Extension Twinning Research Articles

Quasi-in-situ observations of low-angle grain boundaries, twins and texture evolution during continuous annealing in a cold-rolled Mg-Zn-Gd alloy

The evolution of texture, low-angle grain boundaries and twins in a cold-rolled Mg-Zn-Gd alloy during continuous annealing at 250 °C were tracked by quasi-in-situ X-ray diffraction and quasi-in-situ electron backscatter diffraction method, and the influence of extension twins on texture was discussed. The results show that during the whole annealing process, the main texture component is always split toward transverse direction (TD) and the tilting angle hardly changes. However, the peak position changes between the two TD texture components. During continuous annealing, most pre-existing low-angle grain boundaries retain their original misorientation (including angles and axes) and hardly migrate. In addition, a new low-angle grain boundary occurs when two grains with similar orientation grow to connect with each other. Boundaries of pre-existing extension twins tend to migrate with different velocity. Moreover, with the migration, extension twins grow and gradually swallow their matrix grains completely, and thus re-orientate the TD orientations of twinned-grains from one side to another. Considering that nuclei display no preferred orientation between +TD and –TD, the changing peak position is considered to be caused by the migration of ETBs.

A comprehensive analysis on microstructure evolution of Mg-5.65Zn-0.66Zr alloy during hot deformation

The microstructure evolution of Mg-5.65Zn-0.66Zr (wt.%) alloy was studied based on the hot compression tests. The results indicated that the flow stress increased rapidly to a peak point at the initial stage, and then it gradually decreased. Moreover, high temperature and low strain rate resulted in the decreasing of flow stress. All samples exhibited a necklace grain structure because of the occurrence of partial dynamic recrystallization (DRX). High temperature increased both the size and fraction of DRXed grains, while high strain rate showed an opposite tendency. At the conditions of 350 °C/0.001 s−1 and 350 °C/0.1 s−1, the twins were not exhibited and DRX played a dominant role. Importantly, the obvious split of basal texture was observed. The pyramidal <c+a> slip with high value of Schmid factor was active in large deformed grains, which corresponded to the peak split point in (0001) pole figure. A mechanism about the grain rotation was proposed to explain the relationship between the pyramidal slip and the split of basal texture. Finally, it was found that large number of {10–12} extension twins were formed during the initial stage at condition of 300 °C/1 s−1, and the number of twins decreased with the increase of strain. The twins greatly contributed to the fast formation of basal texture and grain rotation. Moreover, the non-basal slips were active in twining region, which could facilitate the nucleation of DRX.

Characterization of newly developed friction stir-arc welding method for AM60/AZ31 dissimilar Mg alloy

The dissimilar AM60/AZ31 Mg alloys joints were obtained via friction stir welding (FSW). Then, the upper and lower surfaces of the FSW joint were remelted by using tungsten inert gas (TIG) welding. The macrostructures, microstructure evolution and mechanical properties of the two kinds of joints were investigated. The results indicated that there was a TIG weld zone in both the upper and lower parts of the nugget zone (NZ) of the FSW joint after the arc heating. Further, the grain size in the NZ-middle grew to a certain extent, especially the area near the NZ-middle-center. Texture analysis showed that the texture in the NZ-middle was modified after the arc heating, especially in the TIG weld zones. However, there were no significant changes in texture and grain size near the NZ interface. The tensile results showed that the tensile strength and yield strength of the two joints were similar, but the joint elongation after arc heating was largely improved, by about 57%. The equivalent strength of the two kinds of joints was due to the similar texture and grain size near their NZ interfaces. The higher elongation was attributed to the increase of the Schmid factor values (basal slip and extension twinning) caused by the weakening of the texture, the growth of the grain size and the annihilation of the initial residual dislocation in the NZ-middle. As a result, the difficulty of deformation in the NZ-middle was reduced, which weakened the localized strain of the joint. This work could provide a new method for weakening the strain localization of FSW Mg joints.

Non-uniform deformation behavior of dissimilar friction stir welded AM60/AZ31 joint and its influence on fracture

In this work, the defect-free dissimilar AM60/AZ31 Mg joint was obtained by friction stir welding (FSW). Tensile tests were employed under different stress states to investigate the tensile behavior of the joint. The joint microstructures before and after deformation were characterized via electron backscatter diffraction to reveal the relationship between the microstructure and the deformation and fracture behaviors. The results indicated that a significant and complex evolution of grain orientation generated in the joint after FSW. The grain size of the weld bottom was smaller than that of the upper-middle part. However, compared with the base materials, the grains in upper-middle part were not refined obviously for the high welding heat input. In addition, there was a triple junction region (TJR) with significant texture fluctuations in the upper area of the weld. Further, the reasons for the fracture of joint approximately along the centerline of nugget zone (NZ) in the back face and both the NZ and the TJR interfaces in the front face were explained. During the tensile process, cracks initiated at the bottom of NZ-center in the back face due to the appearance of compression twins and double twins. Cracks developed towards the AM60 side owing to its larger grain size. Further, near the NZ boundary was a favorable orientation for basal slip and extension twinning, which was more prone to deformation and thereby made cracks terminate here. Meanwhile, there were significant differences in texture distribution and grain size in the different parts of TJR. This resulted in an incompatible deformation between the micro-regions, making cracks easily develop along the interface of these micro-regions.

Influence of shear deformation during asymmetric rolling on the microstructure, texture, and mechanical properties of the AZ31B magnesium alloy sheet

In this study, the changes in the microstructure, texture, and mechanical properties of the AZ31B magnesium alloy sheet after asymmetric rolling (ASR) in different shear deformations during each pass are reported. A high shear strain is induced along the sheet thickness when the velocity ratio of the upper and lower rollers is 1.8. In the case of the sheet processed via ASR in the reverse shear deformation along the rolling direction (RASR), the grain refinement, weak basal texture, and improved structural homogeneity result in better mechanical properties than those of the sheet processed via symmetric rolling (SR). However, in the case of the sheet processed via ASR in the single shear deformation along the rolling direction (UASR), shear bands containing dynamically recrystallized grains and larger numbers of dislocation are formed, tilting the peak intensity of the pole figure away from the normal direction. Moreover, the shear band with an average grain size of 5 μm causes severe structural inhomogeneity, which results in bad mechanical properties than the other two sheets. Furthermore, the twin mode changes from the double twin of the SR sheet to the extension twin of the ASR sheet under shear strain. The extension twin contributes to the optimization of the mechanical properties. Finally, the strength and elongation of the sheets processed via RASR are simultaneously improved to 250.3 MPa and 12.5%, respectively.

Effect of yttrium addition on dynamic mechanical properties, microstructure, and fracture behavior of extrusion-shear ZC61 + xY (x = 0, 1, 2, 3) alloys

In this study, extrusion-shear ZC61 + xY (x = 0, 1, 2, 3) magnesium (Mg) alloys were prepared by a novel severe plastic deformation method, i.e., extrusion-shear, which integrates direct extrusion with equal channel angular pressing. The effect of yttrium (Y) addition on dynamic mechanical properties, microstructure, and fracture behavior of ZC61 + xY (x = 0, 1, 2, 3) alloys was investigated by Split-Hopkinson Pressure Bar setup under a strain rate of 2100 s−1 along extrusion direction. With the increase in the content of added Y from 0 to 3 wt%, due to the strengthening of secondary phases and grain refinement, the yield strength of Mg alloys was enhanced from 142 to 342 MPa, and the ultimate compression strength of Mg alloys was improved from 582 to 701 MPa. During dynamic compression, the <c + a > pyramidal slip and extension twinning were the dominant deformation modes, which coordinated plastic deformation of ZC61 + xY (x = 0, 1, 2, 3) alloys. Analysis of the fracture behavior of four Mg alloys samples indicated that the fracture mode of ZC61 alloy was the brittle and melt mixed-mode; however, the fracture behavior of ZC61 + xY (x = 1, 2, 3) alloys was the brittle and ductile mixed fracture. With the increase of Y content from 1 to 3 wt%, the absorption energy density was observably increased, which was mainly responsible for the change of fracture mode from brittle fracture to ductile fracture.

Effect of Initial Texture on the Evolution of Microstructure and Texture During Rolling of Commercially Pure Titanium at Room and Cryogenic Temperature

The evolution of texture and microstructure of commercially pure titanium during unidirectional rolling at room and cryogenic temperature was studied for two different initial orientations, namely RD (rolling direction)-split basal (BA orientation) and prismatic-pyramidal (PP orientation) till 90 pct rolling reduction. Effect of micro-mechanisms of deformation by slip and twinning on texture evolution and grain fragmentation was studied by employing characterization tools like bulk texture measurement using X-ray diffraction and electron back scatter diffraction. Both orientations yield split TD (transverse direction) texture for 90 pct rolling reduction at both the temperatures. Orientation BA showed severe twinning that saturated at deformation of 50 pct followed by slip dominated deformation, while orientation PP showed slip dominated deformation with marginal twinning throughout the rolling reduction at room temperature. Profuse twinning led to homogeneous grain refinement in orientation BA, while orientation PP showed heterogeneous grain refinement due to absence of twinning in some grains at the final stage of deformation. Rolling at cryogenic temperature enhances grain refinement due to extensive twinning that leads to weakening of texture in BA orientation and increase in intensity of texture for PP orientation compared to room temperature rolling. The evolution of microstructure and texture contributed to similar hardness for 90 pct room and cryogenic temperature rolled BA orientation, while 90 pct room temperature rolled PP orientation showed lower hardness.

Bimodal Phenomenon of the Stress–Strain Curve During Hot Compression of LA43M Mg–Li Alloy

In this study, the samples of LA43M Mg–Li alloy were compressed to the true strains of 0.12, 0.16, 0.36 and 0.60 under 300 °C and the strain rate of 0.1 s−1. Under this condition, the stress–strain curves present a special bimodal phenomenon at the early deformation stage, which is caused by twinning and dynamic recrystallization. In the process of hot compression, extension twins generated first. Twinning accommodated strain and the glide of twinning dislocations dissipated local strain energy, resulting in the first local stress drop. Then extension twins coarsened and devoured all the matrix, leading to the change of grain orientation. Subsequently, dislocations accumulated at the grain boundaries and resulted in the increase in stress. With further strain, dynamic recrystallization occurred, discontinuous dynamic recrystallization is the main dynamic recrystallization mechanism. The generation of dynamic recrystallization resulted in the strain softening and leaded to the second local stress drop. Twinning retards the occurrence dynamic recrystallization and has a crucial influence on the microstructure development during the hot compression of LA43M.

Onset of detwinning in Mg-3Al-1Zn alloy: A synchrotron-based X-ray diffraction study

We investigate the onset of detwinning in magnesium alloy Mg-3Al-1Zn under continuous loading with real-time in situ synchrotron X-ray diffraction. Detwinning of the {101¯2} extension twins activated both by tension parallel to and compression perpendicular to the c-axis fibers is explored. The experimental results reveal that detwinning of the {101¯2} extension twins occurs immediately upon unloading, regardless of whether the twins are activated by tension parallel to or compression perpendicular to the c-axis fibers.

Evolution of Microstructure, Texture, and Mechanical Properties of As-Extruded ND/ZK60 Composite during Hot Compression Deformation

The effects of temperature, strain rate, and strain on the microstructure, texture, and mechanical properties of as-extruded nanodiamond reinforced ZK60 composite during hot compression was systematically studied. The results revealed that the precipitating MgZn2 and the nanodiamond (ND) particles distributed in the grain interiors hindered the motion of dislocations. The ND particles act as nucleation points and promote the dynamic recrystallization (DRX) of the composites during the hot compression deformation, the flow stress of ND/ZK60 increases with strain rate increases and temperature decreases. {101¯2} extension twins are nucleated and grown in the coarse grains as the compressive strain increasing. Meanwhile, the fine grains of DRX generate and present first an increasing then a decreasing trend. The result of Schmid factor and kernel average misorientation indicates that high-density dislocation caused by dislocation climbing and cross slip aggregated in composites with increasing strain. Therefore, the work hardening trend of the composite is strengthened.

Ex-situ EBSD investigation of the reduced c/a values and work hardening behavior in Mg-4Li-1Al-0.5Y alloy under hot compression

The microstructural and textural evolution of the annealed Mg-4.26 wt% Li-1.05 wt% Al-0.52 wt% Y (denoted as Mg-4Li-1Al-0.5Y) alloy at 200 °C and strain rate of 0.1 s-1 was investigated via ex-situ electron backscatter diffraction (EBSD). Microstructural analysis revealed that the axial ratio (c/a) of the annealed Mg-4Li-1Al-0.5Y alloy decreased to 1.605, which reduced the critical shear stress (CRSS) of non-basal slip and promoted the activation of pyramidal <c + a> slip in the early stage of deformation (ε = 0.02). In addition, a weak {112‾0} prismatic texture was formed in the investigated alloy, and the hot compression curve exhibited a significant work-hardening characteristic with a yield strength of 92 MPa. As the strain increased, the texture strength gradually decreased, and the number of low-angle grain boundaries (LAGBs) increased. In addition, the area fraction of {101‾2} extension twins decreased, while no {101‾1} compression twins or {101‾1}-{101‾2} double twins were observed. Moreover, the high dislocation activities indicated that the decrease in the axial ratio (c/a) promoted the non-basal slip more actively and inhibited the activity of the twins to some extent, which resulted in significant work hardening of the Mg-4Li-1Al-0.5Y alloy during the compression process.

Effect of Final Rolling Temperature on Microstructures and Mechanical Properties of AZ31 Alloy Sheets Prepared by Equal Channel Angular Rolling and Continuous Bending.

The effects of final rolling temperature on the microstructures, texture and mechanical properties of AZ31 Mg alloy sheets prepared by equal channel angular rolling and continuous bending (ECAR-CB) were investigated. Extension twins {10–12} could be observed in the ECAR-CB deformed sheets. The increase in the number of {10–12} extension twins with increasing final rolling temperature might be attributed to the larger grain size and faster grain boundary migration. For all the ECAR-CB sheets at different final rolling temperatures, the deformation texture contains a basal texture component and a prismatic texture component, whereas the annealing recrystallization texture becomes a non-basal (pyramidal) texture with double peaks tilting away from normal direction (ND) to rolling direction (RD). With increasing final rolling temperature, the tilted angle of double peaks of annealing recrystallization non-basal texture increases. In addition, the plasticity and formability of ECAR-CB-A (ECAR-CB and then annealing) AZ31 Mg alloy sheets at room temperature can be improved by increasing the final rolling temperature.

The engulfment of precipitate by extension twinning in Mg–Al alloy

To understand the strengthening mechanism of precipitate against twin-mediated plasticity in magnesium–aluminum alloys, the measurements of the increase in the critical resolved shear stress due to impediment on twin thickening are accomplished based on atomistic simulation, specifically at a significantly low strain rate of 1.7 × 106 s−1. Results show conspicuous coupled effect of temperature and strain-rate on the precipitation strengthening. The strengthening effect vanishes if coherent twin boundary migration is purely stress-driven without thermal activations, due to the back-stress dominating over Orowan stress. At a finite temperature the strengthening effect becomes observable, and is distinctly amplified by high strain rates.

Texture modification and mechanical properties of AZ31 magnesium alloy sheet subjected to equal channel angular bending

Analysis of the recently proposed equal channel angular bending (ECAB) process is provided on thin hot-rolled AZ31 magnesium alloy sheets. In particular, effects of deformation temperature and strain path on the texture evolution and mechanical properties are systematically investigated under single pass ECAB at various temperatures and multi-pass ECAB process that involves changes in strain paths. It is found that simultaneous activation of multiple twinning types is successfully introduced during ECAB, which results in obvious tilted component of basal texture. Attributed to the domination of extension twins, weaker basal textures are detected after both single pass ECAB at 150 °C and three cross passes ECAB at 200 °C. After annealing, the basal texture is further weakened via twin-related recrystallization and the annealed microstructure is featured with mixture of basal and non-basal orientated grains. Additionally, the effect of grain orientation on the mode of plastic deformation and the roles of grain orientation and grain boundary on the local strain accommodation are coherently studied. This study reveals that over 60 % increase of uniform elongation with marginal reduction of tensile strength less than 5 % can be achieved for single pass ECAB at 150 °C and three cross passes ECAB at 200 °C, which is the result of larger fraction of grains favored with extension twinning and better local strain accommodation.

Comparative study of anisotropy, strain hardening and twinning behavior in AZ40 and AE42 alloys

Compressive strain hardening and microstructure evolution in AZ40 alloy without rare earth (RE) elements and AE42 with RE elements have been examined and comparatively discussed. Compressive stress-strain curves indicated strain hardening rates in the early regime of of radial direction (RD) samples were significantly higher than these of extrusion direction (ED) samples for both AZ40 and AE42 alloys. AE42 magnesium alloys showed a much higher plastic anisotropy during compression due to texture hardening effects and profuse extension twinning. Thicker {10–12} twins with a higher area fraction were observed in AE42 alloy and intersecting multiple {10–12} twin variants were revealed in AZ40 magnesium alloys. {10–12} twins transversed low angle grain boundaries in AE42, while thin extension twins terminated at grain boundaries in AZ40.

Microstructure, Texture and Mechanical Properties of AZ31 Magnesium Alloy Fabricated by High Strain Rate Biaxial Forging.

High strain rate biaxial forging (HSRBF) was performed on AZ31 magnesium alloy to an accumulated strain of ΣΔε = 1.32, the related microstructure, texture and mechanical properties were investigated. It was found that the microstructure evolution can be divided into two steps during HSRBF. In the early forging processes, the refinement of the grain is obvious, the size of ~10 μm can be achieved; this can be attributed to the unique mechanisms including the formation of high density twins ({102} extension twin and {101}-{102} secondary twin) and subsequently twining induced DRX (dynamic recrystallization). The thermal activated temperature increases with the increase of accumulated strain and results in the grain growth. Rolling texture is the main texture in the high strain rate biaxial forged (HSRBFed) alloys, the intensity of which decreases with the accumulated strain. Moreover, the basal pole rotates towards the direction of forging direction (FD) after each forging pass, and a basal texture with basal pole inclining at 15–20° from the rolling direction (RD) is formed in the full recrystallized HSRBFed alloys. The grain refinement and tiled texture are attributed to the excellent strength and ductility of HSRMBFed alloys with full recrystallized structure. As the accumulated strain is ΣΔε = 0.88, the HSRMBFed alloy displays an outstanding combination of mechanical properties, the ultimate tensile strength (UTS) is 331.2 MPa and the elongation is 25.1%.

Twin nucleation and variant selection in Mg alloys: An integrated crystal plasticity modelling and experimental approach

Extension twin nucleation and variant selection in magnesium alloy WE43 is investigated in experimentally characterised and deformed microstructures replicated in crystal plasticity models. Total stored (dislocation) energy density is found to identify the experimentally observed locations of twins which are not otherwise explained by global Schmid factors or local resolved shear stress criteria. A critical total stored energy of the order 0.015 Jm-2 is determined below which twin nucleation does not occur. The total stored energy density explains the locations of the observed twins and the absence of twins in parent grains anticipated to be favourable for twin nucleation. Twin variant selection has been shown to be driven by minimising locally stored shear energy density, while the geometric compatibility and strain compatibility factors only aid in partial prediction. All experimentally observed variants were correctly determined.

Microstructure, texture, mechanical properties and biodegradability of extruded Mg–4Zn‒xMn alloys

The influence of 0.5 and 1.0 wt% Mn addition on the microstructure, texture, mechanical properties and biodegradability of the extruded Mg–4Zn alloy was investigated. The addition of Mn resulted in significant grain refinement as well as the formation of a strong fiber texture. The results of grain boundary misorientation analysis indicated that extension twinning plays an important role in dynamic recrystallization at elevated temperatures. Tensile test results showed substantial improvement in the yield stress (YS) from 198.4 MPa for the base Mg–4Zn alloy to 233.4 and 246.7 MPa for the Mg–4Zn–0.5Mn and Mg–4Zn–1Mn alloys, respectively. The same trend was observed in the ultimate tensile strength (UTS), while the high elongation to fracture of 33% was almost independent of the Mn content. The contributions of various strengthening mechanisms were measured to elucidate the main strengthening mechanisms. The obtained results indicated that the grain boundary, texture, and solid solution hardening have the most important contributions to the obtained overall YS, the effect of particles and dislocation strengthening being minute. The in-vitro degradation was evaluated by performing electrochemical and hydrogen evolution analyses. It was observed that the Mg–4Zn–0.5Mn alloy exhibits the lowest degradation rate, due to the optimum amounts of second phase particles, twins, and lower grain orientation spread (GOS) value.

Microstructure and texture optimization by static recrystallization originating from {10-12} extension twins in a Mg-Gd-Y alloy

During the deformation of Mg alloys, {10–12} extension twin often contributes to the formation of basal texture but rarely assists the nucleation of recrystallization, i.e., effective grain refinement, therefore it seems to make against the improvement of formability and mechanical properties. In this work, {10–12} extension twin has been creatively utilized as a preference nucleation site for static recrystallization (SRX), achieving grain refinement and orientation randomization in a Mg-Gd-Y alloy using multi-directional impact forging (MDIF) and subsequent annealing treatment. Effect of {10–12} extension twin on SRX behavior has been investigated by annealing treatment at 450 °C using quasi-in-situ optical microscopy (OM) and quasi-in-situ electron back-scattering diffraction (EBSD). The microstructural evolution during annealing shows that several SRX gains can nucleate from the grain boundary of untwinned grains, but they only have few influences on the final microstructure due to their limited volume faction and sluggish growth. In contrast, a large number of SRX gains can initiate from {10–12} extension twin and grow up without the confine of twin boundaries. Finally, they consume their parent grains and make the main contribution to grain refinement. This should be attributed to those pinned {10–12} twin boundary, by interacting with various dislocation slips during the MDIF process, which can operate like grain boundary, store enough strain energy, and promote the nucleation of SRX during annealing. On the other hand, SRX grains usually keep initial random orientation and further randomize the forging texture during annealing treatment.

Shock-induced deformation twinning and softening in magnesium single crystals

Magnesium is widely regarded as an excellent structural material, primarily because it forms the basis for a range of light-weight high-strength alloys. Recently, high-strain rate deformation of magnesium has received a great deal of attention due to the complicated deformation modes that involve combinations of dislocation slip and deformation twinning. In this study, single crystal magnesium samples were shock-compressed along the c- and a-axis, then released back to ambient conditions. Post-mortem transmission electron microscopy revealed that extension twins developed for both c- and a-axis shock loading. Also, the nanoindentation hardness values for these shocked samples were compared to those for samples compressed under quasi-static conditions; it was found that the hardness decreased with increasing strain rate for both c- and a-axis loading. Molecular dynamics simulations were performed to elucidate the detailed mechanisms of deformation twinning in terms of inertial confinement of sample geometry and different stress relaxation speed between impact and lateral directions. The conversion from work-done to heat was discussed to explain the influence of shock-induced heating on the residual hardness. These results give new insights into the residual mechanical response in shock-compressed materials and may help to develop a more fundamental understanding of shock phenomena in metallic materials.