Extension Twinning Research Articles

Deformation dynamics in pre-twinned Mg-3Al-1Zn alloy: A synchrotron X-ray study

We investigate the deformation dynamics in pre-twinned Mg-3Al-1Zn alloy under uniaxial compression based on in situ synchrotron X-ray diagnostics. Stress-strain curves, strain field mapping and X-ray diffraction are achieved simultaneously. The same measurements are conducted on the as-received Mg alloy for comparison. Massive {101¯2}–{101¯2} double twinning is observed in preexisting twins of the pre-twinned specimen, and abundant {101¯2} extension twins are activated in the as-received specimen. The {101¯2} extension twinning in the as-received specimen can rapidly boost strain hardening rate by slowing down the process of strain localization. The {101¯2}–{101¯2} double twinning does not lead to significant reduction in the rate of strain localization, and thus the corresponding increase in strain hardening rate is modest.

Hot Rolling of Magnesium Single Crystals

To analyze the effect of the initial orientation in the activity of twinning and texture development, magnesium single crystals were rolled at 400 °C (nominal furnace temperature) in two specific orientations. In both orientations, the rolling direction of the sheet (RD) was parallel to the c-axis. For orientation 1, the 112¯0 direction was parallel to the normal direction (ND), and for orientation 2, it was parallel to the 101¯0 direction. The samples were rolled at 30%, 50% and 80% of thickness reduction. After rolling, all the samples were quenched in water to retain the microstructure. The microstructure and texture evolution were characterized by X-ray diffraction and Electron Backscatter Diffraction (EBSD). The initial single crystals were turned into polycrystals, where most grains had their c-axis almost parallel to the ND, and this reorientation was explained by extension twinning. The active twin variants in orientation 1 aligned the basal plane ~30° from the sheet plane and caused a weaker basal texture compared to orientation 2, where the twin variants aligned the basal plane almost parallel to the sheet plane. Strain localization inside contraction twins was observed, and consequently, non-basal grains nucleated inside these twins and weakened the final basal texture only in orientation 1.

A novel dynamic extrusion for microstructure tailoring and evading strength-ductility trade-off in AZ31 magnesium alloy

This study demonstrates that dynamic extrusion is a promising method for improving both strength and ductility in magnesium (Mg) alloy. The dynamic extrusion process introduces numerous {10-12} extension twins and high-density dislocation structures in the fine-grained AZ31 Mg alloy at high strain rate (~105 s−1) and room temperature. After annealing, we note that the high-density twins can effectively enhance the strain hardening and strength due to that twin boundary is inherently resistant to dislocation motion. Remarkably, the excellent ductility has been achieved because of high density deformation twins promote more dislocation to nucleate form twin boundaries, and offer more slip systems for dislocation motion. Our findings suggest dynamic extrusion increases the twin density of the initial microstructure, thereby simultaneously increasing both of the strength and ductility in Mg alloy. Ultimately, our work develops a novel strategy or technique to properly tailor microstructure to achieve a substantial improvement of mechanical properties in Mg alloys.

Twin induced Strain Hardening, Grain Fragmentation, and Texture Evolution during Cold Compression of CP-Ti

This work is a systematic investigation of the twin induced strain hardening behavior, grain fragmentation, and texture evolution during titanium’s cold compression. The compression tests were stopped at intermediate strains for quantitative microscopy by electron backscattered diffraction. The microstructural examination indicates the evolution of {1012} extension twins (ET), {1122} contraction twins (CT) and the interaction between them. The strain hardening rate and its derivative plot could be corroborated with the microstructural features and distinguishes regions of slip and twin dominating zones. The texture evolution indicates that dislocation slips were active apart from deformation twins during deformation. Texturally hard ET evolves, broadens, and interacts in texturally soft grains at low and intermediate strains. The ET regions are suitable for the nucleation of texturally soft CT with further strain. During deformation, ET-ET ∼56.8〈1010〉, CT-CT ∼77〈1010〉, ET-CT ∼87〈4221〉 interactions boundaries and CT-ET ∼44.5〈5140〉 double twins develop.

In situ monitoring of dislocation, twinning, and detwinning modes in an extruded magnesium alloy under cyclic loading conditions

This work investigates the microscopic deformation mechanisms of an extruded, precipitation-strengthened AZ80 magnesium (Mg) alloy subjected to strain-controlled low-cycle fatigue using in situ neutron diffraction measurements. Results demonstrate that the plastic deformation during cyclic loading is dominated by the alternating {10.2} extension twinning and detwinning mechanisms. The observed deformation mode is strongly texture and precipitate dependent. For the initial texture, the tested material has two major texture components which result in the occurrence of extension twinning during both compression and reverse tension in the first two cycles. The prolonged detwinning process in the following cycles is proposed to relieve the shear stress field of {00.2} grains, leading to the disappearance of twinning. The precipitation strengthening results in an increase of the critical resolved shear stress (CRSS) by ~33 MPa for the extension twinning in this AZ80 alloy. The synergistic effects of the initial texture, precipitation strengthening, and load sharing of various grain families and phases contribute to the complicated evolution of dominant deformation mechanisms, among which elevated dislocation activities are believed to be responsible for the relatively poor low-cycle-fatigue lifetime when compared to other Mg alloys.

Understanding the interaction of extension twinning and basal-plate precipitates in Mg-9Al using precession electron diffraction

Precession electron diffraction is used to characterize the interaction between {101¯2} tensile twins and basal plate-like precipitates in a post-deformed, precipitate-dispersed Mg-9Al micropillar. We observed a heterogeneous distribution of precipitates in the micropillar sample, which enabled the study of the different stages involved in twin-precipitate interactions. We show that twin nucleation was promoted, taking place on the surface, as well as from the interior of the micropillar. Twin tip propagation and twin growth were hindered by the precipitates. Twin tips either were arrested by precipitates and new twins formed on the other side of the precipitate, or continued to grow around the precipitate without the re-nucleation events. During twin thickening, the precipitates did not significantly alter the shear-dominant twin boundary migration mechanism, as evidenced by the relatively flat twin boundaries around the partially embedded precipitates. However, the twin boundary migration was retarded by the precipitates, especially in regions confined by closely-spaced precipitates.

Research on deformation mechanism of AZ31 magnesium alloy sheet with non-basal texture during uniaxial tension at room temperature: A visco-plastic self-consistent analysis

The relationship between activities of involved deformation mechanisms and the evolution of microstructure and texture during uniaxial tension of AZ31 magnesium alloy with a rare non-basal texture has been thoroughly investigated in the present study by means of electron backscattered diffraction (EBSD) measurement and visco-plastic self-consistent (VPSC) modeling. These results show that except basal <a> slip and prismatic <a> slip, { 10 1 ¯ 2 } extension twin (ET) also plays a significant role during plastic deformation. With the increasing tilted angle between loading direction and rolling direction (RD) of sheet, the activity of { 10 1 ¯ 2 } ET possesses a decreasing tendency and its role in plastic deformation changes from the one mainly sustaining plastic strain to the one mainly accommodating local strain between individual grains. When { 10 1 ¯ 2 } ET serves as a carrier of plastic strain, it mainly results in the formation of basal texture component ( c -axis//ND, normal direction). By comparison, when the role of { 10 1 ¯ 2 } ET is to accommodate local strain, it mainly brings about the formation of prismatic texture component ( c -axis//TD, transverse direction). At large plastic deformation, the competition between basal <a> slip and pyramidal <c+a> slip is responsible for the concentration of tilted basal poles towards ND within all deformed samples. The larger difference is between the activities of basal <a> slip and pyramidal <c+a> slip, the smaller separation is between these two tilted basal poles. Besides, VPSC modeling overestimates volume fractions of { 10 1 ¯ 2 } ET in samples with angle of 0 to 30° between loading direction and RD of sheet because interactions between twin variants are not included in VPSC modeling procedure at the present form. In addition, as compatible deformation between individual grains cannot be considered in VPSC modeling, the predicted volume fractions of { 10 1 ¯ 2 } ET in samples with angle of 45 to 90° between loading direction and RD of sheet are smaller than the correspondingly measured results.

Inelastic Deformation Mechanisms in Shock Compressed Polycrystalline Pure Magnesium at Temperatures Approaching Melt

In this paper, we investigate inelastic deformation mechanisms in polycrystalline commercially-pure (99.9%) magnesium samples shock-compressed using reverse-geometry normal plate-impact experiments at elevated temperatures. The favorability of orientation for the possible deformation modes (i.e. basal slip, prismatic slip, pyramidal I & II slip and extension/contraction twinning) is investigated using Schmid Factor analysis using the EBSD data. The study is an extension of a recent study by the authors [Wang et al. in J Dyn Behav Mater 3:497–509, 2017], and is motivated by the need to better understand the underlying inelastic deformation mechanisms operative in shock-compressed Mg at incipient plasticity at near-melt temperatures. Electron Backscatter Diffraction analysis of the as-received polycrystalline magnesium samples at room temperature reveal that the $$\langle 10\bar{1}0\rangle$$ directions (normal to the prismatic planes) are parallel to the impact direction, while the [0001] poles are aligned with the radial directions (RD). Because of the unfavorable orientation for basal slip and the high Critical Resolved Shear Stresses (CRSS) for the other non-basal slip systems, extension twinning is observed to be the preferred deformation mode in the samples. In addition, results from samples tested at 400 °C, 500 °C, 610 °C and 630 °C, indicate progressive grain coarsening and a notable increase in extension twin activity at the two highest test temperatures used in the study, i.e. 610 °C and 630 °C. In general, for polycrystalline hcp metals, an increase in grain size and test temperatures are understood to promote dislocation-mediated slip and not dislocation twinning; however, in the present experiments, at the two highest test temperatures a notable increase in extension twin area fraction is observed, indicating an increased resistance (suppression) of dislocation slip at incipient inelasticity. These results, along with the experimentally measured particle velocity profiles at the two highest test temperature experiments (610 °C and 630 °C), provide evidence for transition in dislocation-based slip mechanisms from being thermally activated to viscous drag mediated under the normal shock compression and elevated temperature conditions of the present experiments.

Analysis of the twin variant selection in polycrystalline cobalt

The samples of polycrystalline cobalt were strained in tension in the second stage of work hardening in order to activate twinning. The identification of twins and their orientation was analyzed using electron backscatter diffraction. The study of twin variant selection was carried out based on two parameters, namely the Schmid factor (SF) and the geometric compatibility factor (m'). The results indicate that the grains contained the $$\left\{ {10\overline{1}2} \right\}$$ extension twins in most cases while the $$\left\{ {11\overline{2}1} \right\}$$ extension twin mode was present in few cases, as well. In the grains containing only one twin variant, the SF analysis correctly predicted the activated twins in most cases. In the grains with variant pairs in para-position, nearly half of the studied twins had the highest SF of all possible variants, which means that they followed the Schmid law. On the other hand, for twin pairs in meta- or ortho-position, a different behavior was observed. Twins in these configurations were selected rather by the strain compatibility criterion than by the Schmid law, as SF values of the activated twins were significantly lower than those of the twins in para-position. Of these two configurations, the ortho-position one is highly preferred. The combined analysis using SF and m' parameter, predicted the occurrence of specific twin modes and variants only partially. A local analysis considering the effect of neighboring grains should be developed to account for the cases where the two parameters fail to explain the occurrence of particular twins, which seems to be related to the presence of a high fraction of high-angle grain boundaries.

Microstructure and texture evolution of cold-rolled low-Ni Cr–Mn–N austenitic stainless steel during bending

Microstructure and texture evolution of a low-Ni Cr–Mn–N austenitic stainless steel were analysed during three-point bending process. The electron back-scattering diffraction (EBSD) was used to characterise the microstructural evolution with respect to two regions encompassing (I) near the inner corner and (II) near the outer corner. The fraction of deformation-induced martensite of either types (e- and α′-martensite) was analysed in both regions. The region (I) possessed a considerably higher fraction of e-martensite than that in the region (II). In the region (II), the fraction of austenite 60°/〈111〉 twins was increased. Moreover, in the region (I), the $$\left\{10\bar{1}2\right\}\langle \bar{1}011\rangle$$ extension twinning was depicted in e-martensite. The mean kernel average misorientation of e-martensite experienced a trivial change with increasing bending angle after reaching to a saturation point. Upon bending, texture measurements of austenite returned the αγ-fibre orientations with the dominant Brass component. However, a decrease in the Cube and S texture components was identified. The e-martensite texture revealed strong intensities along the $${\left\{hkil\right\}}_{\upvarepsilon }$$ -fibre. Besides, the α′-martensite developed $$\left(001\right){\left[0\bar{1}0\right]}_{{\alpha^{\prime}}}$$ and $$\left(001\right){\left[001\right]}_{\mathrm{\alpha^{\prime}}}$$ orientations with a trace of $$\left(113\right){\left[4\bar{7}1\right]}_{{\alpha^{\prime}}}$$ orientation.

High strain rate deformation mechanism of Mg–2.5Zn–4Y magnesium alloy containing LPSO phase

ABSTRACT Dynamic compression experiments were carried out on Mg–2.5Zn–4Y magnesium alloy containing long period stacking ordered phase (LPSO) to understand its deformation mechanism using ‘Split Hopkinson Pressure Bar’. High strain rates ranging from 700 to 2100 s−1 were applied along Extrusion Direction. The results demonstrate that maximum compression stress increases with increasing strain rate. In the mean the alloy shows a positive strain rate strengthening effect. The deformation mechanism of Mg–2.5Zn–4Y magnesium alloy with LPSO phase is mainly 〈c + a〉 pyramidal slip supplemented with deformation by extension twin in association with kinking of LPSO phase. The impact fracture surface of the investigated magnesium alloy suggested quasi-cleavage fracture.

Strength-ductility balance of Mg-1.03Ca-0.47Mn Mg alloy sheet produced by rolling and annealing

In present work, the high strength-ductility balance of Mg-1.03Ca-0.47Mn Mg alloy sheet was successfully fabricated by rolling and annealing. After rolling, though the yield strength (YS) and ultimate tensile strength (UTS) of sheet were improved to 235 MPa and 253 MPa, the appearance of deformation-induced grain subdivision mechanism, shear bands and {10–12} extension twins also introduced strong (0001) basal texture with basal pole inclining to rolling direction which led the elongation (EL) decreased to 3.8%. As-annealed sheet occurred static recrystallization which led to the formation of finer grain structure than as-rolled sheet, and the strong (0001) basal texture of as-rolled sheet was transformed into weak (0001) peak texture with basal pole spreading widely along TD. Therefore, as-annealed sheet exhibits high strength-ductility balance with the YS of 235 MPa, UTS of 297 MPa and EL of 16.4%.

Critical assessment of the strain-rate dependent work hardening behaviour of AISI 304 stainless steel

Abstract In the present work, the effect of strain rate on the micro-mechanism of plastic deformation and ensuing work hardening behaviour of AISI 304 stainless steel has been critically investigated. The defining role of the early stage of deformation ( γ → α ′ in 3% strained specimen at HSR was higher contributing to lower transition strain from stage 1 to 2 of work hardening compared to SSR, the extent of work hardening was higher in the latter case during the stage 2 due to a combined effect of extensive twinning and deformation induced martensite formation, which was limited in HSR due to occurrence of cross-slip. These differences in the work hardening mechanisms were manifested as an improvement in UTS/YS ratio from 2.66 to 2.99 in HSR and SSR specimens, respectively. Furthermore, changes in the mechanism of plastic deformation was also reflected as ~4 fold increase in dislocation density for SSR specimen compared to HSR specimen at any given strain level and nearly 27% increase in hardness at the point of fracture. A significant improvement in UTS/YS ratio and ductility for SSR specimen left its signature on the fracture surface in form of three-fold increase in the number density of finer dimples compared to HSR specimen.

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.

Study on high speed impact deformation behaviors of Mg-1Al-4Y by SHPB

This study aims to investigate the deformation behaviours of Mg-1Al-4Y alloy by SHPB apparatus under different strain rates at room temperature. Texture evolution analysis of Mg-1Al-4Y specimens were carried out by X-ray diffraction and electron back-scattered diffraction technology. The macro-texture of the specimen in the transverse direction was gradually biased towards [20–21] with the increasing of the strain rate. Furthermore, a large amount of {10–12} extension twins formed in the grains at low strain rate, and partial twin-shaped nuclei grew up to meet, resulting in formation of boundaries with ∼60° misorientation. The grain boundary orientation distribution indicated that the grain size of Mg matrix became smaller and smaller with the growth of twins, which was not conducive to the continued nucleation of twins. Meanwhile, the rotation of grains decreased the Schmid factor of the twins, which led to an obviously reduction in the fraction of the twin nucleation. Therefore, the dynamic deformation mode of Mg-1Al-4Y gradually changed from twin deformation to twin deformation and dislocation slip with the increasing of strain rate.

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.