Twin Variant Selection Research Articles

A Study of {10-12} Twinning Activity Associated with Stress State in Mg-3Al-1Zn Alloy during Compression

An eight-sided prism sample, obtained from a hot-rolled AZ31 magnesium alloy sheet, was compressed at room temperature along the transverse direction to investigate the influence of local strain on twinning behavior using electron backscatter diffraction (EBSD) measurements, hardness distribution, and metallographic observations. The octagonal surface of the sample was divided into distinct regions based on hardness distribution and metallographic observations. Combined analysis of the Schmid factor (SF) and the strain compatibility factor (m’) was employed to study twin variant selection. Basal on SF ratio distribution, the Schmid factor criterion, can predict over 75% of observed twin variants in regions A and D (normal stress samples). In contrast, 64% of twin variant selection behavior in region C (shear stress sample) can be effectively explained using a pure shear model. Twin variants with high strain compatibility factors may prefer activation to reduce stress concentration. The strain compatibility factor is more appropriate than the Schmid factor for analyzing the effect of local strain on the selection behavior of twin variants.

The anisotropy of deformation twinning in bcc materials: Mechanical loading, temperature effect, and twin–twin interaction

By integrating atomistic simulations with theoretical calculations, we investigate the prevalence of {112} deformation twins in bcc materials and their sensitivity to the loading direction as well as temperature. Our atomistic simulations reveal the copious occurrence of {112} twins involving either hcp or fcc intermediate phases. The hcp and fcc cases occur in bcc titanium alloy during [100] compression and tension, respectively. Similarly, they can also manifest in bcc iron under different temperature conditions. Furthermore, by calculating the correspondence matrix, we identify the fcc and hcp cases as the normal deformation twin mode and the 1/2 atoms-shuffle mode, respectively. The twinning modes have a significant influence on twin–twin interactions and the final microstructure. Our theoretical calculation confirms that the selection of specific twin modes and twin variants is governed by the correlation between their deformation path and mechanical loading. The results underscore the crucial role of mechanical loading and temperature in activating the specific twin modes, thereby providing a novel avenue for engineering twin microstructures through carefully designed thermomechanical processing techniques.

The Characteristic of {101¯2} Twin of Ti-10V-2Fe-3Al under Planar Wave Detonation.

The microstructure evolution of the twin of TB6 (Ti-10V-2Fe-3Al) under planar wave detonation was studied. The initial microstructure of the alloy consists of an α and β phase. It is found that twin deformation is operated in only the α phase due to the limited slip system in this phase. α grains are mainly rotated from {101¯0} to {0002} during the deformation due to the {101¯2}<101¯1¯> twin. Twin variant selection is found in this study, and the orientation of all {101¯2} twins is oriented at {0002} in different α grains with different deformation degrees. The twin variant selection is well explained based on the strain relaxation along the loading axis and the Schmid factor for twinning shear.

Effect of stress-states on non-classical twinning in three-point bending of magnesium alloys

The influence of stress state on the drivers of non-classical twin nucleation in three-point bending of Magnesium alloy AZ31 is presented in this study. Deformed microstructures and textures at the tensile and, for the first time, shear regions are investigated in detail. The experimental observations coupled with the strain-gradient crystal plasticity analyses show that the shear nucleates more twins compared to the tensile and compressive stress-states at near yield-point. Stored energy density provides mechanistic insight into the local drivers of non-classical twin nucleation observed in the shear stress-state region. Further, the twin variant selection follows the locally stored shear energy density criterion, as opposed to the twin resolved shear stress, indicating that their nucleation is driven by local energy and defect sources.

Twin nucleation from disconnection-dense sites between stacking fault pairs in a random defect network

Deformation twinning, an important plastic deformation mode, critically influences the strength and ductility of Mg. As such, a fundamental understanding of twin nucleation will provide us with an important insight into the conditions that favor the onset of this deformation mode. To reveal the twin nucleation mechanism, we construct a defect network without making assumptions about the types and relative densities of dislocations present. The generated defect network features a high density of I1 stacking faults as well as disconnections between them. Deformation modeling results suggest that this structure enables a twin nucleation event from disconnection-dense sites between I1 stacking faults pairs. By probing twin nucleation and early-stage growth inside of the random defect network, we propose a geometry-based twin variant selection rule as well as a pure-shuffle twin nucleation and early-stage growth mechanism underpinning twinning in Mg.

Microstructure Evolution and Deformation Behavior of Extruded Mg-5Al-0.6Sc Alloy during Room and Elevated Temperature Tension Revealed by Ex-Situ EBSD and VPSC.

In this study, the microstructure evolution and deformation behavior of the extruded Mg-5Al-0.6Sc (AS51) alloy during tensile testing at room temperature (RT) and 250 °C were investigated by electron backscattered diffraction (EBSD) characterization and Visco Plastic Self Consistent (VPSC) simulation. The results showed that a continuous hardening behavior of the alloy occurred during the deformation at RT, and a certain softening was caused by the occurrence of dynamic recovery (DRV) and dynamic recrystallization (DRX) in the late stage of deformation at 250 °C. The primary deformation mechanism at both RT and 250 °C was dislocation slip, with prismatic <a> slip being the dominant deformation mode, and no significant changes in grain size or texture type occurred. By identifying the activated twin variants, the results indicated that the selection of twin variants was closely related to the local stress concentration. The relatively low activation frequency of extension twinning at 250 °C is partly attributed to the fact that the consumption of dislocations by DRV and DRX can effectively relax the local stress concentration. Meanwhile, the DRX mechanism during the deformation of the alloy at 250 °C was mainly discontinuous dynamic recrystallization (DDRX), with a low recrystallization fraction.

Microstructure and twinning behavior of b.c.c tantalum under dynamic plastic deformation

In this paper, dynamic plastic deformation (DPD) of tantalum (Ta) was achieved by split Hopkinson pressure bar and a variety of DPD conditions were set to systematically investigate the microstructure of the deformed samples, focusing on the twinning behavior. Results showed that the deformation conditions have a significant effect on the number and distribution of {112}<111>twins. Compared with {100} and {111} grains, {110} grains are more prone to twinning due to the fewer slip systems. As the strain rate increases or the deformation temperature decreases, the number of twins increases dramatically. In addition, the texture of Ta samples gradually changes from {110} to mixed {100} and {111} textures with the increasing deformation, and the number of twins first increases and then decreases. After calculation, it is found that the selection of twin variants in Ta under DPD not only follows Schmid law but is also influenced by strain coordination. The research in this paper deepens the understanding of tantalum as a material for armour-piercing projectile.

Role of yttrium content in twinning behavior of extruded Mg—Y sheets under tension/compression

The influence of Y content on the grain-scale twinning behavior in extruded Mg—xY (x=0.5, 1, 5, wt.%) sheets under uniaxial tension and compression along the extruded direction was statistically investigated. An automatic twin variant analysis was employed, based on large data sets obtained by electron backscatter diffraction (EBSD), including 2691 grains with 977 twins. The {10▪2} tension twinning (TTW) dominance and prevailing anomalous twinning behavior (Schmid factor (m) <0) under both tension and compression were found. The anomalous twinning behavior was more pronounced as Y content increased under tensile loading, indicating a promoted stochasticity of twin variant selection for more concentrated Mg—Y alloys. However, the trend for the Y-content dependent anomalous twinning behavior was opposite in compression. The fractions of the anomalous TTWs were found to be well correlated with the maximum Schmid factor (mmax) values of basal slip and prismatic slip in the corresponding parent grains for compression and tension, respectively, indicating that twinning and dislocation slip might be closely related in the present Mg—Y alloys.

The effects of microstructure on deformation twinning in Mg WE43

The interplay between microstructure and deformation twinning in a WE43-T6 Mg alloy under uniaxial compression was investigated using a combination of scanning electron microscopy with digital image correlation (SEM-DIC) and crystal plasticity finite element (CPFE) simulation. To improve understanding of the statistical characteristics of deformation twin formation, microstructural effects were characterized in over 1000 grains through metrics including the nominal Schmid Factor, grain size, geometric compatibility factor (m'), residual Burgers vector, and the strain accommodated by neighboring grains. There was a strong correlation between the nominal Schmid Factor and both twin activation and variant selection, but this was not fully deterministic. Deformation twinning also exhibited a strong dependence on existing slip and twinning in the neighboring grain and on the m' value. Within the range of grain sizes present in this condition, grain size was determined to have minimal effect on deformation twinning. Statistical analysis of CPFE simulations was used to further investigate microstructural effects on twinning, and qualitatively captured the effect of the nominal Schmid Factor.

Selection behavior of {10−12} tensile twin variants and its contribution during plastic processing of magnesium alloy

{10−12} tensile twins are successfully prefabricated in the magnesium alloy sheet by bending and straightening. The relationship between twin morphologies and twin variants is established and the effects of the Schmidt factor, geometric compatibility parameter, and local strain field on the twin variant selection are comprehensively explored. The results show that the twin plates in the parallel relationship in the same matrix grains belong to the same variant usually with a high Schmidt factor, while the twin plates that orientate at a certain angle are the parasite variants usually corresponding to low Schmidt factor with respect to the applied stress. Whether the selection behavior of twin variants conforms to Schmidt's law is determined by the local strain field at the position of the twins rather than the matrix grains. The degree of strain compatibility between twins will change the result of the twin variants selection caused by the strain field. Compared to the local strain field, the geometric compatibility parameter between transgranular twins has a greater impact on the choice of twin variants. The twins are preferentially nucleated and have a positive effect on activating the Pyramidal II<a+c> slip of the matrix grains. • A comprehensive analysis of the factors influencing the twin variants selection. • In-grain misorientation quantified local stress field. • Tensile twinning activates the pyramidal II<a+c> slip of the matrix.

Twinning character and texture origination in the randomly-oriented AZ31B magnesium alloy during cold-rolling

Based on generalized Schmid factor (G-SF) calculation, the twinning character and texture development in the rolling plate of randomly-oriented AZ31B magnesium alloy were investigated in this work. The results showed that when the rolling reduction/nip angle is low, the twin type selection is independent of crystal orientation or G-SF due to the significant difference of critical resolved shear stress (CRSS) between extension twinning and contraction twinning. The twin variant selection does not intimately follow the Schmid criterion. In comparison, the extension twin variant selection departs more from the G-SF prediction than the contraction twin variant selection. The orientations of the matrix grains develop gradually tending to distribute around the normal direction (ND), which promote the activation of contraction twinning. The basal texture results only from the orientation development of the matrix grains, depending on the basal <a> slip and the pyramidal <c+a> slip, which was explained in terms of a crystal rotation by the dislocation interactions.

Deformation twins stimulated by {11[formula omitted]2} twinning in adjacent grain in titanium

Deformation twins are likely nucleated at and impeded by grain boundaries (GBs), developing shear transformation bands. Knowing the accommodated deformation in adjacent grains to the twinning shear is essential for understanding mechanical properties and the evolution of microstructure and texture of polycrystalline aggregates. Based on EBSD analysis, we characterize and statistically group accommodated deformation modes in the neighbor to {112¯2} compression twins (CTWs) in titanium with respect to the misorientation axis/angle. Two types of CTWs are preferably activated in adjacent grains with low misorientation angles, two types of extension twins (ETWs) are favorably activated in adjacent grains with high misorientation angles, and some of {112¯2} CTWs terminate at GBs with high misorientation angles. We discuss the preference of accommodated deformation modes and the selection of twin variants based on the SDGA criterion which couples the displacement gradient accommodation (DGA) and stress driven accommodation (SDA) criteria. Although misorientation angle of GBs is widely used to predict the preferred deformation modes, we demonstrate that misorientation axis that was often ignored, must be considered in determining the preferred deformation modes especially for large misorientation angles. Finally, we predict the misorientation angle/axis pairs with respect to the activity of four twinning modes and two slip systems as the best accommodation mechanism in adjacent grains. The results are useful for the implementation of intergranular deformation mechanisms in multiscale modeling and the design of HCP materials with specific GB characteristics.

{10–12} twinning relationship in Mg-Gd-Y(-Sn)-Zr alloys under uniaxial compression at room temperature

After homogenization treatment, uniaxial experiments at room temperature with strain rate of 1 s−1 are carried out on Mg-Gd-Y(-Sn)-Zr alloys, and the maximum strain is controlled to 8%. Plenty of {10–12} tensile twins appear in the deformed microstructure of Mg-Gd-Y-Zr and Mg-Gd-Y-Sn-Zr alloys. The addition of Sn significantly improves the nucleation ability of twins and effectively reduces the lamellar thickness. In Mg-Gd-Y-Zr alloy, the twins are mostly coarse parallel, while in Mg-Gd-Y-Sn-Zr alloy, they are mostly thin cross. A great quantity of closed regions and twin boundaries introduced by high-density thin cross twins are the main reasons for the high flow stress of Mg-Gd-Y-Sn-Zr alloy. In addition, the analysis of twin variant selection behavior, twin interaction and dislocation transmutation reaction in this paper is conducive to consummate the theory of room temperature deformation of magnesium alloys.

Quasi-in-situ investigation on extension twinning behavior of extruded ZC61 alloy during dynamic compression

In this study, extension twinning nucleation and growth of extruded ZC61 alloy were investigated using a quasi-in-situ electron backscatter diffraction method during interrupted dynamic compression along the extrusion direction. The experimental results show that 63.2% of extension twinning nucleation behavior satisfies the Schmid law during high strain-rate compression. Furthermore, the nucleation of twins and selection of twin variants are influenced by the strain accommodation factor. Non-Schmid twin variants have the highest m' and active twinning systems of neighboring grains can be activated in the same twin variant form. In addition, prismatic and pyramidal slips in neighboring grains play a positive role in low SF twin nucleation.

A statistical analysis of compressive deformation mechanisms in an extruded Mg–3Y sheet

A statistical analysis of plastic deformation mechanisms was performed on an extruded Mg-3 wt%Y sheet during room temperature uniaxial compression along the extrusion direction. Grain-scale slip trace analysis and twin variant analysis were performed grain-by-grain to reveal the active slip/twinning systems based on quasi-in-situ SEM and EBSD. The initial microstructure was fully recrystallized exhibiting typical rare-earth texture characteristics. The true stress-true plastic strain curve showed a sigmoidal shape indicating a profuse twinning activity which was consistent with the twin analysis. After 10% strain, there were 78 out of ~700 grains exhibited slip trace. In addition to the dominated basal <a> slip (79.5%), considerable non-basal slips including 2nd pyramidal <c + a> slip (11.5%) following prismatic <a> slip (9%) were active. For twinning activity, among the ~250 grains analyzed at 10% strain, all the identified ~150 twins were tension twins with an area fraction of 19%. The twin variant selection did not solely follow the Schmid law, i.e. 42.3% twins exhibited low Schmid factor values (m<0.2) and 5.4% m values was negative. However, the proposed normalized m (mnor) can correlate the variant selection more effectively, and over 80% twins exhibited large mnor values. Twin transmission at grain boundary was observed for over 50% twins, and only 43.6% of them had high Luster-Morris parameter values (m’>0.75). However, the transmitted pairs with both large mnor sum and normalized m’ were preferred, and the fraction was over 64%. The twined number fraction was closely related to the maximum m values of prismatic slip. The present work implied that the twinning behavior is greatly dependent on the local conditions, and slip and twinning are closely related.

Variant selection of primary–secondary extension twin pairs in magnesium: An analytical calculation study

Twining is an important deformation mode in magnesium. In a deformed magnesium sample, an extension twin crystal, i.e., {101¯2} twin, can form inside a {101¯2} primary twin, which is named {101¯2}–{101¯2} secondary twin. These secondary twins often appear at the intersection of two primary twins, and form primary–secondary twin pairs. Experimental observations show that the most frequently observed primary–secondary twin pairs have a unique misorientation, i.e., twin variant selection exists. Such variant selection of the primary–secondary twin pairs is studied in this work. The crystallographic analysis reveals that the twin planes of the primary and secondary twins that form a twin pair have coincident intersection lines with the boundary where the twin pair adjoins. An analytical calculation method based on Eshelby's inclusion theory is developed, and the calculation results show that only for this unique misorientation, the stress fields concentrated at the rims of the primary and the secondary twins are mutually favoured. The analysis is further extended to the incoming–outgoing twin pairs across ordinary grain boundaries, and compared with the commonly used geometrical compatibility factor m′. It is found that m′ only gives good prediction for twin transmission when the shear stress component on the twin plane along the twin shear direction of the incoming twin is the major contributor to the resolved shear stress of the outgoing twin. When other stress components play a dominant role, m′ becomes ineffective in prediction, which is the case for the primary–secondary twin pairs.

A three-dimensional mechanistic study of the drivers of classical twin nucleation and variant selection in Mg alloys: A mesoscale modelling and experimental study

This work presents a detailed investigation of twin inception to identify site of twinning, variant type selected, and the strain to nucleate it within a full 3D reconstructed microstructure obtained using 3D-EBSD. Microstructurally-faithful 3D crystal plasticity analysis provides quantitative insight to show that stored energy density is a key factor (rather than stress or other criteria) which identifies the experimentally observed twin nucleation site (which supersedes twin inception), and the strain necessary to drive nucleation. 3D (as opposed to 2D surface) analysis has been shown to be essential. The critical energy density for twin nucleation was found to be ∼0.015 Jm-2 in Mg alloy AZ31. Further, at the predicted nucleation site, the experimentally observed twin variant is shown to be driven by the local twin resolved shear stress.

Strain Accommodations among Twin Variants in Ti and Mg

The selection of twin variants has a great influence on deformation texture and mechanical property in hcp metals where slip systems are limited and twinning types are abundant during deformation. Local strain accommodations among twin variants are considered to shed light on variant selection rules in Ti and Mg alloys. Five kinds of strain accommodations are discussed in terms of different regions that are affected by the twinning shear of primary twin. These regions contain (I) the whole sample, (II) neighboring grain, (III) adjacent primary twin in neighboring grain, (IV) adjoining primary twin within the same parent grain, and (V) multi-generation of twinning inside the primary twin. For a potentially active variant, its operation needs not only relatively higher resolved shear stress but also easily accommodated strain by immediate vicinity. Many of the non-Schmid behaviors could be elucidated by local strain accommodations that variants with relatively higher SFs hard to be accommodated are absent, while those with relatively lower SFs but could be easily accommodated are present.

Phase field modeling of diamond-silicon carbide ceramic composites with tertiary grain boundary phases

Phase field simulations are used to study effects of microstructure features on overall strength and ductility of polycrystalline ceramic composites. The material of present interest is a diamond-silicon carbide (SiC) blend with a grain boundary (GB) phase consisting of much smaller SiC grains, graphitic inclusions, and porosity. Homogenized properties—elastic moduli and surface energy—used in the phase field representation of the GB phase are obtained from a novel approach involving bounds from elastic homogenization theory. Extensive parametric calculations on synthetic microstructures with smoothed polyhedral grains are used to deduce trends in structure-property-performance relations. Results suggest that peak strength, for a fixed fraction of GB phase, can be increased by the following prescriptions: increasing bulk diamond content, reducing porosity, reducing graphite content, and distributing any unavoidable defects uniformly rather than randomly. For the assumed constitutive behavior of the GB phase, graphite appears less deleterious than pores of the same average volume fraction, and median defect concentration appears to be more important than randomization of distributions at low volume fractions. More complex interrelations among microstructures, model features, and material responses are also revealed, for example effects of anisotropic fracture, inelastic dilatation associated with crack opening, and twin variant selection in the SiC phase.

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.