Double Twins Research Articles

Suppressed transformation of compression twins to double twins in Mg by activation of non-basal slip

The transformation of compression twins (CT) to double twins (DT) is studied in a dual-textured Mg-6.5%Zn(wt.) alloy during deformation along the extrusion axis. After 7.3% compression, 85% of CT are transformed to DT. This ratio drops to 22% and 36% during tension although the applied stresses and strains in tension were much higher. The Schmid factor of the actual DT variants was very low (and often negative) in tension and compression and could not explain the differences in DT activity. It is shown that the suppressed CT→DT transformation during tension is accompanied by the activation of 〈a〉 non-basal slip -rather than 〈a〉 basal slip- which presumably hinder the transformation because the dissociation of 〈a〉 basal dislocations is necessary to nucleate and grow extension twins in primary CT. These findings point out an effective strategy to suppress DT by activating 〈a〉 non-basal slip, which may be useful to design Mg alloys with high ductility.

Unexpectedly formed strong basal texture in a rolled Mg-Zn-Ca-Mn alloy sheet

The reheating at high temperature, which contributed to the significant texture weakening in rolled Mg alloys, was used to control the textural features of a Mg-Zn-Ca-Mn alloy sheet. The grain coarsening was apparent when the alloy was reheated at high temperature of 450 °C, facilitating the formation of double twinning and shear bands during rolling. Double twinning and shear bands generally cause the weakening of basal texture after post-annealing; however, the basal texture became strong in the Mg-Zn-Ca-Mn alloy sheet rolled with reheating at 450 °C. The electron backscattered diffraction revealed that the non-uniformly distributed dislocations caused such unexpected textural development. In contrast, when the reheating was performed at 350 °C, the alloy formed uniform deformation-microstructure after rolling, contributing to the apparent texture weakening by the static recrystallization at double twinning during post-annealing.

Effects of surface roughness on bending properties of rolled AZ31 alloy

This study investigated the effects of mechanical-polishing-induced surface roughness and the direction of polishing lines on the bending properties of a rolled AZ31 alloy. To this end, three-point bending tests were performed on one sample without polishing lines (SS sample) and two samples with polishing lines—one in which the polishing lines were parallel to the rolling direction (RS-RD sample) and the other in which they were parallel to the transverse direction (RS-TD sample). In all three samples, macrocracks were formed in the width direction on the outer surface, where tensile stress was predominantly generated in the longitudinal direction. However, the macrocracks formed in the SS sample were curved because of the merging of uniformly formed fine microcracks, whereas those formed in the RS-TD sample were linear owing to the formation of relatively coarse microcracks along the polishing lines. The bendability of the samples was in the order of SS > RS-RD > RS-TD, and their limiting bending depths were 4.8, 4.6, and 4.4 mm, respectively. In the presence of mechanical-polishing-induced surface roughness, polishing lines perpendicular to the direction of the major stress (i.e., tensile stress along the longitudinal direction) resulted in a greater degree of stress concentration on the outer surface of the bending specimen. This higher stress concentration promoted the formation of undesirable {10–11} contraction and {10–11}–{10–12} double twins—which typically act as crack initiation sites—and thereby facilitated crack generation and propagation. Consequently, the surface roughness caused premature fracture during bending deformation, which, in turn, caused deterioration of the bendability of the rolled Mg alloy.

Dynamics of Four Triple Systems

Orbital motions in four hierarchical stellar systems discovered by speckle interferometry are studied. Their inner orbits are relatively well constrained, while the long outer orbits are less certain. The eccentric and misaligned inner orbits in the early-type hierarchies ϵ Cha (B9V, central star of the 5 Myr old association, P = 6.4 yr, e = 0.73) and I 385 (A0V, P ∼ 300 yr, e ∼ 0.8) suggest past dynamical interactions. Their nearly equal masses could be explained by a dynamical decay of a 2+2 quadruple progenitor consisting of four similar stars. However, there is no evidence of the associated recoil, so similar masses could be just a consequence of accretion from the same core. The other two hiearchies, HIP 32475 (F0IV, inner period 12.2 yr) and HIP 42910 (K7V, inner period 6.8 yr), have smaller masses and are double twins where both inner and outer mass ratios are close to one. A double twin could either result from a merger of one inner pair in a 2+2 quadruple or can be formed by a successive fragmentation followed by accretion.

Twinning-induced plasticity with multiple twinning modes and disclinations in Mg alloys

Twinning-induced plasticity plays a critical role in determining the deformation behaviors of hexagonal close-packed metals (e.g., Mg and Ti), due to the lack of five independent slip systems required for a general deformation. In particular, twinning modes that can accommodate c-axis strain (tensile or compressive) are especially important, which are necessary complementary deformation modes to <a>-type dislocations. However, only limited types of extension twins, e.g., {101¯2} type twin, have been unambiguously identified in the literature, which is theoretically inadequate to accommodate a complex deformation with c-axis extension. Using topological defect theory, here we show that another extension twinning mode, {112¯6} twin, can be formed through reactions between disclinations and {101¯2} twins in Mg-alloys. Based on symmetry of the deformation space, we demonstrate that the {112¯6} twin originates from the inter-connection of correlated deformation paths, which can provide a tensile strain of 5.2% along c-axis and a compressive strain of 4.5% along 〈112¯0〉. {112¯6} twins can be formed through three formation mechanisms, i.e., direct formation, twin-twin intersection, and double twinning, which are validated by a combination of experimental characterizations and phase field simulations. Our work not only suggests a symmetry-based method to analyze twinning-induced plasticity, but also provides a new insight into the deformation mechanisms and mechanical properties of Mg-alloys.

Triple and double twin interfaces in magnesium—the role of disconnections and facets

Twin boundaries have been shown to deviate from the twinning planes in hcp metals, and facets have often been observed in twin interfaces. This study presents a twinning disconnection-based model for faceting in single, double and triple twin boundaries in magnesium. Primary twinning disconnections predicted via symmetry arguments are shown to produce commensurate facets in single twin boundaries, which are then transformed into commensurate facets in double twin boundaries via the action of secondary twinning disconnections. In contrast, it is shown that for triple twin boundaries with tension-compression-tension twinning sequence, no commensurate facets can be produced by the action of tertiary twinning disconnections. The effect of facets on the macroscopic orientation of twin interfaces is discussed. Theoretical findings are validated by a transmission electron microscopy study of a hot rolled Mg-1.18wt%Al-1.77wt%Nd alloy. Single and double twins are observed, as well as rare triple twins, and the interface between the matrix and a triple twin is captured for the first time. Facets consistent with theoretical predictions are imaged via high-resolution TEM and macroscopic deviations of the boundaries from the primary twinning planes are measured.

The effects of strain rate and strain on the deformation-induced grain refinement process and hardness evolution of a polycrystalline cobalt

In this study, the effects of strain rate and strain on the evolution of microstructure and hardness of a polycrystalline cobalt subjected to high-speed impact and quasi-static compression, with the strain rate of ∼ 103 s−1 and 3 × 10−3 s−1, respectively, were investigated. During high-speed impact, the high strain rate promoted the activation of various types of twinning: 101̅2, 101̅1, 112̅1 and 112̅2 twinning, as well as the 101̅1−101̅2 double twinning. Meanwhile, the temperature rise during high-speed impact activated the incomplete dynamic recrystallization. Deformation twinning, deformation-induced martensitic phase transformation and dynamic recrystallization contributed to the grain refinement. During quasi-static compression, dislocation slip and twinning dominated the deformation. Only 101̅2 twins with different orientations were triggered. The number of twins increased with the increase of strain, but was less than that in the high speed compressed sample. The high speed compressed samples exhibited higher hardness values compared with the quasi-statically compressed samples. The more pronounced hardening effect mainly accounted for the higher density of twins produced during high-speed compression. Although the recrystallization during high-speed compression may lead to strain softening, such softening effect was overwhelmed by the strengthening effect of grain refinement.

Recrystallization mechanisms and texture evolution of AZ31 alloy by gradient caliber rolling

A single-pass gradient caliber rolling of a conical-shaped AZ31 Mg alloy was performed at 723 K to obtain a transitional microstructure with strain ranging from 0 to 30%. The recrystallization mechanisms involved and the evolution of texture during the deformation were specifically investigated. The results show that the recrystallization extent increases with strain. The isolated fine grains at 5% strain were formed by the particle stimulated nucleation, which tended to randomize the texture. A large number of {10 1¯ 2} extension twins at 10% strain effectively divided and refined the grains, but no twin-induced dynamic recrystallization was found. Some {10 1¯ 1}-{10 1¯ 2} double twins and {10 1¯ 1} compression twins were activated at 15% strain, which to a certain extent facilitated dynamic recrystallization nucleation. Meanwhile, the continuous dynamic recrystallization and discontinuous dynamic recrystallization mechanisms also occurred at various strains. In particular, the discontinuous dynamic recrystallization was more prominent due to the higher deformation temperature, and the recrystallization grains were evidently different from the orientations of their parent grains. In addition, with increasing strain, the overall texture component of the deformed samples gradually transitioned from the initial (0001) basal texture to the [11 2¯ 0]-[10 1¯ 0] non-fiber component, then tended to form the [11 2¯ 0] fiber orientation eventually.

Influence of Ca addition on microstructural characteristics and mechanical properties of Mg–5Bi–3Al alloy extruded at extremely high speed

In this study, the effects of Ca addition on the microstructural characteristics and mechanical properties of a Mg–Bi–Al alloy extruded at high speed were investigated by analyzing extruded Mg–5Bi–3Al (BA53) and Mg–5Bi–3Al–0.5Ca (BAX530) alloys fabricated at a speed of 70 m/min. The homogenized BA53 billet contained fine and coarse Mg3Bi2 particles, whereas the homogenized BAX530 billet contained only coarse Mg2CaBi2 particles. Both alloys exhibited fully recrystallized microstructures and similar grain sizes upon extrusion at the high speed. Ca-induced texture weakening did not occur in the extruded BA53 alloy because the added Ca was completely consumed during the formation of Mg2CaBi2 particles. Abundant fine Mg3Bi2 particles were uniformly distributed throughout the extruded BA53 alloy, whereas the extruded BAX530 alloy contained coarse Mg2CaBi2 particles. The addition of Ca to the BA53 alloy decreased the tensile strength by ∼10%, primarily due to weakened particle hardening. Under tension, an increased number of {10–11} contraction and {10–11}-{10–12} double twins were formed and a larger number of twin variants were activated in the extruded BAX530 alloy than in its Ca-free counterpart; consequently, Ca addition enhanced the tensile elongation of the extruded alloy by ∼21% via suppressing the local strain concentration. Thus, the addition of a small amount of Ca can enhance the tensile ductility of high-speed-extruded Mg–Bi-based alloys with high Bi contents without altering the alloy texture.

Dynamic microstructure evolution and mechanical properties of dilute Mg-Al-Ca-Mn alloy during hot rolling

The dynamic microstructure and texture of dilute Mg-0.50Al-0.71Ca-0.33Mn (wt.%) during hot rolling at two slab temperatures were investigated by electron backscattered diffraction (EBSD) and high resolution transmission electron microscopy (HRTEM). The results show that the development of the rolling microstructures is to first form {10–12} extension twins in the original grains, thereby forming the extension-twinned regions, and then to further form {10–11} - {10–12} double twins and kinks in the extension-twinned regions, and finally to form continuous dynamic recrystallized (continuous DRXed) grains in the double twins and the cross parts of the shear-deformed coarse extension-twinned regions. These extension twins, double twins and kinks show a decisive effect on the formation of rolling texture. The number of {10–11} - {10–12} double twins and resultant the continuous DRX process are strongly affected by the rolling slab temperature and the reduction thickness per pass. By optimizing the rolling conditions, texture and microstructures of the multi-pass rolled Mg-Al-Ca-Mn alloy sheets are successfully modified. Although the total alloy content is only 1.5 wt.%, these Mg-Al-Ca-Mn alloy sheets show much higher strength than the commercial Mg-3Al-1 Zn (wt.%) (AZ31B) sheet.

Effect of rolling reduction on deformation mechanism and twinning behavior of WE43 magnesium alloy

The effect of rolling reduction of the last pass on the dislocation slip and twinning behavior during direct hot rolling of a cast WE43 magnesium alloy at 480 °C was investigated. The results showed that prismatic 〈a〉 slip was always the main deformation mode during rolling at 480 °C. In addition, the activated twinning type was associated with rolling reduction. The extension twinning was activated at a slight rolling reduction (2%), while compression twinning and - double twinning were activated at larger rolling reduction (12% and 20%). Schmid factor calculation showed that the activation of extension twin variants followed the Schmid Law, whereas the activation of compression twin variants did not follow it. Even if the rolling reduction reached 20%, almost no dynamic recrystallization (DRX) grains were found, presumably because the amount of deformation required for DRX to occur was not reached.

Atomic-scale study of the {[formula omitted]} twinning and {[formula omitted]}-{[formula omitted]} double twinning mechanisms in pure titanium

The {112¯1} twinning and {112¯2}-{112¯1} double twinning mechanisms in pure Ti are systematically investigated using molecular dynamics (MD) simulations. MD simulation results indicate that a {112¯1} twin embryo is composed of a group of pyramidal stacking faults (PSFs) with 6d{112¯1} or 8d{112¯1}, caused by the successive slip of partial dislocations with the Burgers vector of 136〈112¯6¯〉M. The migration of {112¯1} twin boundaries (TBs) is mainly carried out by the slip of twinning dislocations (TDs) b⇀1 = 16(4γ2+1)<1¯1¯26> ±12<11¯00>. Two {112¯2}-{112¯1} double twinning mechanisms are discovered. One is that a {112¯1} twin forms and grows at the {112¯2} twin tip through the dissociation of the TD b⇀1(112¯2) at the {112¯2} twin tip, forming the {112¯2}-{112¯1} double twin structure. The other is that a basal <a>T dislocation interacts with a {112¯2} TB, forming a twin embryo composing of a group of PSF with the 6d{112¯1¯} and leaving a step at the {112¯2} TB, eventually forming the {112¯2}-{112¯1} double twin structure. The interfacial structures of {112¯1} twins have the terraced character with short coherent (112¯2)||(112¯0) facets and coherent twin boundary (CTB) segments. In addition, (0001)||(112¯4) facets with a disclination dipoles feature can form at twin tips. The coherent twin interfaces form at their intersected interfaces during the {112¯2}-{112¯1} double twinning process. Our present findings may provide clues for designing Ti alloys with excellent strength–ductility properties through obtaining a high-volume fraction of coherent interfaces in terms of {112¯2}-{112¯1} double twins.

Compressive deformation behavior of AZ31 alloy at 21K: an in-situ neutron diffraction study

Deformation behavior during uniaxial compression at 21 K and 298 K in a commercial AZ31 magnesium alloy was studied by in-situ neutron diffraction. Decreasing the deformation temperature resulted in a slight increase in yield stress (115 MPa →139 MPa), but a remarkable enhancement in both the fracture stress (365 MPa → 551 MPa) and fracture strain (12.8%→16.5%). The low temperature sensitivity of the {101¯2} extension twinning which governed the macroscopic yielding led to the slight increase in yield stress. At 21K, basal slip was suppressed, while the extension twinning was promoted resulted in higher twin volume fraction. In the late stage of deformation, the {101¯1}-{101¯2} double twinning was suppressed, which is considered to be the reason to delay the fracture at 21 K.

Bending-deformation-induced inhomogeneous aging behavior and accelerated precipitation kinetics of extruded AZ80 alloy

In this study, the effects of bending deformation on the precipitation behavior of an extruded AZ80 alloy are investigated by subjecting a plate of the alloy to three-point bending and subsequent aging at 200 °C. In the upper region of the bent specimen (denoted as CZ), numerous {10−12} extension twins are formed. In the lower region of the bent specimen (denoted as TZ), small quantities of {10−12} extension twins, {10−11} contraction twins, and {10−11}–{10−12} double twins are formed, and most of the bending deformation is accommodated by dislocation slip. In the central region of the bent specimen (denoted as NZ), few twins are formed, and the microstructural characteristics of this region are almost identical to those of the initial material. When the bent specimen is aged, the peak-aging time of the CZ and TZ (8 h) is 60% shorter than that of the NZ (20 h), which is attributed to the accelerated precipitation caused by the abundant twins in and high internal strain energy of the CZ and TZ. In the CZ, nucleation and growth of continuous Mg 17 Al 12 precipitates (CPs) occur rapidly, especially inside the {10−12} twins, and this accelerated CP formation inhibits the formation of discontinuous Mg 17 Al 12 precipitates (DPs). Because of the high internal strain energy of the TZ, promoted formation of CPs occurs in the early stage of aging, which, in turn, suppresses the formation of DPs. In contrast, the NZ, because of its low internal strain energy and insufficient twins, exhibits slower CP formation behavior and has a considerably larger amount of DPs than the CZ and TZ. Although the bending deformation before aging induces inhomogeneous precipitation behaviors, the prebending deformation shortens the overall peak-aging time of the material and increases its peak hardness. • An extruded AZ80 alloy plate is subjected to bending and subsequent aging. • The bent plate has an inhomogeneous microstructure and internal strain energy. • Its upper and lower regions show a faster aging rate than its central region. • Continuous and discontinuous precipitation behaviors are different in each region. • Bending deformation shortens the peak-aging time and improves the peak hardness.

Effect of Al addition and annealing on the microstructure evolution of Mg-3Sn Mg alloy subjected to high-speed rolling

The microstructure evolution of Mg-3Sn, Mg-3Sn-3Al-Zn (ATZ331), and Mg-3Sn-6Al-Zn (ATZ631) alloys during high-speed rolling (HSR) and annealing was studied. High-speed rolling was carried out on the three alloys at a rolling speed of 1100 m/min, a rolling temperature of 400 °C, and a reduction of 40% during a single pass. The results showed that the microstructure of the samples after HSR contained many twins and was dominated by {10−12} extension twins (ETs) and supplemented by {10−11} contraction twins (CTs) and {10−11}-{10−12} double twins (DTs). After adding Al, the content of these different types of twins was reduced. The twinning fraction decreased significantly with 3 wt% Al addition, while the twinning fraction decreased slightly with 6 wt% Al addition. The high-speed rolled alloys were annealed at 370 °C for 1 min, 2 min, 3 min, 5 min, and 8 min. When the annealing time was 1 min, the recrystallization fraction of the ATZ631 alloy was low, indicating that increasing the Al content to 6 wt% inhibited recrystallization. When the annealing time increased to 3 min, the recrystallized grains in the ATZ631 alloy grew, indicating that the grain growth progressed faster in this alloy. When the annealing time was 8 min, the grain size of the three alloys grew. The electron backscattered diffraction (EBSD) results of the alloy annealed for 8 min showed that the twin fraction in the annealed state was lower than that after high-speed rolling. In addition with the increase in Al content, the twin fraction decreased. During high-speed rolling, the maximum intensity also increased with an increase in Al content, were 22.72, 25.09, and 26.35, and the texture of the annealed alloy was weakened. The Al addition also increased the maximum intensity.

Twinning-mediated texture weakening in a basal-textured Mg-6Al-1Zn (mass%) alloy sheet by a novel cold-sample rolling method

In this paper, we report the use of cold-sample rolling (CSR) processing, which induced a high fraction of double twins and a substantial texture-weakening effect in an Al-concentrated commercial Mg-6Al-1Zn (mass%) alloy sheet. A CSR-processed and subsequently annealed sheet showed a weak basal texture with a broad distribution of the (0001) poles toward the rolling and transverse directions. This led to moderate stretch formability with a limiting dome height of 5.5 mm. High-temperature rolling (HTR) processing has been reported to weaken the basal texture in Mg-Al-based alloy sheets because of the activation of non-basal <c+a> dislocations, resulting in texture weakening. However, the processing induced a relatively low fraction of double twins. Consequently, CSR processing led to a relatively high texture-weakening effect. The results showed that twinning-mediated static recrystallization is essential for texture weakening in Al-concentrated Mg-alloy sheets.

Effects of deformation twins on microstructure evolution, mechanical properties and corrosion behaviors in magnesium alloys - A review

Due to lattice reorientation, grain segmentation, induced recrystallization, twins play a very important role in regulating texture, refining grains, improving mechanical properties and corrosion resistance, and has received more extensive attention. Numerous studies have shown that {10-12}<10-11> tensile twins (TTWs) are easily activated in large quantities due to the lower critical resolve shear stresses (CRSS). Introduction of TTWs under uniaxial compression improved the strength, ductility, and formability of magnesium (Mg) alloys. Moreover, TTWs produced by multi-directional impact forging (MDIF) can optimize the microstructure by dividing grains and promoting recrystallization, resulting in significant improvement of mechanical properties. Although {10-11}<10-12> compressive twins (CTWs) and {10-11}-{10-12} double twins (DTWs) can promote dynamic recrystallization (DRX), they are also favorable nucleation sites for cracks. In addition, the type and volume fraction of twins can affect the corrosion resistance, and they also play different roles in the corrosion process of different Mg alloys. Twins have shown great potential for improving structure and properties, but a comprehensive and critical discussion of twins in Mg alloys is still lacking. Therefore, based on previous studies, this article reviews the common types and variants of twins in Mg alloys, influencing factors, and their effects on the microstructure, mechanical properties and corrosion resistance. In addition, some interesting ideas are being proposed for further research.

Microstructural evolution and its effects on the mechanical properties of fine-grained Mg-3Gd during tension

Recrystallized Mg-3 wt. % Gd with an average grain size of 4 μm was prepared and tensioned under different strains (2%, 5%, 10%, and 20%) to investigate the evolution of the deformation mechanism and its effects on the mechanical properties of fine-grained Mg alloy during plastic deformation. The microstructure of the alloy has been characterized using electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). The results show that the dislocation density was ∼0.67 × 1014 m−2 at 2% strain, which was accompanied by a few tension twins and <c + a> dislocations. Upon increasing the strain to 5%, the dislocation density was significantly increased to 1.77 × 1014 m−2, which contributes approximately 60% of the flow stress increment when compared to that observed at 2% strain. Meanwhile, tension twins were more frequently observed, while <c + a> dislocations were detected in most of the grains. The dislocation density was slightly increased to 2.31 × 1014 m−2 in the 10% tensile-deformed Mg-3Gd sample, while other strengthening sources became increasingly important. The proportion of low angle grain boundaries (LABs) was almost tripled and compression or double twins are observed. The dislocation density became saturated at ∼2.00 × 1014 m−2 and the proportion of LABs remained unchanged upon further increasing the strain to 20%, although compression or double twins were more frequently detected, which resulted in a low work hardening rate.

Abnormal Twinning Behavior Induced by Local Stress in Magnesium

This study investigated the twinning behavior with increasing compressive strain in rolled AZ31 alloy. With that purpose, a polycrystalline structure with an average grain size of 30 μm was utilized to perform the uniaxial compression tests. Microstructure evolution was traced by in situ electron backscattered diffraction (EBSD). Multiple primary twin variants and extension double twins were observed in the same grain. A comprehensive analysis of kernel average misorientation (KAM) and Schmid factor (SF) revealed that the nucleation of twins in one special grain is not only based on the SF criterion, but that it is also strongly influenced by surrounding grains. Moreover, the existing primary twins modified the inner and outer strain distribution close to the twin boundaries. With continued compression, the strain inside the primary twins stimulated the nucleation of double twins, while the strain in the matrixes facilitated twin growth. Therefore, the primary twin growth and the new nucleation of secondary twins could take place simultaneously in the same twinning system to meet the requirements of strain accommodation. Twinning behaviors are controlled by the combined effect of the Schmid factor, strain accommodation between surrounding grains, and variation in the local stress state. The local stress exceeded the critical resolved shear stress (CRSS), implying that twin nucleation is possible. Hence, the twinning process tends to be a response of the local stress rather than the applied stress.

Faceting and Twin–Twin Interactions in {1121} and {1122} Twins in Titanium

Twin–twin interactions are an important component of the microstructural evolution of hexagonal close-packed metals undergoing plasticity. These interactions are prevalent because of the predominance of twinning due to limited easy slip modes. Despite their importance, the complexities of the atomic-scale behavior of interacting twins has limited robust characterization. Using interfacial defect theory, we developed a three-dimensional model of twin–twin interactions, double twinning and other complex interfacial reactions that occur between twins acting on different interface planes. Using molecular dynamics, {1122} and {1121} twins in titanium were activated and produced facets, twin–twin interactions and double twins that we characterized with our model. The results showed excellent agreement between the molecular dynamics results and the model. Surprisingly, some highly ordered and mobile boundaries can be produced by these complex reactions, which could provide important insights for higher scale models of plasticity.