Twin Variants Research Articles

In-situ observation on the twinning boundary movement of pure zirconium during three-point bending tests

The movements in two directions of the boundary between twinning variants and matrix were firstly reported using in-situ electron backscatter diffraction (EBSD) during continuous three-point bending tests in nuclear grade pure zirconium. With the combination of schmid factors and the three dimensional model of identified twinning variants, the mechanism of the twinning boundary movement was proposed by follows: twinning {10–12} variants and tensile twinning {11–21} were formed during the first three-point bending test. After that, the matrix moves along the twin boundary through activated PR (prismatic {10-10}) with the further growth of twinning {10–12} variants and tensile twinning {11–21} in the next bending test, resulting in the movements in two directions of the boundary between twinning variants and matrix. Moreover, a new atomic model showing the movement of the twinning boundary was proposed.

An exceptional ductility of AZ31 magnesium alloy sheet achieved by consecutive multi-pass cooperative lowered-temperature rolling

In this study, a novel technique of consecutive multi-pass cooperative lowered-temperature rolling (CMCLR) was proposed for the first time, with the purpose of preparing high performance wrought magnesium alloys. Based on this, a highly ductile magnesium alloy sheet could be effectively produced from ingot. The corresponding microstructure could directly evolve from coarse dendrites to equiaxed grains with an average size of ∼5 μm, which was attributed to the intense dynamic recrystallization (DRX) during isothermal deformation. The equivalent strain gradually decreased from the surface to the interior and edge of the rolled sheet due to the anisotropic plastic deformation. Instead, the temperature of the core of the sheet was slightly higher than that of the surface due to the deformation heat effect. The basal texture in the RD-TD plane generally weakened in the form of the splitting of basal texture toward the RD and the broadening of basal plane away from the pole center due to texture randomization dominated by DRXed grains. The exceptional ductility of more than 28.7% together with acceptable ultimate tensile strength of not less than 267 MPa was ascribed to the progressive work hardening as a result of seamless switching of multiple deformation modes, i.e., basal dislocation-slip-mediated deformation was preferentially operated and then was substituted by twinning-controlled deformation and non-basal dislocation-slip-dominated deformation. The selection of {101‾2} twin variants agreed well with geometrical compatibility factor (m’) criterion instead of Schmid’s law. The activated twin variants nearly engulfed the whole parent grains for the sake of preserving compatible deformation continuity at ambient temperature, giving rise to the formation of TD-split texture. Comparatively, the {101‾2} twinning could randomize the basal texture via reorienting grain orientations which in turn facilitated the reactivation of dislocation slip systems in the case of high temperature deformation.

The evolution of local stress during deformation twinning in a Mg-Gd-Y-Zn alloy

In this study, we perform in-situ tensile testing and high-resolution electron backscatter diffraction (HR-EBSD) to monitor the twin local stresses at different stages: (i) before twinning, (ii) twins that have propagated into the parent crystal, and (iii) thickening of the twin lamella. The resolved shear stress (RSS) at the twin interface is calculated during deformation twinning, which indicates that the RSS evolution is a dynamic process. Negative shear stress is observed during the propagation stage, which provides evidence for the twinning shear transformation-induced stress reversal. Furthermore, a local Schmid factor (LSF) is used to evaluate the extent to which the local stress is dominated by the RSS on the identified twin plane. The experimental results indicate that the LSFs evolve with strain and are at their maximum level before twinning. The initiated twins are correlated with the highest LSFs compared with the unactivated variants. In addition, the product of the Luster Morris m′ value times the LSF of the twin variant may reveal a threshold for the slip-induced twinning process.

Effects of annealing on microstructure evolution and mechanical properties of constrained groove pressed pure titanium

Constrained groove pressing (CGP) is performed on pure Ti followed by intermediate annealing at 450–650°C to fabricate large-scale plate-shaped sheets with improved mechanical properties. The results reveal that intermediate annealing not only improves ductility, enabling the successful achievement of the three-pass CGP at room temperature, but also enhances the tensile strength during the subsequent passes. The process involving three CGP passes with intermediate annealing after the first pass (referred to as PAPP) is superior for strength improvement. Compared to that of pure Ti, the tensile strength of the PAPP specimen annealed at 550°C is improved by 67%. This is ascribed to the recovery of the secondary twins, 85° <11–20> during annealing at 550°C, resulting in the activated 29.5° <1–100> twin variant in the two subsequent CGP passes. Meanwhile, the dislocations within the shear bands recover and form dislocation networks or subgrains at 550°C, and thus the dislocations continue to entangle and pile up at these sub-grain boundaries in the subsequent pressing, which further contributes to the improvement in the tensile strength. However, recrystallisation occurs in the CGP-deformed pure Ti annealed at 650°C, leading to a decrease in the tensile strength.

Towards understanding double extension twinning behaviors in magnesium alloy during uniaxial tension deformation

Multiple {101¯2} twin variants can be activated when the external tension loading is parallel to the normal direction of magnesium alloy plate. In the current work, we report that in addition to abundant primary {101¯2} twins, an atypical double twinning sequence, i.e. {101¯2}-{101¯2} double twin can also occur during uniaxial loading. The results of electron backscatter diffraction (EBSD) observation indicate that these secondary twins prefer to be formed at the regions where two primary {101¯2} twins belonging to different twin variant pairs interact with each other. Meanwhile, abundant dislocations activated can also be found within the primary twin containing secondary {101¯2} twin. Accordingly, the possible formation mechanisms of such secondary {101¯2} twin are proposed and discussed based on the local strain accommodation evaluated by a combination of geometrical compatibility parameter (m’) and Schmid factor. The analysis results suggest that the formation of these secondary twins may be closely related to the minimization of local strain incompatibility at the twinning boundary caused by the basal dislocation within the primary twins.

Characterization of primary, secondary and tertiary {20[formula omitted]1} successive internal twins in the D019 ordered hexagonal α2-Ti3Al phase

Due to the lack of independent slip systems and the difficulty in mechanical twinning, D019 ordered α2-Ti3Al phase is known to be of substantial mechanical anisotropy, which partly accounts for the low ductility of two-phase (α2+γ) TiAl alloys. However, recent studies have shown that in Al-rich α2 phase compression deformation twins can be activated, which could be beneficial for the ductility of the alloys. In the present study, for the first time, we have revealed a novel internal twinning mechanism of α2 phase in a TiAl alloy deformed at high temperature. Three twin generations of a common type {202¯1}<1¯014> were revealed: Secondary twins formed within the primary twin, and tertiary twins formed in the secondary twin. It is important to note that this internal twin structure can only be observed in the <21¯1¯6> direction but not the commonly used <112¯0> direction. The shear plane P of all twin variants is of type {12¯10}. Due to the variation of the twinning elements between the twin variants, the twinning shear is blunted. The interfaces between the twin variants are characterized by high-resolution electron microscopy. Special attention was paid to the structure of the coherent and incoherent twin boundaries. The effect of chemical ordering is discussed. Our results also shed light on the complex twinning mechanisms in hexagonal structures.

Unveiling the microstructure evolution based on deformation mechanisms and dynamic recrystallization in as-extruded AZ31 Mg alloys during uniaxial compression

A series of uniaxial compression experiments were carried out on the as-extruded AZ31 Mg alloy with<10–10>-<11–20>//ED (extrusion direction) double fiber texture at 200–350 ℃. The microstructure evolution during the work hardening and dynamic softening stages were systematically investigated based on deformation mechanisms and dynamic recrystallization (DRX), respectively. Experimental and simulation results demonstrated that in the work hardening stage, the dominant deformation mechanisms were the basal<a>slip and {10−12} twinning at 250 ℃, while the basal<a>slip and prismatic<a>slip 350 ℃. The appearance of profuse {10−12} twins at 250 ℃ not only effectively refined the microstructure but also quickly promoted the formation of strong<0001>//CD (compression direction) texture. Moreover, the activation of twin variants was mainly dependent on their own Schmid factor that was higher in<10–10>//CD grains, resulting in the earlier disappearance of the<10–10>//ED texture component than the<11–20>//ED texture component. In contrast, owing to the limited of {10−12} twins, the samples at 350 ℃ exhibited a weak fiber texture with diffusely distributed of basal plane along the CD. In the dynamic softening stage, high temperature increased both the grain size and proportion of DRXed grains, resulting in a more homogeneous microstructure. More importantly, the continuous DRX (CDRX) transformed into discontinuous DRX (DDRX) as the temperature increased from 250 °C to 350 °C. The similar orientation between the new CDRXed grains and parent grains at 250 ℃ preserved the strong<0001>//CD texture, while at 350 ℃ the basal texture was significantly weakened due to the random orientation DDRXed grains and higher activation of pyramidal<c + a>slip. These findings are beneficial for the optimization of the temperature range of the MDF process.

Growth twinning and morphology of Al45Cr7 and Al13Fe4

Twinned crystal growth is studied in primary Al45Cr7 and Al13Fe4 intermetallic compounds (IMCs) to explore how different twin types and twin variants affect the growth morphology during solidification. In both IMCs, the number of twin variants increased as the cooling rate increased and, by ~ 5 K/s, both IMCs formed cyclic twins with combined icosahedral (Al45Cr7) or decagonal (Al13Fe4) pseudosymmetry. The growth morphology depended on which twin variants were present. When all twin domains shared a common direction that was a rod growth direction in single crystals, twinning did not prevent crystals from growing as rods. This was the case for both IMCs at slow cooling rate and Al13Fe4 at all cooling rates. Cyclic twinning of Al13Fe4 generated many re-entrant corners but resulted in only a modest reduction in rod aspect ratio. In contrast, when twin domains in Al45Cr7 had common directions along multiple pseudo-i(2) axes, crystal growth transitioned from rod-like to a near-equiaxed morphology.

Atomistic and phase field simulations of three dimensional interactions of [formula omitted] twins with grain boundaries in Mg: Twin transmission and dislocation emission

In polycrystals, the interaction of dislocations and twins with grain boundaries (GBs) plays a role in hardening and formability during plastic deformation. While dislocation-GB interactions are relatively well-understood, twin-GB interactions remain mostly unknown. Here, an approach using molecular dynamics and phase-field simulations is followed to study the forward and lateral interactions between {101¯2} twins and tilt grain boundaries in Mg. Molecular dynamics results show that the resolved shear stress on slip/twinning modes of the neighboring grain, not the geometric alignment, is the dominant factor in determining the outcome of the twin-GB interactions. For some lateral interaction configurations, as the misorientation angle increases, the resolved shear stress on the same {101¯2} twin variant of the neighboring grain reduces while it increases for slip or I2 stacking fault emissions or other twin modes such as {112¯1} and {101¯1}, explaining why twin transmission is not seen at high misorientation angles. Furthermore, lateral and forward interactions of the twin with tilt grain boundaries whose misorientation axes are normal to the coherent twin boundary show significantly different outcomes. For the forward interaction, the twin is absorbed and stacking faults are emitted when interacting for low misorientation angles (up to 30°) while the lateral interaction results in twin transmission, nucleation of a {112¯1} twin, and emission of I2 stacking faults. Finally, comparisons between twin interactions with symmetric and asymmetric tilt GBs with different GB structures show similar outcomes.

Deformation substructure development in AlXCoCrFeNi (X = 0, 0.3) high entropy alloy under different influencing factors

Three distinct initial microstructures, including the coarse grains (Al0 sample), fine grains (Al0-T sample) and fine grains with particles (Al0.3-TA sample), are prepared to investigate the deformation substructure development in AlxCoCrFeNi (X = 0, 0.3) high entropy alloy under the different influencing factors (grain size, strain temperature, strain level and particle). Results suggest slip dislocation is recognized in all deformed samples under the different strain conditions, while the strain temperature and the strain level are the critical factors for producing deformation twins for Al0 sample and Al0.3-TA sample, respectively. The effect of slip dislocation on deformation substructure development in Al0 sample is investigated via analyzing how orientation spread builds up inside the grains, confirming three different kinds of dislocation induced deformation patterns. The main difference between annealing twins and deformation twins are systematically investigated, revealing the different morphological features and the different number of activated twin variants.

On the formation of [formula omitted] boundary via [formula omitted]-[formula omitted] twin–twin interaction in magnesium

{101̄2} twinning occurs extensively in Mg to accommodate plastic deformation. With multiple active twin variants, twin–twin interaction occurs and this often forms twin–twin boundaries. In this work, the {112̄2} twin–twin boundary is studied using electron backscatter diffraction (EBSD) analysis and atomistic simulations. EBSD data show that many of the twin–twin boundaries align well with {112̄2} or {112̄6} planes. Further, atomistic simulations reveal dynamically the formation of {112̄2} boundary via the interaction of two non-co-zone {101̄2} twin variants. Moreover, the twinning mode of the {112̄2} boundary is found to be an extension twin with second undistorted plane of {112̄6}. In addition, the {112̄2} boundaries contribute significantly to the 60°〈011̄0〉 peak in the misorientation histogram; they also play an essential role in the unique strong strain hardening under c-axis tension. Our findings are crucial for completing the twinning theories for Mg.

Digital twins and their use in future power systems

The electric power sector is one of the later sectors in adopting digital twins and models in the loop for its operations. This article firstly reviews the history, the fundamental properties, and the variants of such digital twins and how they relate to the power system. Secondly, first applications of the digital twin concept in the power and energy business are explained. It is shown that the trans-disciplinarity, the different time scales, and the heterogeneity of the required models are the main challenges in this process and that co-simulation and co-modeling can help. This article will help power system professionals to enter the field of digital twins and to learn how they can be used in their business.

Effect of grain size on twinning behavior of pure titanium at room temperature

The grain size effect on deformation twinning behavior of pure titanium during uniaxial compression was investigated using electron backscattered diffraction (EBSD) technology. In this work, EBSD statistical analyses on big data sets of twins in pure titanium specimens with different initial grain sizes were carried out. Results show that a greater deformation coordination ability caused by small initial grains results in higher yield strength. Moreover, upon increasing the grain size, more twin variants were activated and more twin lamellae developed. The types of activated twins in specimens with larger grain size were more diverse. The twin growth of different twin modes was affected by grain size differently. The volume of the {10–12} extension twinning was sensitive to the grain size due to the small shear force generated by it per unit volume. In addition to grain size, grain orientation and the amount of deformation also significantly affect twinning activity during deformation. The primary twin lamellae were more abundant in the normal direction (ND) specimens as the deformation increased, while more lamellar thickening occurred in the rolling direction (RD) specimens. Because of more diverse primary twin modes in larger grains, the secondary twinning was also various. The secondary twinning {10–12}→{11–22} later developed. It is speculated that the activation of the secondary twins may be related to the volume ratio of the primary twin to the matrix grain. Simultaneously, the twin-twin interactions were more common in larger grains, restricting twin growth and promoting the material's hardening. The types and structures of twin-twin interactions in specimens with large grain size were also varied.

The cross-transition of deformation twinning in magnesium

Deformation twinning operates via twinning dislocations gliding on a specific twinning plane and along a specific twinning direction. As a consequence, a twin variant is supposed to retain its identity and unable to transform into other variants. However, we demonstrated that for the {101¯2} deformation twinning in magnesium, one twin variant can transform towards its conjugated variant in response to the external applied loading via the participation of prismatic-basal interface migration. We term such transformation as cross-transition of deformation twinning, which is expected to broaden the understanding of twinning mechanism.

Microstructure and twin behavior of Ti-2Al-2.5Zr during cold pilgering

The microstructure and twin behavior of Ti-2Al-2.5Zr tube during cold pilgering were investigated. During the deformation, the intragranular deformation is not uniform, and the microstructure characteristics are similar to low-cycle shear fatigue. Under alternating stress, the rotation tendency of grain is mainly related to the ratio of circumferential to radial strain. As the mainly primary twin, tensile {10–12}〈10–11〉 is activated in large number, which selection of twinning variant conforms to the Schmidt factor law. However, the selection of the secondary twinning variant is related to the slip system activated by the adjacent grains to accommodate the shear strain caused by the twins. The Schmidt factor of the secondary twin is determined by the slip system opened by the adjacent grains.

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.

Offsetting strength-ductility tradeoff in Mg-Sn-Zn-Zr alloy by a novel differential-thermal ECAP process

Equal-channel angular pressing (ECAP) technique is a promising method to enhance the properties of Mg alloys. Herein, a novel differential-thermal ECAP process (DTECAP) with pre-aging treatment (T6) was developed to simultaneously improve strength and ductility of Mg-Sn-Zn-Zr (TZK) alloys. After T6-DTECAP, the TZK alloy exhibited high ultimate tensile strength (UTS) of ~323 MPa and excellent elongation (El.) of ~28.7%, showing superior balance of strength and ductility. Examination microstructures of TZK alloys revealed that the excellent comprehensive properties were mainly associated with the activation of {10–12} tension twin variants and non-basal slips. Moreover, the grain refinement, fine second phases, and transition of strong ED texture to fiber (10–10) < 11–20 > texture along TD were also beneficial for the enhanced properties. This work aims to provide a novel insight for the development of high-performance Mg alloys using T6-DTECAP processing.

A novel method for predicting variant selection of {1 0 [formula omitted] 2} twins in pure hafnium

A novel method for predicting variant selection of twins was built. The results generated in this study were used to identify the twin variants in pure hafnium after indentation deformation (ID). Our results reveal that six {101¯2} tensile twin (TTs) variants were easily formed in the grains located near (0001) orientation having no active slip systems. However, the number of the TTs variants would be reduced in the grains having active slip systems. Moreover, this method can also be used to identify other types of twin variants.

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.

Evolution of microstructure and crystallographic texture of Ni-Mn-Ga melt-spun ribbons exhibiting 1.15% magnetic field-induced strain

The microstructure and texture evolution of 10M Ni-Mn-Ga melt-spun ribbons were thoroughly evaluated by high-energy synchrotron radiation and electron backscatter diffraction. The as-spun ribbons were subjected to annealing treatment in order to tailor microstructure, atomic order degree, and crystallographic texture. The optimum annealing treatment at 1173 K for 72 h produced a homogenous <100> fiber texture and induced grain growth to the size that spans the entire ribbon thickness. This in turn reduced microstructural constraints for twin variant reorientation in the direction perpendicular to the ribbon surface. On the other hand, a homogenous radial microstructure ensured in-plane stress/strain compatibility giving rise to strain accommodation during variant reorientation. Particular attention was also given to the evaluation of atomic order, which to the largest extent controls the characteristic transformation temperatures. It also lowered the twinning stress to a level sufficiently low for martensitic variant reorientation under magnetic field. As a result, 1.15% magnetic field-induced strain without the aid of mechanical training in the self-accommodated state was achieved.