the VS criterion emitted two αWGB colonies on either side of the β/β GB more frequently than those with no particular orientation.
The mechanism of nucleation and growth of α-lamellae when a TA15 Ti alloy with an equiaxed structure is cooled from an α + β phase field were studied by end quenching experiments and thermal simulation tests using a Gleeble-3500. The results showed that for the Ti alloy TA15 the nucleation and growth of α-lamellae involved four steps, including nucleation of αGB, growth of αGB, nucleation of αWGB, and growth of αWGB. Widmanstätten α, αWGB, grew faster than grain boundary α, αGB, and equiaxed α in common cases. It was found for the first time that the mode of nucleation of αWGB for alloy TA15 was interface instability, i.e. αWGB nucleated through surface instability and the protuberance of αGB and equiaxed α, and the αWGB nucleus did not have an independent and complete surface. A new model of the nucleation of αWGB and phase transformation is proposed. The growth of αWGB in the TA15 alloy started from a small protuberance and spread into a β grain with a sectorial morphology, to become lamellar instead of spiculate or oblate cuboid in shape. The nucleation rate of αWGB determined the thickness of αWG, with, to some extent, an inverse relation between the nucleation rate and thickness of αWGB.
The crystallography of alpha(hcp) precipitates formed on the beta(bcc) matrix grain boundaries has been studied with transmission electron microscopy (TEM) in a Ti-15V-3Cr-3Sn-3Al alloy. The alpha precipitates have a near-Burgers orientation relationship with respect to at least one of the adjacent beta grains. Among the possible 12 variants in this orientation relationship, the variant that [11•20]alpha is parallel to the beta closest to the grain boundary plane tends to be preferred by the alpha precipitates. Additionally, further variant selections are made so as to minimize the deviation of orientation relationship with respect to the ``opposite`` beta grain from the Burgers one. Such rules in variant selection often result in the formation of precipitates with a single variant at a planar grain boundary. Prior small deformation of beta matrix changes the variant of alpha precipitates at the deformed portion of grain boundary. It is considered that the stress field of dislocations in the slip bands intersecting with the boundary strongly affects the variants of alpha precipitates. Discussion of these results is based upon a classical nucleation theory.
In the present study, the texture evolution and the role of β grain growth on variant selection during β→α phase transformation have been investigated in Ti-6Al-4V with and without 0.4wt% yttrium addition. The aim of adding yttrium was to control α grain growth above the β transus by pinning grain boundaries with yttria. Both materials were first thermomechanically processed to generate similar starting microstructures and crystallographic textures. Subsequently, both materials were solution heat treated above the β transus followed by slow cooling to promote growth of the α lath structure from grain boundary α. Additional interrupted slow cooling experiments were carried out to identify the α lamellae that nucleate first from β grain boundaries. Detailed EBSD analysis was carried out and it was found that the β heat treatment did not generate new texture components although the intensities of the individual components changed dramatically depending on the alloy/β grain size. Variant selection was assessed by comparing measured α texture components with predicted α texture components based on the high temperature β texture assuming equal variant selection. It was found that with increasing β grain size variant selection intensified favouring the {φ1, Φ, φ2} {90°, 30°, 0°} texture component. Interrupted cooling experiments revealed that α nucleates first on β grain boundaries that are formed by two β grains having a common (110) normal and that these α lamellae display almost exclusively a {φ1, Φ, φ2} {90°, 30°, 0°} orientation. Consequently, the dominance of this variant with increasing β grain size can be related to the relative free growth of this particular α texture component into an “empty” β grain.
The microtexture of secondary α plates in Ti–4.5Fe–6.8Mo–1.5Al has been investigated by electron backscatter diffraction (EBSD) to obtain more insight in the nucleation and variant selection of these α plates. A statistical analysis of the EBSD data shows that for most β grain boundaries the variant selection of the α plates is in agreement with a commonly used variant selection criterion yielding that the α-{0001} pole is nearly parallel to the closest β-{110} poles of the two adjacent β grains. For a small angle between the β-{110} poles nucleation is predominantly observed at both sides of the grain boundary, while with increasing angle some β grain boundaries exhibit nucleation of α plates at only one side. In the β grain interior many so-called Type 2 α–α grain boundaries are observed which are thought to originate from autocatalytic nucleation when a new α plate is formed at an existing α–β interface.
The present study has examined for α/β-Ti alloys the relationship between the morphology and crystallography of Widmanstätten plates of α-Ti in colonies within a prior grain of β-Ti. Thus, optical metallography, scanning electron microscopy and transmission electron microscopy have been used to characterize the morphological features of the microstructure, whereas orientation-imaging microscopy (OM) and transmission electron microscopy (TEM) have been employed to reveal crystallographic information. It has been discovered that within a prior β-Ti grain, although the growth direction of the Widmanstätten plates in given colonies may differ by large angles from α-plates in other colonies, they may exhibit very close crystallographic relationships. For example, inclined α-plates may share common basal planes and be related by a rotation of ~10.5° about the c-axis of the crystals. This phenomenon has been interpreted on the basis of variant selection of the Burgers orientation relationship commonly adopted between the α and β phases in these alloys. Similar relationships have been observed in α-colonies growing either side of a given prior β grain boundary. These latter observations have been used to draw conclusions concerning the precipitation of α on prior β grain boundaries.
The role of starting texture in variant selection has been studied during α → β → α transformation in Ti–6Al–4V. By hot rolling at different temperatures followed by recrystallization, material with either a strong basal texture or a strong transverse texture was generated. Subsequently, both conditions were heat-treated above the β transus followed by slow cooling. The degree of variant selection was assessed by comparing the strength of the measured and predicted α texture from high temperature β texture, assuming equal occurrence of all possible variants during β → α transformation. It was found that, even though the material rolled originally at 800 °C displayed a stronger α texture after β heat treatment, it was the material rolled originally at 950 °C that showed greater variant selection. The variant selection mechanism is discussed in terms of the generated β texture and common 〈1 1 0〉 poles in neighbouring β grains selecting a similar α variant on both sides of the prior β grain boundary. Predictions of possible 〈1 1 0〉 pole misorientation distributions for the two investigated β textures showed that the combination of texture components generated during rolling Ti–6Al–4V at 950 °C increases the likelihood of having β grain pairs with closely aligned (1 1 0) planes compared to rolling at 800 °C. Therefore, it can be proposed that avoiding the generation of certain combinations of β texture components during thermomechanical processing has the potential for reducing variant selection during subsequent β heat treatment.
The evolution of texture in a cold rolled Ti–5Ta–1.8Nb alloy sheet, during the α → β → α transformation has been studied using EBSD and XRD techniques, for different cooling rates. The sheet exhibited a basal plane type texture upon cold rolling, and a sharp ‘{1 1 −2 0}||rolling plane’ transformation texture was inherited after the heat treatment. The microtexture analysis of the lamellar α/β structure, suggested that this transformation texture arises from the {1 1 1}〈1 1 0〉 type of high temperature β texture, obeying the Burgers orientation relationship. The strength of the transformation texture was found to sharply increase with decrease in cooling rates, denoting variant selection. Two types of parent β orientations with [1 1 0] parallel to either rolling directions are possible, and among their α product variants, 3 of them are common, and can be preferentially formed at the prior-β grain boundaries. The role of grain boundary-α in influencing variant selection and the transformation texture for different cooling rates is described in this paper.
The solid-state β→β + α transformation in titanium alloys leads to complex microstructures with feature spanning across a range of length scales. In order to develop a better understanding of the microstructural evolution process, a detailed characterization of the crystallography of α laths formed from the β phase in a candidate α/β Ti alloy, Timetal 550, has been carried out. Specifically, the influence of the orientation relationship (OR) between the grain boundary α (GB α) and the adjacent β grains on the microstructural evolution has been investigated in this alloy employing orientation imaging microscopy (OIM) studies in a high-resolution SEM. The results indicate that the colony microstructure (clustering of α laths belonging to the same variant) tends to develop in the β grain that exhibits the Burgers OR with the GB α allotriomorph, whereas the basketweave microstructure (clustering of multiple variants) develops in the adjacent β grain. Additionally, the multiple variants of α laths forming the basketweave microstructure appear to be related by certain selection criteria.
The phase transformations of Ti-5Al-2Sn-4Zr-4Mo-2Cr-1Fe (β-CEZ) have been studied during continuous cooling after β-solution treatment. For this purpose, electrical resistivity measurements and metallographical examinations have been carried
out, and the continuous cooling transformation (CCT) diagram of β-CEZ alloy has been plotted. The different kinds of β-phase decomposition schemes in β-CEZ alloy during continuous cooling have been investigated in detail. Two main morphological features of the α/β structure are involved, depending on the cooling rate: the basket-weave and the colony structures are observed for high and
low cooling rates, respectively. For the intermediate cooling rates, the two morphologies coexist. Finally, a generalized
scheme of the β → β + α transformation sequences during continuous cooling is presented.
In heterogeneous nucleation on lattice defects in the matrix during diffusional phase transformation and precipitation reactions, the variant of product phase with the specific orientation relationship is strongly selected by the nature of the defects. For dislocations, effective accommodation of the transformation strain by the strain field of dislocations occurs, leading to the variant selection in which the direction of the maximum misfit is nearly parallel to the Burgers vector of the dislocations. For high-angle grain boundaries and subgrain boundaries, precipitates tend to select the variant with a low-energy interface (often the parallel close packed planes) inclined at the smallest angle to the grain boundary plane. On the other hand, precipitates that nucleate on the incoherent inclusions embedded in the matrix do not hold any specific orientation relationship with respect to the matrix. It is important to produce many variants locally for the effective refinement of microstructures that improves the mechanical properties of steel and titanium alloys.
The variant selection occurring in the β-α phase transformation of Ti-64 material after rolling in the β-field was investigated by using texture transformation modelings. In a previous work published in this journal, we observed that the variant selection was very sensitive to the deformation degree imposed at high temperature prior to the transformation. In this contribution, we assumed that crystal defects formed during glides on the 110〈111〉 and 112〈111〉 β slip systems could favor the growth of plates corresponding to specific variants. Moreover, a detailed study of the β → α phase transformation suggested a link between the selected variants and the slip systems activated at high temperature. Therefore, we developed a texture transformation modeling, assuming that a variant is selected if its corresponding slip system was sufficiently activated in the last step of the hot deformation. The activity of the slip systems was evaluated by a rate sensitive Taylor model. The modeling was applied to a Ti-64 material 75% rolled at 1050°C and the simulated texture was in good agreement with the experimental one.
A three-dimensional (3D) continuum stochastic field kinetic model of martensitic transformations which explicitly takes into account the transformation-induced elastic strain is developed. The model is able to predict the major structural characteristics of martensite during the entire transformation including nucleation, growth and eventually formation of internally twinned plates which are in thermoelastic equilibrium with the parent phase. No a priori constraints are made on the possible configurations and sequences of structural patterns formed by orientation variants of the martensite. 3D computer simulations are performed for a generic cubic → tetragonal martensitic transformation in a prototype crystal which is elastically isotropic and elastically homogeneous. The simulations predict that (i) nucleation of martensite in a perfect crystal occurs collectively to accommodate the coherency strain, e.g. the critical nuclei are formed by two internally twinned orientation variants; (ii) the ultimate structure consists of plate-like martensite and retaining parent phase. The martensitic plates consist of twin-related platelets of two orientation variants and the habits of the plates meet the invariant plane requirement. These simulation results are in good agreement with experimental observations.
This study presents in situ observations of the hcp (α) to bcc (β) phase transformation in commercially pure titanium at 882 °C using SEM imaging concurrent with crystal orientation determination using EBSD. Direct observations of the onset of the phase transformation are presented showing the early stages of the growth of β plates within α grains and allotriomorphic β along α–α grain boundaries. Intragranular β plates have a Burgers orientation relationship (OR) with the parent α grain and are lenticular in shape. These plates also have a tent surface relief and surface-traces consistent with habit planes predicted by the phenomenological theory of martensitic crystallography for pure titanium. These features suggest a military component to the growth mechanism. The β allotriomorphs have a Burgers OR with one of the α grains abutting at the boundary, but do not have surface relief characteristic of a military transformation. These are likely to grow by a civilian mechanism. The final stage of the transformation is a process of competitive growth of the two β forms, with the allotriomorphic β dominating by virtue of its faster moving α–β interfaces. Grain growth in the β stability field is more than an order of magnitude faster than that in the α field at temperatures near the phase transformation.