Contraction Twins Research Articles

Microstructure evolution and mechanical properties of commercial pure titanium subjected to rotary swaging

The wide applications of pure titanium (Ti) are greatly limited by its low strength and hardness. In this paper, we employed an industrial method of rotary swaging (RS) to improve the strength of commercial pure (CP) Ti and uncovered the elementary mechanisms governing microstructural changes under RS. Specifically, the {10-12} extension and {11-22} contraction twins take place in the coarse grains at the early stage of RS deformation, which contributes to the increase of yield strength and moderate ductility. As for the late stage, the twins disappear and are replaced by textured nanostructured grains; meanwhile, there exists a hardness gradient along the radial sample direction. High strain rate RS brings about profuse twins and rapid grain refinement at room temperature in contrast to conventional severe plastic deformation. We rationalize that the high yield strength of CP Ti (up to 955 MPa) is attributed to dislocation, grain boundary and texture strengthening. This result may provide some insights in microstructure-based design of metals with superior mechanical properties subjected to this scalable RS technique.

Hot deformation behavior and dynamic recrystallization mechanism of an Mg-5wt.%Zn alloy with trace SiCp addition

The influence of trace (3 vol%) silicon carbide particle (SiCp) addition on the hot deformation behavior of the Mg-5wt.%Zn (Mg–5Zn) alloy was studied through the hot compression test. The activation energy of the Mg–5Zn alloy is decreased by the addition of trace SiCp, which is due to the high dislocations density in particle deformation zone (PDZ) around SiCp stimulates the pipe diffusion. Moreover, the activation energy of the composite also decreases with the increase of the size of PDZ (dPDZ). Additionally, the optimum deformation condition of 3 vol% 20 μm SiCp/Mg–5Zn composite is 693 K and 0.1–0.01s−1 according to the processing map. The instability region of the processing map at 543 K and 1 s−1 disappears with the increase of strain, which is associated with the inhibitory effect of trace SiCp on {101¯2} extension twins and promoting effect on the dynamic recrystallization (DRX) within {101¯1} contraction twins and {101¯1}-{101¯2} double twins by introducing flow localization. Moreover, the DRX behavior of the 3 vol% 20 μm SiCp/Mg–5Zn composite was analyzed by EBSD technology. The DRX mechanisms of the composite at 543 K and 1 s−1 are continuous dynamic recrystallization (CDRX), particle stimulated nucleation (PSN) and twining dynamic recrystallization (TDRX). With the increase of temperature, the DRX mechanisms are discontinuous dynamic recrystallization (DDRX) and PSN.

Texture and Microstructure Evolution During Single-Point Incremental Forming of Commercially Pure Titanium

In the present investigation, the evolution of texture and microstructure during incremental sheet forming was investigated. Hot-rolled sheets of commercially pure titanium were subjected to single-point incremental forming to obtain truncated pyramid geometries. It was observed from texture measurements that there was splitting of basal poles along the transverse direction. Moreover, a significant number of twins was observed in the deformed microstructure with the fraction of extension twins being highest compared to other variants. The crossing of deformation twins, formation of twin lamellar structure and formation of secondary extension twins within primary contraction twins were additionally observed. The state of deformation in the pyramid walls was analyzed via finite element method which in turn was used as input for carrying out crystallographic texture simulations via visco-plastic self-consistent simulations. It was observed that the state of deformation in the wall regions is plain strain compression plus through-thickness shear.

Deformation behavior of Mg–Zn–Zr magnesium alloy on the basis of macro-texture and fine-grain size under tension and compression loading along various directions

The deformation behavior of an extruded Mg–Zn–Zr alloy in the form of plate/sheet was investigated on the basis of fine-grained and initial macro-texture along various directions under uniaxial compression and tension test. The initial macro-texture revealed that 60.4% of grains have c-axes of hexagonal closed packed crystal structure along the normal direction and 17.4% grains have c-axes parallel to the transverse direction. The results showed that the flow stress and strain hardening rate under compression and tension were significantly affected by the loading directions and the initial texture. The ratio of tensile yield strength to the compressive yield strength along normal direction extruded direction, and transverse direction was found 0.85, 1.5, and 0.93, respectively. The practically symmetric yielding behavior along normal and transverse direction was caused by the fine-grained size, twinning dependencies on strain path loading, and texture. The anisotropy ∼1.5 along extrusion direction is the cause of the large fraction of {101¯2}<101¯1> extension twinning under compression and no twinning activity in tension along ED in the early stages of deformation in large grains. The strain hardening curves under tension and compression shows almost similar behavior in the normal direction. For extrusion direction and transverse direction, strain hardening curves exhibited three and two regimes under compressive loading which leads to a large difference in ultimate compressive and tensile strength. It is also revealed that {101¯2}<101¯1> extension twinning was favorable, while {101¯1}<101¯2> contraction twinning and {101¯1}−{101¯2} double twinning were unfavorable for strain hardening. The underlying deformation mechanisms under different loading paths were discussed on the basis of X-ray diffraction and optical microscopic analysis.

In Situ Characterization of the Effect of Twin-Microstructure Interactions on {1 0 1 2} Tension and {1 0 1 1} Contraction Twin Nucleation, Growth and Damage in Magnesium

Through in situ electron backscatter diffraction (EBSD) experiments, this paper uncovers dominant damage mechanisms in traditional magnesium alloys exhibiting deformation twinning. The findings emphasize the level of deleterious strain incompatibility induced by twin interaction with other deformation modes and microstructural defects. A double fiber obtained by plane-strain extrusion as a starting texture of AM30 magnesium alloy offered the opportunity to track deformation by EBSD in neighboring grains where some undergo profuse {1 0 1 2} twinning and others do not. For a tensile loading applied along extrusion transverse (ET) direction, those experiencing profuse twinning reveal a major effect of grain boundaries on non-Schmid behavior affecting twin variant selection and growth. Similarly, a neighboring grain, with its ⟨c⟩-axis oriented nearly perpendicular to tensile loading, showed an abnormally early nucleation of {1 0 1 1} contraction twins (2% strain) while the same {1 0 1 1} twin mode triggering under ⟨c⟩-axis uniaxial compression have higher value of critical resolved shear stress exceeding the values for pyramidal ⟨c + a⟩ dislocations. The difference in nucleation behavior of contraction vs. compression {1 0 1 1} twins is attributed to the hydrostatic stresses that promote the required atomic shuffles at the core of twinning disconnections.

Lithium Cluster Segregation in Coherent Contraction Twin Boundaries of Magnesium Alloys

Modification of twin boundaries has been deemed as a potential strategy to achieve high strength in combination with good ductility for many engineering alloys. Herein, we report a unique Li cluster segregation phenomenon in coherent contraction twin boundaries of Mg alloys. The structures at the atomic scale have been investigated by transmission electron microscopy. Meanwhile, the driving force for segregation and strengthening mechanisms have been elucidated by first-principles calculations and molecular dynamics simulations, respectively. Experimental results show that the segregated Li cluster with a hexagonal close-packed structure is coherent with a contraction twin. Subsequently, theoretical calculations reveal that the Li cluster is prone to occupy the interface vacancy in the contraction twin boundaries, following which it grows along the twin boundaries preferentially. Finally, both dynamics simulations and experimental observations demonstrate that the formation of Li cluster in contraction twin boundaries can not only directly pin dislocation movement analogous to precipitation strengthening, but also improve the critical shear strain of contraction twin boundaries, inhibiting their deformation or widening. The finding of cluster segregation in twin boundaries also occurs in other systems, which might open an avenue for twin boundary engineering to tune the mechanical properties of metals and ceramics.

Influence of Initial Textures on Microstructure and Mechanical Properties of Commercially Pure TA2 Titanium Sheet Pre-Strained by Cryorolling

Titanium alloys are one of the most important structural materials for the aerospace applications, which bring active study on microstructural and mechanical optimization of the alloys. In the present study, commercial TA2 titanium sheets with different initial textures were rolled to a strain of 4% at liquid nitrogen temperature. Microstructures of the rolled sheets were characterized using a scanning electron microscope equipped with an EBSD detector and their mechanical properties were tested through quasi-static uniaxial tension tests at room temperature. The influence of the initial texture on the microstructure and mechanical performance of the cryorolled sheet were discussed. Twinning is found to occur in the TA2 sheets after rolling even at a small strain. Initial texture is an important factor affecting both numbers and types of the twins. {11–22} contraction twinning is always the dominating type of twins for all three sheets having different initial textures. A lot of {11–22} and {11–24} twins can be found in the sheets with the initial texture of C-axes//ND, while the sheet with an initial texture of C-axes away from the ND contains few twins, with a high density of dislocations developed instead. Along with contraction twinning, many {10–12} and {11–21} twins are detected in the sheet with the initial texture of C-axes⊥ND. The cryorolled sheet having the densest twins shows the best mechanical properties, indicating a beneficial effect of a small pre-strain.

Mechanical properties and strengthening mechanisms of shock-modified titanium alloy

High strain-rate plastic deformation treatment is applied to modify the mechanical properties of a titanium alloy via gas gun loading. The yield strength, ultimate tensile strength (UTS) and fracture strain derived from stress-strain curves are 1.02–1.18 GPa, 1.23–1.41 GPa and 7.4–10.9%, respectively, for the impact velocities explored. Compared to the untreated titanium alloy, the specimen shocked at 609 m/s exhibits the best strength-toughness tradeoff: the UTS (1.41 GPa) is increased by 14.6%, and the fracture strain (9.7%) is decreased by 11.2%; the relatively small reduction in ductility is related to the {101¯1} contraction twinning and high dislocation density.

Zr50Cu25Ni7.5Co17.5 high-temperature shape memory alloy with excellent thermal stability and large recovery strain, and the associated microstructural deformation mechanism

In this study, we produced a novel Zr50Cu25Ni7.5Co17.5 high-temperature shape memory alloy with the aim of achieving high thermal stability and a good shape memory effect. Differential scanning calorimetry and the compression cycle were utilized to determine the phase transformation temperature and shape memory effect of the alloy, and transmission electron microscopy (TEM) was employed to investigate the microstructure and clarify the corresponding deformation mechanism. We found that the Zr50Cu25Ni7.5Co17.5 alloy exhibited excellent thermal stability and achieved the best maximum recovery strain of 6.87% (8% pre-strain) observed to date, making it a promising high temperature shape memory alloy. The TEM results indicated that, during the primary stage of the compression cycle, a small number of the (001) compound twins were de-twinning and numerous contraction twins formed. As the stress increased, increasing numbers of nanoscale (021) and (111) type-I deformation twins formed, and the shift displacement of the type-B planar defect expanded from 3d ~ (001) to 8d ~ (001). Furthermore, the newly found (111) type-I deformation twin and the extension of the type-B planar defect that contributes to the large recovery strain in the Zr50Cu25Ni7.5Co17.5 alloy were observed.

Effect of deformation speed on microstructure evolution and mechanical property of Mg-Gd alloys

The effect of deformation speed on microstructure and mechanical property was investigated in Mg1Gd and Mg3Gd alloys, and annealing of high-speed deformed samples was studied. Twinning was a dominant deformation mechanism after low-speed and high-speed compressions, while twin variant and type diversities were observed. Low-speed and high-speed deformed samples showed difference in twin amounts instead of twin types. Low-speed deformed samples exhibited more 101¯2 tension twins (including unusual tension twins), while amount of 101¯2-101¯2 twin boundaries soared in high-speed deformed samples that contained less tension twins. 101¯1 contraction twins and 101¯1-101¯2 double twins were relatively less activated after both low-speed and high-speed deformations. During annealing, hardness fluctuated because of competition between twin boundary segregation and recovery, while a higher alloying content would lead to a higher hardening rate as well as an extended hardening stage.

Microstructure evolution and fracture behavior of Mg-9.5Gd-0.9Zn-0.5Zr alloy subjected to different heat treatments

Microstructure evolution and fracture behavior of Mg-9.5Gd-0.9Zn-0.5Zr alloy subjected to different heat treatments were systematically investigated using SEM and TEM as well as tensile testing. It was found that the microstructure of the as-cast alloy consisted of α-Mg matrix, net-like eutectic compounds (α-Mg + Mg3 (Gd, Zn)), cubic GdH2 phases and lamellar 14H LPSO phases. After solution treated at 515 °C for 24 h, two different cooling processes were used to elucidate the effect of cooling rates on the precipitation microstructure. With a hot water quenching, those secondary phases were completely dissolved. Instead, grey-like patches were observed within the α-Mg matrix, which were proposed to be rod-like Zn2Zr3 phases around the α-Zr particle. In contrast, with a furnace cooling, the formation of 14H LPSO and several cubic Mg3 (Gd, Zn) phases was observed. Furthermore, after hot water quenching, the subsequent ageing treatment parameters were also optimized to be 225 °C for 48 h. In the peak-aged condition, a denser and uniform distribution of basal precipitates γ″ and several basal precipitates γ′ together with Zn2Zr3 and ZnZr2 phases were observed. The samples after the solution treatment (for both hot water quenching and furnace cooling) showed a much higher ductility than the as-cast alloy, while the tensile yield strength (TYS) and ultimate tensile strength (UTS) remained unchanged. After the peak-ageing, a significant increase in the TYS and UTS but a great loss in ductility was observed. In the as-cast alloy, the initiation of microcracks occurred from the net-like eutectic compounds, which was believed to be one of the most important reasons for the tensile fracture, and showed a co-existence of intergranular and transgranular fracture behavior. After the solution treatment, with a hot water quenching, the fracture can be mainly related with failures along contraction twins, and then showed a transgranular fracture behavior. While, with a furnace cooling, due to the presence of the kinked 14H LPSO phases, the fracture was caused by the broken of 14H LPSO phases, and then lead to a transgranular fracture. The peak-aged alloy exhibited a brittle intergranular fracture, which can be related with failures along soft precipitation free zones (PFZs).

Influence of yttrium addition on recrystallization, texture and mechanical properties of binary Mg–Y alloys

In the present study, the influence of yttrium (Y) addition on the recrystallization, texture and mechanical properties of Mg–Y alloys has been examined. In particular, the properties of two binary Mg–Y alloys viz. Mg–5Y and Mg–10Y have been compared. The addition Y appears to enhance the propensity of contraction twinning and double twinning. This also results in variable stored energy prior to recrystallization in the Mg–5Y and Mg–10Y alloys. The results suggested that increasing the Y content substantially retards the recrystallization kinetics, which was attributed to the increase in the extent of pinning of the grain boundaries due to an increase in the solutes. Moreover, increasing the amount of Y was also found to significantly weaken the texture in both the as-rolled and annealed conditions. Overall, Mg–10Y alloy exhibited much better mechanical properties as compared to Mg–5Y alloy.

Microstructural and textural investigation of an Mg–Dy alloy after hot plane strain compression

Abstract The microstructure and texture development in a Mg–0.41Dy (wt%) alloy after plane strain compression (PSC) at 300 °C under a strain rate of 10−2 and 10−4 s−1 up a final true strain of −1.2 were investigated using electron backscatter diffraction (EBSD) and X-ray diffraction. At high strain rate of 10−2 s−1, the microstructure exhibited massive twins mainly {10 1 ¯ 2} extension, {10 1 ¯ 1} contraction and {10 1 ¯ 3}-{10 1 ¯ 2} double twin due to the random texture of the as-cast alloy. Meanwhile, at a low strain rate of 10−4 s−1, the microstructure was characterized by dynamic recrystallization at the {10 1 ¯ 2} extension, {10 1 ¯ 1} contraction twins and grain boundaries. Twin dynamic recrystallization (TDRX), rotational dynamic recrystallization (RDRX) and discontinuous dynamic recrystallization (DDRX) were the main mechanisms responsible for the formation of recrystallized grains. The texture was characterized by the formation of three fibers: basal , 1 ¯ 0>// CD and 2 ¯ 0> // CD where CD is the compression direction. Moreover, the texture was less sensitive to the deformation conditions since the recrystallized grains showed the same orientation than twins. The changes of the mechanical properties of the alloy were ascribed to the resulting microstructure due to the twinning and dynamic recrystallization.

Strong strain path dependence of strain localizations and fracture in magnesium AZ31 sheet

Abstract This study compares the deformation and fracture behavior of a basal-textured Mg AZ31 alloy sheet under uniaxial tension and biaxial stretching by using an in-plane biaxial test setup capable of observing and measuring the deformation at both meso (millimeter) and microstructure scales. Strain distributions at the mesoscale and accompanying fracture surfaces indicate a significant dependence on strain path. At the microscale, limited slip activity in biaxial case promotes contraction twins, where severe strain localizations (emax/emean ≈ 20) to the twins and their boundaries cause mainly transgranular fracture. This leads to a brittle, and a more pronounced shear-type fracture under biaxial stretching. In uniaxial case, considerable tensile twinning activity reorients the initial texture for slip activity. Strain localizations (emax/emean ≈ 2) to the grain interiors and boundaries initiate mainly intergranular fracture. Samples fail by displaying both brittle and ductile fracture structures, with smaller shear lips compared to the biaxial case.

Structural characteristics of {10[formula omitted]1} contraction twin-twin interaction in magnesium

Twin interface characteristics play a crucial role in elucidating twin propagation, twin-twin and twin-dislocation interactions, and then affect their mechanical properties. Herein, focusing on magnesium as a model of a hexagonal close-packed structure, we investigate the structural characteristics of \\{101¯1} contraction twin, depending on crystallographic analysis, molecular dynamics simulations and transmission electron microscopy observations. The geometrical features reveal that it is preferential to form acute twin boundaries during the processes of twin-twin interactions. Subsequently, the dynamics simulations results demonstrate that the propagation process of {101¯1} contraction twin is mainly related to the migration of {101¯3} twin boundaries in twin tip, and its thickening process is dominantly associated with the movement of multiple-layered steps. Additionally, two twin-twin interaction modes- tip-twin boundaries crossing transmission and tip-tip collision-have been successfully constructed, dependent on the preferential acute growth mode of contraction twins. Finally, typically structural characteristics on both free-growth of single contraction twin and twin-twin interactions have been confirmed in the ultrahigh pressure pure magnesium by transmission electron microscopy observations. This work clarifies fundamental physics of {101¯1} contraction twins in hexagonal close-packed metals, which can also be applied to other systems to explain twin strengthening mechanisms.

Deformation and Failure Mechanisms in a Magnesium Alloy Under Uniaxial Compressive Loading

We have investigated the deformation and failure mechanisms of a rolled AZ31B magnesium alloy under both quasi-static ( $$10^{-3}\,{\text {to}}\,10^{-2}\,{\text{s}}^{-1}$$ ) and dynamic ( $$10^{3}\,{\text{s}}^{-1}$$ ) compressive loading. The observed anisotropy in the plastic response originates from the different in-plane and out-of-plane deformation mechanisms activated when the loading orientation changes. For plasticity driven by extension twins, extension twin???twin boundaries were found to dominate in the late stage of deformation. Severe localization was observed after deformation mediated by pyramidal slip and contraction twinning compared to the deformation mainly caused by extension twinning. The fracture process was established as a function of both loading rate and loading orientation. Moreover, the intermetallic inclusions in the material, which appeared to be hard and brittle, might lead to macroscopic fracture under compressive loading.

Microstructure and mechanical property of multi-pass low-strain rolled Mg-Al-Zn-Mn alloy sheet

Abstract Effect of different rolling reductions on the microstructure and mechanical properties of Mg-6Al-3Zn-0.1Mn wt.% alloys were investigated by means of scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). The results show that after rolling the as-extruded Mg-Al-Zn-Mn alloy, more second phases are dynamically precipitated. The grains are also gradually refined with increasing the total reduction due to the dynamic recrystallization, and the average grain sizes are estimated to be ∼7.2 μm, ∼3.7 μm, ∼1.2 μm for the as-extruded Mg alloy, ∼34% and ∼60% rolled samples, respectively. On the other hand, the high-density contraction twins have been activated in as-rolled Mg samples, which can effectively separate the Mg matrix and a similar grain refinement effect can be afforded. Moreover, the dislocations are significantly multiplied during the rolling process. Consequently, the tensile strength of the Mg alloy is gradually increased, and the ∼60% rolled Mg alloy exhibits the optimal mechanical properties, with the ultimate tensile strength (UTS) of ∼363 MPa and the elongation of ∼7.2%.

A comparative study between {11−22} twinning and {10−12} twinning variant selection mechanisms during uniaxial compression in pure titanium

The effect of initial texture on the twinning behaviors of pure titanium during uniaxial compression was investigated by electron backscattered diffraction (EBSD) technology. The {11−22} and {10−12} twinning variant selection behaviors were investigated by the Schmid factor (SF), strain compatibility factor (m′) and twinning strain tensor. For {11−22} contraction twins, multiple twin variants are more likely to form within a grain. For {10−12} extension twins, however, single variant is usually observed within one grain. The variant selection of {10−12} twinning is more favorable to follow the SF law than that of {11−22} twinning. However, both {11−22} twinning and {10−12} twinning variant selection behaviors can be well explained by the twinning strain tensor. The activated {11−22} twinning or {10−12} twinning variants can well accommodate the deformation along the compression direction. For intragranular cases, when multiple twins are generated within a grain, most of the activated variants have high SF values, and both {11−22} twinning and {10−12} twinning activities show stronger orientation dependence. For intergranular twin pairs, the strain compatibility is related to their formation sites. The twin pairs forming in the common boundaries of the two grains have good strain compatibility. However, twin pairs at those triple junctions show poor strain compatibility.

Effect of rolling with different amounts of deformation on microstructure and mechanical properties of the Mg–1Al–4Y alloy

In this study, the Mg–1Al–4Y alloy was successfully prepared by smelting and casting. Then, the alloy was rolled at different amounts of deformation (40%, 60%, and 80%) at 500 °C. Microstructure observations were carried out by X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). The effects of different amounts of rolling deformation on the microstructure and mechanical properties of Mg–1Al–4Y alloy were investigated. The results indicate that in the as-cast Mg–1Al–4Y alloy, the Al2Y phase is dispersed in the magnesium matrix, and the grain size of the alloy decreases from 201.06 μm to 35.07 μm with increasing rolling deformation. The rolled Mg–1Al–4Y alloy has a strong {0001} basal texture. As the amount of rolling deformation increases, the texture intensity gradually increases, but the texture type does not change substantially. Moreover, the Mg–1Al–4Y alloy is prone to {101¯2} extension twinning, {101¯1} contraction twinning, and {101¯1}-{101¯2} double twinning during rolling. Compared with the as-cast Mg–1Al–4Y alloy, the tensile strength and yield strength of the Mg–1Al–4Y alloy after 80% deformation were as high as 257 MPa (104%) and 185 MPa (an increase of 330%).

The Activation and Evolution of Twinning during Bending of Friction Stir Welded AZ31 Magnesium Alloys

AZ31 magnesium alloy joints obtained by friction stir welding with rotation speed of 1400 rpm and welding speed of 200 mm/min were subsequently subjected to the three-point bending process. The bending behavior, microstructure evolution and twinning mechanism were investigated. The results indicate that the stress-strain curve appeared as power-law shape during tension and the stress-strain curve appeared as work hardening shape during compression. However, the stress-strain curve during bending is different and macrographs of face and base bending indicated that the severe strain localization was present during bending of FSWed AZ31 magnesium alloy joint. Three concave regions formed due to texture distribution and stress state in the weld zone. In those regions, the grains had favorable orientation with c-axis parallel to the direction of tensile stress and abundant twins were activated. It can be proved by electron backscatter diffraction (EBSD) analysis—two twinning mechanisms were activated during bending—that is, ~56° { 10 1 ¯ 1 } contraction twin and ~86° { 10 1 ¯ 2 } expansion twin, in which { 10 1 ¯ 2 } twinning was main plastic deformation mechanism of joint and the number of twins was proportional to the compressive stress in corresponding areas. The twinning resulted in lattice rotation about 86° around &lt; 1 2 ¯ 10 &gt; direction and changed the orientation distribution of original crystal.