Secondary Twins Research Articles

Enhancement on mechanical properties via phase separation induced by Cu-added high-entropy alloy fillers in metastable ferrous medium-entropy alloy welds

This study evaluated the weldability of metastable ferrous medium-entropy alloys using high-entropy alloy fillers with various Cu contents and elucidated the strengthening mechanisms at room and cryogenic temperatures by focusing on the formation of a Cu-rich phase (FCC2) in the weld metal (WM). The WM with higher Cu content exhibited more dual face-centered cubic (FCC) phases with dendrite-shaped compositional heterogeneity due to phase separation driven by higher mixing enthalpy. The FCC2 induced greater lattice distortion, resulting in higher nano-hardness and stronger solid-solution hardening. In addition, the increased presence of FCC2 in the WM promoted grain refinement and the formation of additional grain boundaries within individual grains. After cryogenic deformation, the base metal and heat-affected zone in transverse welds underwent martensitic transformation from metastable FCC (transformation-induced plasticity), while the diluted WMs exhibited twinning-induced plasticity because of their higher stacking fault energy (SFE) compared to the base metal. The combination of these deformation mechanisms enhanced the cryogenic tensile properties without compromising ductility. Furthermore, the cryogenic tensile properties of weld with higher Cu content were further improved due to the increased formation of deformation twins in the WM. The consumption of solute Cu to form more Cu-rich FCC2 resulted in compositional variations in the matrix (FCC1), leading to a further reduction in the SFE of FCC1 and the activation of additional twin formation. Additionally, FCC2 with a relatively higher SFE refined secondary twins as well as contributed to further dislocation accumulation. This study suggests that the formation of Cu-rich phase in the WM can enhance mechanical properties at both room and cryogenic temperatures and provides a valuable approach for the future development of welding materials to improve the mechanical properties of FCC-based welded structures.

Interplay between deformation and fracture mechanism in gradient nanostructured austenitic stainless steel

The plastic deformation parameters are crucial for the grain size of gradient nanostructured (GNS) materials, and the multiscale grain structure within the GNS layer has a significant impact on fracture behavior. In this study, the detailed depth-dependent microstructures of GNS310S stainless steel fabricated by surface mechanical rolling treatment (SMRT) were investigated, and the relationship between the geometrically necessary dislocation (GND) density and the strain gradient was established through the local misorientation obtained by electron backscatter diffraction. The critical deformation parameters (strain gradient, shear strain, and strain rate) of SMRT-induced grain refinement at different scales were quantitatively evaluated. The tensile results show the yield strength and tensile strength of GNS310S are improved by 103 % and 23.1 % respectively compared to CG310S. Additionally, we elucidate the underlying tensile fracture mechanism of GNS310S stainless steel through detailed microstructural analysis. Due to the poor ductility of the near-equiaxed nanograins, cracking occurred on the surface of the SMRT specimen at low strain during the tensile testing. These cracks evolved into competitive shear offsets and multi-cracks with increased strain on the specimen surface, eventually leading to a hybrid fracture mode. In contrast, the subsurface nanotwinned lamellae layer displayed good ductility during tension, which can accommodate the deformation through lamellae widening and forming secondary twinning. This nanotwinned lamellae layer can suppress the inward propagation of surface cracks when the strain was not large. This work provides new insights into the fabricating parameters of the GNS stainless steel, as well as the deformation and fracture mechanisms.

Synergistic interaction behavior of deformation mechanisms in a new metastable β-Ti alloy with high strain hardening rate

The present study successfully designed a novel metastable β titanium alloy, Ti–8.8Cr–1.9Sn, exhibiting exceptionally high strain-hardening rate (∼2.4 GPa) as well as significant total elongation (∼48.2 %). Detailed analysis of the microstructure revealed that the successive activation of primary mechanical twins and primary SIM α" within β phases, along with secondary mechanical twins and secondary SIM α" within twinning zones during deformation, which results in a highly intricate network of deformation microstructures. Meanwhile, during deformation the microstructure undergoes dynamic refinement through the continuous occurrence of mechanical twinning and phase transformations, leading to a reduction in dislocation mean free path, thereby enhancing the strain-hardening rate. Importantly, the microstructural heterogeneity is formed due to generation of numerous heterogeneous interfaces, including β/α'', β/twin, α''/twin as well as twin/twinnano, thereby inducing additional HDI hardening resulting from the formation of GNDs for accommodating strain gradients near these interfaces. Furthermore, the relative mobile dislocation density ρm/ρm0 is significantly high during deformation, due to a reduced exhaustion rate and a high nucleation rate of mobile dislocations. The values of activation volume Va and V∗ typically range from 1b3–100b3, indicating the occurrence of cross-slip and dislocation nucleation. The interaction and accumulation between GNDs and mobile dislocations further contribute to the improvement of strain hardening rate. Therefore, the synergistic interaction activities of multiple deformation mechanisms lead to enhanced strain-hardening rate along with the large plasticity.

Influence of martensite and grain size on the mechanical behavior of austenitic Fe–C–Mn–Ni–Al medium Mn steels with TWIP mechanism under uniaxial tension/compression and micromechanical creep

Medium Mn steels with induced plasticity mechanisms have effectively addressed the high-strength and ductility requirements for structural applications. However, there is still uncertainty about the impact of residual martensite on medium Mn steels with an austenitic matrix under uniaxial tension and compression. Furthermore, research on the influence of grain size through incomplete recrystallization in medium Mn alloys is scarce. In this context, our study focused on design and manufacturing a medium Mn Fe–C–Mn–Ni–Al alloy through controlled atmosphere casting incorporating martensite, aiming to analyze its effects under uniaxial stress conditions. Thermomechanical and annealing heat treatments were applied to control grain growth and nucleation of new grains, allowing us to explore how grain size affects the twinning process and mechanical reinforcement in the alloy. Microstructural characterization techniques, including optical and scanning electron microscopy, were employed to assess the mechanical response and damage mechanisms. Mechanical tests included tension, compression, and nanoindentation. The results indicated that residual martensite generates an accelerated damage mechanism under tension due to co-deformation at the interface with austenite, resulting in crack nucleation and rapid propagation through mode I fracture. Consequently, ductility in tension significantly decreases, while no apparent failure is observed during the compression deformation process. Furthermore, incomplete recrystallization was observed to enhance the creep response in both tension and compression. The strain hardening rate results revealed the activation of primary twinning in tension and both primary and secondary twinning in compression for annealed samples. Nanoindentation tests confirmed the presence of twinning and revealed that diffusion creep and grain boundary sliding are the predominant creep mechanisms.

Twins and β0 precipitation effects on the lamellar degradation in TNM alloy during creep

In this paper, the microstructure instability of the TNM alloy during creep at 800 °C under 150 MPa and 250 MPa was investigated. The results indicate that, both β0 precipitation (inside lamellae) and primary γ twins formation are initiated during the second stage of creep. Compared to low creep stress conditions (150 MPa), high creep stress conditions (250 MPa) accelerate the initiation of the third stage of creep, inducing the precipitation of ω0 phase and secondary twins. The nanoscale ω0 particles formed inside β0 or at the interface of the β0/β0. The intersection of secondary γ twins and α2 lamellae leads to stress concentration and dislocation accumulation, providing nucleation points for β0 phase and promoting the precipitation of β0 within α2 lamellae. The precipitation of the secondary γ twin introduces a unit dislocation of 1/3 [111]γ in the (111) atomic planes, promoting the movement of adjacent atomic planes within the (111) atomic planes. The 1/3 [111]γ dislocations will dissociate into two partial dislocations via 1/3 [111]γ→1/4 [11 1‾]γT +1/9 [0 1‾1]γM. The adjacent atomic planes of the γ matrix and secondary γ twin can transformation to β0 through shear along 1/9 [0 1‾1]γM and 1/4 [11 1‾]γT, respectively. The size and content of β0 phase significantly increase in the third stage of high creep stress, accelerating the decomposition and fragmentation of the α2 lamellae, leading to rapid creep failure.

Molecular Dynamics Study on the Mechanical Behaviors of Nanotwinned Titanium

Titanium and titanium alloys have been widely applied in the manufacture of aircraft engines and aircraft skins, the mechanical properties of which have a crucial influence on the safety and lifespan of aircrafts. Based on nanotwinned titanium models with different twin boundary spacings, the impacts of different loadings and twin boundary spacings on the plastic deformation of titanium were studied in this paper. It was found that due to the different contained twin boundaries, the different types of nanotwinned titanium possessed different dislocation nucleation abilities on the twin boundaries, different types of dislocation–twin interactions occurred, and significant differences were observed in the mechanical properties and plastic deformation mechanisms. For the {101-2} twin, basal plane dislocations were likely to nucleate on the twin boundary. The plastic deformation mechanism of the material under tensile loading was dominated by partial dislocation slip on the basal plane and face-centered cubic phase transitions, and the yield strength of the titanium increased with decreasing twin boundary spacing. However, under compression loading, the plastic deformation mechanism of the material was dominated by a combination of partial dislocation slip on the basal plane and twin boundary migration. For the {101-1} twin under tensile loading, the plastic deformation mechanism of the material was dominated by partial dislocation slip on the basal plane and crack nucleation and propagation, while under compression loading, the plastic deformation mechanism of the material was dominated by partial dislocation slip on the basal plane and twin boundary migration. For the {1124} twin, the interaction of its twin boundary and dislocation could produce secondary twins. Under tensile loading, the plastic deformation mechanism of the material was dominated by dislocation–twin and twin–twin interactions, while under compression loading, the plastic deformation mechanism of the material was dominated by partial dislocation slip on the basal plane, and the product of the dislocation–twin interactions was basal dislocation. All these results are of guiding value for the optimal design of microstructures in titanium, which should be helpful for achieving strong and tough metallic materials for aircraft manufacturing.

Superior combination of strength and ductility in Fe-20Mn-0.6C steel processed by asymmetrical rolling

A Ce-alloyed twining-induced plasticity steel with the gradient structure was prepared through asymmetrical rolling. The primary role of Ce addition was to refine grains, and the asymmetrical rolling also effectively reduced grain sizes, and increased dislocation density. Under the combined effects of asymmetrical rolling and Ce addition, the steel exhibited superior combination of strength and ductility, with the yield strength of 1320 ± 5 MPa, ultimate tensile strength of 1775 ± 10 MPa and good elongation of 27 ± 1 %. Dislocation strengthening was the key factor of the increasing yield strength. As grain size decreased, the thicknesses of twins were significantly refined to several nanometers, accompanied with the activation of secondary twins. The promoted TWIP effect effectively enhanced the strain hardening ability of the steel.

Effect of grain size and segregation on the cryogenic toughness mechanism in heat-affected zone of high manganese steel

High manganese steels have been nominated as desirable materials used for various cryogenic applications due to their excellent cryogenic toughness caused by twinning. High manganese steel welded joints were obtained through gas metal arc welding, and the microstructural differences at three subzones of heat-affected zone (HAZ) were utilized. The aim was to investigate the effect of grain size and Mn/C/Cu segregation on the cryogenic toughness mechanism of high manganese steel welded joints. The results indicate that the average impact absorbed energies at −196 °C for 1.0, 3.0, and 5.0 mm from the fusion line (denoted by FL + 1, FL + 3, and FL + 5) is 118.6 J, 102.4 J, and 117.2 J, respectively. Compared with base metal, the embrittlement occurred in HAZ. Large grains improve toughness, while small grain boundaries, segregation, and precipitation in HAZ deteriorate toughness. Larger grain size facilitates the primary twins with thinner inter-twin spacing and twin thickness, and the high density of these primary twins inhibits the activation of secondary twins. This, in turn, enhances cryogenic impact toughness. Although the stacking fault energy (SFE) throughout the welding joint lays within the range for the twinning-induced plasticity effect, the enrichment of Mn/C/Cu slightly increases the local SFE and critical resolved shear stress for twining within the segregation zone. This phenomenon makes the activation of primary twins more challenging. Furthermore, the C segregation at grain boundaries leads to the precipitation of fine Cr23C6 carbides. These carbides serve as crack initiation points during impact tests, contributing to degradation in cryogenic impact toughness.

Microstructure and texture transformation of wedge-shaped rolled 3%Y2O3p/ZGK200 composites

The microstructure and texture transformation of 3%Y2O3p/ZGK200 composites after wedge-shaped rolling were investigated. The 3%Y2O3p/ZGK200 composites could be wedge-shaped rolled for 50 % reduction without obvious edge cracks. Basal <a> slip, prismatic <a> slip and pyramidal <c+a> slip detected by in-grain misorientation axes (IGMA) combined with Schmid factor (SF) analysis were abundantly activated during the whole rolling process. Different from the activation of all kinds of dislocation slips, many {10 1‾ 2} extension twins and few {101‾ 1}-{101‾ 2} secondary twins were activated with a reduction of 10 %. With increasing reduction from 30 % to 50 %, {101‾ 1}-{101‾ 2} secondary twins and {101‾ 1} compression twins started to play more important roles. Finally, the rolled deformed grains were elongated along rolling direction (RD) with abundant twins. During the whole rolling process, few recrystallized grains were found in the composites due to the effect of Gd addition. The {101‾ 2} extension twins whose c-axis oriented along normal direction (ND) should be attributed to the formation of basal texture, which was kept unchanged during the whole rolling process.

Towards Understanding {10-11}-{10-12} Secondary Twinning Behaviors in AZ31 Magnesium Alloy during Fatigue Deformation.

Tensile-compression fatigue deformation tests were conducted on AZ31 magnesium alloy at room temperature. Electron backscatter diffraction (EBSD) scanning electron microscopy was used to scan the microstructure near the fatigue fracture surface. It was found that lamellar {10-11}-{10-12} secondary twins (STs) appeared inside primary {10-11} contraction twins (CTs), with a morphology similar to the previously discovered {10-12}-{10-12} STs. However, through detailed misorientation calibration, it was determined that this type of secondary twin is {10-11}-{10-12} ST. Through calculation and analysis, it was found that the matrix was under compressive stress in the normal direction (ND) during fatigue deformation, which was beneficial for the activation of primary {10-11} CTs. The local strain accommodation was evaluated based on the geometric compatibility parameter (m') combined with the Schmid factor (SF) of the slip system, leading us to propose and discuss the possible formation mechanism of this secondary twin. The analysis results indicate that when the local strain caused by basal slip at the twin boundaries cannot be well transmitted, {10-11}-{10-12} STs are activated to coordinate the strain, and different loading directions lead to different formation mechanisms. Moreover, from the microstructure characterization near the entire fracture surface, we surmise that the presence of such secondary twins is not common.

Cross-slip of extended dislocations and secondary deformation twinning in a high-Mn TWIP steel

Twinning-induced plasticity (TWIP) steels have become important materials in industry owing to the good combination of strength and ductility and high strain hardening rate. The excellent mechanical properties are highly related to the glide of dislocation and deformation twinning. However, the cross-slip behavior of the extended dislocation and the mechanism of deformation twinning are still controversial. Here, the partial dislocation motion and austenite twinning of a high-Mn steel at the early stage of deformation were investigated using in-situ tensile transmission electron microscope (TEM) technique. Results show that a large number of plane glide and cross-slip of extended dislocations can occur at the early stage of deformation. Extended dislocation nodes can be formed as a result of the reaction between adjacent extended dislocations on the same glide plane. In-situ tensile TEM experiments confirm two cross-slip models of partial dislocation: (1) the Friedel-Escaig model, cross-slip based on constriction of extend dislocation and re-dissociation; (2) the Fleischer model, cross-slip involving Lomer-Cottrell dislocation. Based on experimental results and energy calculations, it can be confirmed that the formation mechanism of austenite primary deformation twin induced by partial dislocations is different from that of secondary deformation twin. Grain boundary emits partial dislocation into the grain to form stable stacking fault which induces austenite primary deformation twin. The formation of secondary deformation twin is related to cross-slip of extended dislocation. Only the cross-slip of an extended dislocation containing a 90° partial dislocation can induce the formation of secondary deformation twin by introducing the Frank partial dislocation.

Effects of cryogenic pre-deformation on the microstructure and mechanical properties of Ti–Mo alloys

In the present study, pre-deformation at liquid nitrogen temperature (77 K) was applied to introduce twins into Ti–Mo alloys with different contents of molybdenum. The microstructural evolution and mechanical properties with different cryogenic pre-deformation amounts (from 0 to 9 %) were characterized and analyzed. The results show that the twinning activities and the evolution of twinning systems are mainly influenced by β phase stability and cryogenic pre-deformation amount. After cryogenic pre-deformation, the primary and secondary twinning of {332}<113>twins are observed in Ti–12Mo and Ti–15Mo alloys but absent in Ti–20Mo alloy. With the cryogenic pre-deformation amounts increasing, the width and quantity of twins increase constantly. Moreover, {112}<111>twinning is activated in Ti–15Mo alloy pre-deformed by 6 % and 9 %. Twins and high-density dislocations induced by cryogenic pre-deformation cause a significant enhancement in strength of all these three alloys. However, the effect of cryogenic pre-deformation on the elongation of alloys is complicated. For Ti–12Mo alloys, the yield strength increases monotonically (from 527 to 778 MPa) with the increase of cryogenic pre-deformation reduction, while the elongation is completely opposite (from 47 to 20 %). For Ti–15Mo alloys, the optimal matching of strength and ductility is achieved when the amount of cryogenic pre-deformation is 6 %, with a tensile strength of 815 MPa and an elongation of 24 %. Compared with the other two alloys, Ti–20Mo alloy gets a smaller increase in strength and a slight decrease in elongation with the amount of cryogenic pre-deformation increasing.

Delving into the intrinsic co-relation between microstructure and mechanical behaviour of fine-/ultrafine-grained TWIP steels via TEM and in-situ EBSD observation

To illustrate the microstructural factors of grain refinement for enhancing mechanical properties, the fine-/ultrafine-grained TWIP steels with a product of strength and elongation of ∼71 GPa·% were first prepared by combining rolling and stress relief annealing. Subsequently, the evolution of dislocations, stacking faults, and associated substructures of the fine-/ultrafine-grained TWIP steels was analysed by using in-situ EBSD tensile tests and TEM characterisation of the interrupted strain experiments. The results reveal that the excellent mechanical properties of the TWIP steels are attributed to dislocations and associated dislocation cells, dislocation walls, dislocation tangles, stacking faults and associated Lomer-Cottrell locks (LCs), nano-twins, primary and secondary twins and their interactions during plastic deformation. The density of geometrically necessary dislocations (GNDs) was evaluated based on the modified Ashby's model and compared with experimental results, indicating that grain size heterogeneity can promote the accumulation of GNDs, which facilitates the generation of subgrains and new boundaries to reduce the mean free path (MFP) of dislocations, thus enhancing strain hardening. Meanwhile, the interaction of lamellar primary and secondary twins in fine grains and the generation of stacking faults and nano-twins in ultrafine grains at higher strains can further promote strain hardening to elevate strength. Furthermore, the effects of grain orientation and grain size on the activation and evolution of dislocations and twins were elucidated. In ultrafine grains, twinning is strongly inhibited due to the elevated critical shear stress for twinning, resulting in more stacking faults and nano-twins, but fewer dislocation cells. The present work contributes to an in-depth understanding of the mechanical properties of fine-/ultrafine-grained materials to exploit their potential for industrial applications.

Insight into double twinning behavior of Mg–Gd rolled alloy at the nano scale: Role of micro-texture entropy

101¯1}− {101¯2} double twinning is a classical twinning mode with diverse variants in magnesium and its alloys. However, its variant selection criterion and its influence on the mechanical behaviors are still in debate. In this study, we proposed a view of micro-texture entropy to elucidate the double twinning behavior during large strain rolling of Mg–1.5Gd (wt%) alloy. Combined with the high spatial resolution scanning electron microscopy-transmission Kikuchi diffraction (SEM-TKD) technique, the detailed microstructure and micro-texture of the double twins in early stage of evolution were detected, and new insight into the variant selection and evolution of double twins was obtained. The observations suggest that the nucleation events of different double twin variants are influenced by their micro-texture entropy, the variants with lower micro-texture entropy are preferred. While the growth stage of double twins is affected both by their micro-texture entropy and the geometric factors between the primary and secondary twin systems; the former will influence their growth potential and the later can affect their growth space. A novel {101¯1}− {101¯3} double twin was also triggered to release the high local stress concentration during large strain rolling process.

Dynamic Compression and Constitutive Model in Fe-27Mn-10Al-1C Duplex Lightweight Steel

Fe-Mn-Al-C lightweight steels have been of significant interest due to their excellent mechanical properties and unique microstructures. However, there has been limited focus on the dynamic deformation. Here, we systematically investigate the mechanical responses over various strain rates and corresponding microstructure evolution in quasi-static and dynamic compression to reveal the transition of deformation mechanisms. The present lightweight steel exhibits a significant strain rate effect, with the yield strength increasing from 735.8 to 1149.5 MPa when the strain rate increases from 10−3 to 3144 s−1. The deformation in ferrite under high-strain-rate loading is dominated by wave slip, forming a cellular structure (cell block). Meanwhile, the deformation in austenite is dominated by planar slip, forming dislocation substructures such as high-density dislocation walls and microbands. In addition, the deformation twinning (including secondary twinning)- and microband-induced plasticity effects are responsible for the excellent dynamic compression properties. This alloy delays damage location while maintaining high strength, making it ideal for shock loading and high-strain-rate applications. The Johnson–Cook (J–C) constitutive model is used to predict the deformation behavior of lightweight steel under dynamic conditions, and the J–C model agrees well with the experimental results.

Fine grains with high-density annealing twins and precipitates inducing favorable strength and excellent plasticity in laser powder bed fusion-fabricated Inconel 718 via deep cryogenic and heat treatments

Tailoring high-density annealing twins in laser powder bed fusion (LPBF)-fabricated alloys based on their intrinsic residual stress requires high annealing temperatures and/or long-term annealing, resulting in the abnormal growth of large recrystallized grains, which is detrimental to mechanical properties. This work proposes a new strategy for achieving a favorable strength–plasticity synergy of the LPBF-fabricated Inconel 718 superalloy by performing a deep cryogenic treatment (DCT) with the subsequent heat treatment (including annealing and double aging) to tailor fine grains with “high-density annealing twins + precipitates” architectures and compares the obtained material with an alloy subjected to a direct heat treatment without a prior DCT. The obtained results reveal that the additional internal stress generated during DCT increases the stored energy and dislocation density, which provide a sufficient driving force for activating high-density annealing twin boundaries (63.2 %) with fine grains (31.6 μm) within a short annealing time. The more homogeneous tailored microstructure with the “finer grains + high-density twins + precipitates” architectures decreases the mean free path of slipping dislocations, promoting intensive interactions with dislocations and inducing a strong strain hardening effect. The multiple deformation modes of stacking faults coupled with Lomer–Cottrell locks, thin primary deformation twins, and secondary twins activated during tensile loading, sustaining a strong work hardening ability and delaying the plastic instability, which exhibits a high strength (yield strength of 1088 MPa and tensile strength of 1369 MPa) and excellent plasticity (elongation of 30 %). This work not only describes a feasible method for simultaneously enhancing the strength and plasticity in additively manufactured (AM) alloys but also provides new insights into increasing the fraction of twins at a small grain size to improve the grain boundary-related properties without destroying the AM alloy shape.

Dynamic compression responses of heterogeneous-structured CrMnFeCoNi high-entropy alloy at cryogenic temperatures

Dynamic compression experiments are conducted on a heterogeneous-structured CrMnFeCoNi high-entropy alloy (HEA) at different strain rates (1000–5000 s−1) and temperatures (123–293 K) with a split Hopkinson pressure bar system. Yield strength and strain hardening rate increase considerably with decreasing temperature at 5000 s−1 (low-temperature strengthening), but are insensitive to strain rate at 173 K, which can be well described by the Zerilli–Armstrong model. Compared to typical aluminum alloys and homogeneous-structured CrMnFeCoNi HEA, the heterogeneous-structured HEA exhibits higher thermal sensitivity and strain hardening rate. Microstructural characterizations show that dislocation density in the fine-grained domains increases more drastically than in the coarse-grained domains, inducing high deformation gradient between the coarse- and fine-grained domains. At 123 K, nano-scale primary and secondary deformation twins form in grain interior, along with a great number of stacking faults on twin boundaries; the twin width and spacing in coarse grains are considerably higher than in fine grains. In addition, the twin density at 123 K is significantly higher than that at 293 K. Overall, the increased deformation twinning and deformation gradient contribute to pronounced strengthening and strain hardening with decreasing temperatures in the heterogeneous-structured HEA under dynamic compression.

Strength-ductility synergy in twinned titanium fabricated via dynamic equal channel angular pressing and heat treatment

Dynamic equal channel angular pressing (DECAP) and heat treatment are applied to pure Ti to achieve a strength-ductility synergy. DECAP induces a complex multiscale twin structure in Ti including four primary twin types, secondary twins and tertiary twins with multiple variants, and an increased yield strength (58% higher than the as-received Ti). Subsequent annealing at 200 ℃ and 400 ℃ leads to improved ductility and strength. The yield strength (σy) and total elongation (ɛt) for the Ti sample processed with DECAP and 400 ℃ annealing are 555 MPa and 30%, respectively, and its σyɛt product is ∼45% higher than the as-received Ti. DECAP and subsequent annealing lead to a superior strength-ductility synergy in twinned Ti, providing a novel approach to produce high strength and ductility materials via dynamic deformation and subsequent heat treatment.

Effect of grain size and temperature on deformation mechanism of commercially pure titanium

To investigate the effects of grain size and processing temperature on the deformation behaviors of commercially pure titanium (CP-Ti), the tensile tests were conducted at room temperature (RT) and liquid nitrogen temperature (LNT) on the samples with the average grain size of 2, 9, 23, and 51 μm, respectively. The experimental results based on EBSD and TEM characterizations showed that dislocation slip was the main deformation mechanism at RT, while abundant deformation twins including {102} extension twins, {11} compression twins and some {11}−{102} secondary twins were observed under LNT condition. The transition of deformation modes from dislocation slip to dynamic twinning contributes to the outstanding mechanical properties of the samples with a larger grain size at LNT. Additionally, a modified Hall−Petch relationship was proposed to study the quantitative contribution of average grain size and deformation twins on yield stresses of CP-Ti under LNT condition.

{1[formula omitted]01} tension twins and {1[formula omitted]01}-{2[formula omitted]01}/{2[formula omitted]01}-{1[formula omitted]01} double twins in the D019 ordered hexagonal α2-Ti3Al phase

Compared to the disordered HCP metals, the α2-Ti3Al phase is reluctant to twin due to its D019 ordered structure. Recently, abundant {202¯1} compression twins were observed in this phase. However, other type of twin has not been reported besides the {202¯1} ones. In this study, we observed the {11¯01} tension twins in the ordered α2-Ti3Al phase for the first time in the α2+γ two phase TiAl alloy. In addition, the {11¯01}<1¯102> tensile mode occurs together with {202¯1}<1¯014> compression twins already known for the α2 phase, and these two modes are often twinned internally. Two types of double twins ({11¯01}-{22¯01} and {22¯01}-{11¯01}) were observed. The structure of the various interfaces was examined with high-resolution electron microscopy. The factors governing the evolution of the observed twin architecture are discussed with particular emphasis on deformation induced internal stresses and the interchange shuffles necessary to restore the D019 ordering of the α2 phase in its twinned counterpart. The transformation matrix was calculated for the observed two types of double twin structures, which explains the reorientation of the grains and the shear continuity, further confirming the double twin mechanism. The variant selection mechanism of the secondary twin is also discussed. These results indicate that abundant twinning activity can be initiated in the ordered α2 phase under severe deformation.