Secondary Twins Research Articles

Water-assisted exfoliation of epitaxial CdTe film from mica analyzed with azimuthal RHEED

Semiconductor thin films grown on weakly interacting substrates have recently attracted much attention due to the fact that one can peel/release the film from its substrate for flexible electronic and optoelectronic applications. In this work we report separable CdTe(111) epitaxial films grown on mica substrate via vapor transport deposition. The CdTe films were separated from the mica substrate using double-sided Cu tape, exposing the CdTe interface-surface. Azimuthal reflection high-energy electron diffraction (ARHEED) was used to characterize the fresh CdTe interface-surface and the original CdTe surface. Dramatic differences between these two surfaces were observed. (1) Diffraction streaks were observed from the interface-surface, indicating a smooth surface. Diffraction spots were observed from the surface, indicating a rough surface. (2) The rough CdTe surface has twin and secondary twin structures, which are difficult to be observed with conventional X-ray diffraction. (3) The full-width-at-half maximum (FWHM) analysis of the streaks and spots in RHEED patterns reveals that despite the smoothness of the interface-surface, there is a lower degree of lateral long-range order as compared with the rough surface. This etchant-free method to create smooth interface-surface and ARHEED characterization may be applicable to other films that are detachable from weakly interacting substrates.

Effects of the aluminum concentration on twin boundary motion in pre-strained magnesium alloys

We investigated the effects of Al concentration on the reciprocated motion of twin boundaries in pre-strained Mg-Al-Zn alloys through a combination of applied compression and tension, in-situ electron-backscattering diffraction observations, and high-angle annular dark-field scanning transmission electron microscopy observations. The twin growth was restricted by increased Al concentration, which resulted in the occurrence of smaller-sized twins. The reverse motion of twin boundaries was also restricted, resulting in the formation of higher fractions of secondary twins and 2–5° boundaries during reverse tension. The secondary twins and 2–5° boundaries mainly contributed to the increased ultimate tensile strength of the pre-strained Mg alloys. This effect is more significant in Mg alloys with larger pre-compression. Moreover, the increased amount of the Al solute atoms, rather than the precipitates, mainly contributed to the increased strengthening effect on the twin boundary motion. Our research contributes to development of high-strength Mg alloys by stabilizing twin boundaries.

Cyclic deformation responses in α-phase of a lamellar near β-Ti alloy

A lamellar near β-Ti alloy was subjected to cyclic tensile loading at room temperature. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) investigation revealed that the deformation mechanisms of α-phase were found to be changing with increasing loading cycles, from mechanical twinning to martensite transformation. To be specific, at the beginning of straining, the formation of mechanical α-twins was the dominant deformation mechanism to accommodate the applied strain. As the increasing cycles, the secondary twin and hierarchical structure formed in α laths. During further straining, the noticeable groove was observed at the α/β grain boundary. Meanwhile, dispersed nanosized α" martensite phase formed in the α plate and the number of α-twins sharply decreased at this stage. This work will provide a basis for the enhancement of load bearing capabilities of near β-Ti alloys.

Microstructure evolution of ultra-high-strength twinning-induced plasticity (TWIP) steel under dynamic loading

To prepare ultra-high-yield strength twinning-induced plasticity (TWIP) steel and reveal its work hardening mechanism at different strain rates from the microcosmic range, the microstructure evolution mechanism of Fe–20Mn–0.6C TWIP steel was investigated at strain rates of 10−4–103 s−1 using a high-speed tensile testing machine and a transmission electron microscope. The results show that the strain rate and deformation had a significant effect on the twin morphology of TWIP steels. At a strain rate of 102 s−1, secondary deformation twins were developed, which intersected with the initial deformation twins and increased the resistance of dislocation movement, as well as the plasticity. TWIP steel at a strain rate of 102 s−1 had a higher twin formation speed than that at 100 s−1. At the same amount of deformation, the twin boundary fraction was higher and increased linearly at a strain rate of 102 s−1, while the rule of twin growth at 100 s−1 was conformed to S-curve change of DoseResp model.

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%.

The Effect of Cerium and Twins Fraction on Corrosion Resistance of AZ31-1Ca Sheet Alloys

In this study, in the range of 0.2% to 1.0% Ce elements were added to AZ31-1Ca alloys produced by low pressure die casting method. Hot rolling process was applied to these produced alloys at 400 °C with 1,5 m/min rolling speed at 15% deformation rate per pass. The corrosion resistance of samples extracted from sheets that have included different twins fraction after various Ce adding were investigated at 3.5% NaCl solution for 72 hours according to ASTM G31 of Standard Practice for Laboratory Immersion Corrosion Testing of Metals. The effect of secondary phases, grain size and twins on immersion corrosion resistance of investigated alloys was studied systemically on the base of microstructure analysis.

Evolution of Twinning During Cyclic Loading of Magnesium Alloy ZM21 Examined by Quasi-in-Situ EBSD

Most complex engineering components experience varied loading in use. We conducted successive experiments of cyclic tensile deformation followed by electron-backscattered diffraction imaging of the same area on each of several ZM21 magnesium alloy sample to investigate the microstructure and texture evolution during cyclic loading. In this way we correlated the evolving deformed microstructure of cyclically loaded magnesium to the initial grain orientations. To investigate the effects of the strain path, the behaviour of samples with and without pre-compression was compared. Twins can propagate from grain to grain and twin chains will form in the material without any pre-deformation. Twin chains are a result of twin transmission at grain boundaries and twin transmission frequency decreases with increasing grain boundary angles. In the absence of pre-compression, the fraction of extension twins increases from 0.003 to 0.019 after two cycles of tensile loading to a strain of 0.03, after which it changes only slightly as strain is increased. Twin chains in the pre-compressed specimen are activated without any obvious dependence on the matrix orientation. The fraction of extension twinning is 0.2 for the pre-compressed specimen. After a cycle of tensile loading to a total plastic strain of 0.015, 90% of the extension twins undergo detwinning. The detwinning starts just after reverse loading. Secondary twins forming within primary twin interfaces start to nucleate in the microstructure of the pre-compressed material at an early stage (at plastic strain values of 0.056). This is different from the nucleation of secondary twins in the material without pre-compression that occurs at a strain of 0.104. The early formation of secondary twinning is due to the presence of residual primary twins in the microstructure of the pre-compressed material as a result of detwinning of the initial twins.

Interactions between 〈a〉 dislocations and three-dimensional [formula omitted] twin in Ti

〈a〉dislocations on basal or prismatic planes and {112¯2} compression twins are commonly activated in deformed Titanium (Ti). In the present work, their interactions are investigated by both crystallographic analysis and atomistic simulations. For a three-dimensional {112¯2} twin, we firstly analyze seven possible twin boundaries (TBs) bonding two low index planes in matrix and twin. Next, we focus on the two lower energy boundaries, {112¯2}M/T∥{112¯2}T/M coherent twin boundary (CTB) and {1¯21¯1}M/T∥{12¯11¯}T/M. Depending on dislocation character and boundary type, we define four types of interactions between 〈a〉 dislocations and these TBs. Further, we predict possible dislocation reactions on/across TBs using crystallographic analysis according to the deformation compatibility and the change in elastic energy, such as twinning/detwinning of the primary twin, slip transmission and secondary twinning, for each type of interaction. Molecular dynamics (MD) simulations are then conducted for all interactions under pre-selected loadings in order to explore the dynamic process associated with each of these interactions and examine the predicted reactions. MD simulations predict that the interaction between 〈a〉 dislocations and some facets can lead to the formation of secondary twins and 〈a〉 dislocations on basal or prismatic planes in twins, and reveal the possibility of forming 〈c〉 and 〈c+a〉 dislocations in twins. Moreover, some of the possible reactions take place on lateral TBs other than CTBs.

Superior-tensile property of CoCrFeMnNi alloys achieved using friction-stir welding for cryogenic applications

The friction-stir weld (FSW) was investigated based on the relationship between microstructural and mechanical properties at room and cryogenic temperatures for rolled and cast Co0.2Cr0.2Fe0.2Mn0.2Ni0.2 high entropy alloys (HEAs). The rolled and cast HEA welds exhibited good weldability without welding defects. The grain size (GS) of the stir zone (SZ) for rolled and cast HEAs was smaller than that of the base metal (BM). The W, Cr, and C particles caused by the wear of the WC-Co tool formed fine M23C6 carbides, due to the heat generated during the FSW, and accelerated the recrystallization via the particle stimulated nucleation (PSN) effect. Therefore, the area with M23C6 carbide exhibited a finer grain size compared with the area without M23C6 carbide. The rolled HEA was fractured in the SZ owing to the thinning phenomenon, and the cast HEA was fractured in the BM owing to the GS of the SZ, which was much finer than the GS of the BM. However, both HEA welds had larger room temperature strength than BM, and the cryogenic strength was also improved owing to the primary twin, secondary twin, and tangled dislocation. The PSN effect due to the carbides contributed to the increase in strength. Therefore, the FSW of the rolled and cast HEAs produced in this study is suitable for cryogenic applications.

Plastic deformation via hierarchical nano-sized martensitic twinning in the metastable β Ti-24Nb-4Zr-8Sn alloy

A superelastic metastable β Ti-24Nb-4Zr-8Sn (wt.%) alloy is analyzed in the solution treated state after different applied strains in order to reveal evolution of microstructures and explore plastic deformation mechanisms. Deformation bands are observed by optical microscopy, EBSD and TEM. After 3% of strain, only few thin {332}<113>β twins are observed, while after 5% of strain, complex hierarchically β twinning structures composed of primary and secondary bands are observed by TEM. Indeed, very thin secondary {332}<113>β twins are observed (about 50 nm in average) and their massive occurrence leads to the loss of the initial {332}<113>β twinning relationship between matrix and primary bands which is transformed passively to an unusual type II <531>β twinning relationship. Similarly, the orientation relationship between matrix and secondary twins tends to another abnormal type I {541}β twinning relationship. As in situ synchrotron XRD experiments show that almost all the β phase is transformed into α" martensite under loading condition, a crystallographic reconstructing method is then used to determine the real α″ orientation before stress releasing. The relaxed {332}<113> β twins is found to correspond perfectly to the reversion of {130}<310> α″ martensitic twins occurring under loading condition. The unusual twinning relationships between matrix and primary/secondary twins are shown to correspond to type II <512>α" and no twinning relationships in α" martensite, respectively. Schmid factor analyses show that the activation of primary twins obeys the Schmid law. However, the activation of secondary twins disobeys the Schmid law due to the variant selection of α" martensite occurring during primary twinning.

Dislocation dissociation induces secondary twinning in titanium

The twin–twin interaction phenomenon in Ti is studied by incorporating experimental observation and theoretical analysis. Secondary $$ \{ {10\bar{1}2} \} $$ twins are stimulated in primary $$ \{ {11\bar{2}2} \} $$ and $$ \{ {11\bar{2}4} \} $$ via twinning dislocation dissociation. Primary twin variants are found either share the same $$ \langle {10\bar{1}0} \rangle $$ zone axis or with different zone axes. Schmid law, dislocation theory and crystallographic analysis are used to investigate the formation processes and physical mechanisms of the twin–twin interaction phenomenon. Results show that non-Schmid secondary twins are stimulated under the effect of primary twinning interactions. Besides to secondary twinning dislocations, glissile and sessile dislocations generate as a result of primary dislocation dissociation. Glissile dislocation glide on the easy slip prismatic plane, while sessile dislocation accumulates and leads to the increase of local strain.

A transmission electron microscopy study of strain-induced secondary twin and epsilon-martensitic transformation in Fe-15Mn-2Cr-0.6C-0.06N austenitic steel

Strain-induced twin formation and γ(f.c.c.)-austenite to ε(h.c.p.)-martensite phase-transformation are important processes for mechanical properties of high-Mn austenitic steel. One-directional primary twin and ε-martensite (Tε) are sequential and bi-sequential structures of stacking faults (SFs) formed with the passage of 1/6 <112> Shockley partial dislocations along every and every other {111} plane of the f.c.c. structure, respectively. As strain increased, secondary Tε, which do not satisfy the Shoji-Nishiyanma (S-N) orientation relationship, were developed by the intersection of two primary Tε-bands. The sequential process for the formation of secondary Tε-phase was studied by selective area diffraction patterns (SADPs) and high-resolution scanning transmission electron microscopy (STEM) analyses and compared with two atomistic models: (i) rigid-body rotation model and (ii) epsilon martensite {11¯02}ε twinning-shear model. The rotation of the slip plane within a grain, referred to as the disclination process, is also developed by forming the secondary Tε-band at the intersection of two primary Tε-bands. The thickness change of four arms of primary Tε-bands contributes to the rotation of each subgrain of the γ-matrix.

Local microstructure and micromechanical stress evolution during deformation twinning in hexagonal polycrystals

Abstract

Size-dependent formation and thermal stability of high-order twins in hierarchical nanotwinned metals

Introducing hierarchical twins into nanotwinned (NT) materials is regarded as an effective way to further improve their mechanical properties. It can be imagined that, with the increase of the order of hierarchical twins, it is insufficient to solely take single twin spacing into consideration. For example, the effect of the spacings of primary and secondary twins should be considered together for tertiary twinning. By virtue of theoretical modelling and atomistic simulations, we investigate the influence of low-order twin spacings on high-order twinning. The optimization strategy of high-order twin density and spacings with respect to low-order twin spacings are proposed. It is demonstrated that there exists a trade-off between high-order twin density and twin spacing which can be tuned by the low-order twin spacings. In addition, the atomistic deformation mechanisms related to low-order twin spacings are discussed. Different size-dependent propagation behaviors of partial dislocations are unveiled, relying on the combination of low-order twin spacings. At last, the great thermal stability of high-order twins is also verified, which is attributed to a strong pinning effect of partial dislocations onto low-order twins, leading to a special stress partitioning phenomenon. Our findings may provide a theoretical benchmark for the fabrication of high-order hierarchical nanotwinned (HNT) structures and thus, assisting the design of high-performance mechanical materials.

Modeling twinning, detwinning, and dynamic recrystallization of magnesium alloys

Abstract

Effect of Rotary Swaging on Microstructure, Texture, and Mechanical Properties of a Mg‐Al‐Zn Alloy

The effect of rotary swaging on the microstructure, texture, and mechanical properties of the magnesium alloy Mg‐4.4Al‐0.9Zn‐0.4Mn is studied. Repetitive processing, conducted at progressively dropped deformation temperature (from 400 down to 200 °C), leads to an increase in the cumulative strain ε. Rotary swaging of the alloy is shown to lead to microstructure fragmentation due to intensive twinning on various crystallographic planes. A high density of twins observed at the final stage of deformation, at ε = 2.77, leads to a decrease in the distance between boundaries (including both twins and grain boundaries) to ≈3 μm. With decreasing temperature, 0.2‐μm‐wide secondary deformation twins form within the primary twins with a width in the range of 1.5–2.5 μm. The texture analysis shows that, upon deformation, the number of orientations increases as does their scatter. Rotary swaging at 350 °C to ε = 2.77 gives rise to an increase of both strength and tensile ductility of the alloy. The high strength characteristics are achieved due to the formation of a subgrain structure and profuse twinning. The increased tensile ductility is associated with the activation of prismatic slip in addition to developed basal slip.

Effect of local stresses of primary twinning on double twinning, twin intersection and their competition

Taking into account the local stresses resulting from the primary twinning with transformation strains, the competition between double twinning {10–11}–{10–12} and (10–12)–(–1012) (CDTW and EDTW) is theoretically modeled. Using electron backscatter diffraction technique, the statistical fractions, thickness-length ratios and their critical values of CDTW and EDTW within grains of as-cast AZ31 magnesium alloy sheets at the low strain stage of hot rolling processing are obtained to validate the theoretical results. In addition, the competition between the DTW and twin intersection are also discussed in detail. The results show that: (1) The secondary twin growth complies with the classical Hall-Petch relationship. Due to the difference in the driving forces resulting from different transformation strains between the primary extension and contraction twinning, the secondary extension twin of EDTW is more difficult to grow compared to that of CDTW for primary twin bands with small thickness-length ratios, which is contrary to the regulation for primary twin bands with large thickness-length ratios; (2) The greater the thickness-length ratio of the primary twinning is, the smaller the critical shear stress of the secondary twinning is. This is opposite to the effect of thickness-length ratio on the critical shear stress of the incident twinning in the twin intersection. It is indicated that the thickening of the primary twinning provides the higher driving force for the formation of DTW, however hinders the twin intersection more evidently; (3) Within coarse grains, at the low strain stage, the formation of DTW can dominantly induce dynamic recrystallization and continue plastic deformation, while the stress concentration resulting from the twin intersection is inclined to cause brittle fracture. Therefore, based on the effect of the local stress, the selection mechanism of the thickness-length ratio on the DTW and twin intersection is of great significance to determining the plastic performance of magnesium alloys during hot working. In the current study, massive control factors of the thickness-length ratio of the primary twinning are concluded, which provides some corresponding schemes of material design.

Microstructure and mechanical properties of cold drawing CoCrFeMnNi high entropy alloy

The present study investigated the effects of cold drawing deformation on the microstructure and mechanical properties of CoCrFeMnNi high entropy alloy. The results showed that at low strain (≤0.28), the plastic deformation is dominated by planar slip with the formation of slip bands, Lomer-Cottrell locks and Taylor lattices. While at medium to high strain (between 0.28 and 1.96), the microstructure consisted of dislocation cells, deformation twins (including the primary and secondary twins) and irregular dislocation (i.e. randomly distributed dislocation clusters). At high strains (＞1.96), deformation is mainly mediated by twinning and shear banding causing the formation of lamellae structures parallel to metal flow direction. Texture evolution showed that the <111> fiber texture was the stable texture component and the <100> fiber texture became unstable at low strain. At strains higher than 1.96, the texture component content remained unchanged even under further increased strain, and the volume fractions of <111>, <100> and complex texture components were 57.4%, 30.7% and 11.9%, respectively. Additionally, the microhardness and yield strength increased monotonically with increased strain. At the early stages of deformation, activation of planar slip causes the initial HEA to display a very high strain hardening rate. The hardness value saturated at 430 HV at the strain of 2.77, which is about 3 times larger than the initial hardness value. Pre-strain by cold drawing deformation can significantly enhance the yield strength of the CoCrFeMnNi high entropy alloy by twinning and texture strengthening.

Scale law of complex deformation transitions of nanotwins in stainless steel

Understanding the deformation behavior of metallic materials containing nanotwins (NTs), which can enhance both strength and ductility, is useful for tailoring microstructures at the micro- and nano- scale to enhance mechanical properties. Here, we construct a clear deformation pattern of NTs in austenitic stainless steel by combining in situ tensile tests with a dislocation-based theoretical model and molecular dynamics simulations. Deformation NTs are observed in situ using a transmission electron microscope in different sample regions containing NTs with twin-lamella-spacing (λ) varying from a few nanometers to hundreds of nanometers. Two deformation transitions are found experimentally: from coactivated twinning/detwinning (λ < 5 nm) to secondary twinning (5 nm < λ < 129 nm), and then to the dislocation glide (λ > 129 nm). The simulation results are highly consistent with the observed strong λ-effect, and reveal the intrinsic transition mechanisms induced by partial dislocation slip.

Coupled nonlinear elasticity, plastic slip, twinning, and phase transformation in single crystal titanium for plate impact loading

High purity single crystal titanium (Ti) under shockwave loading in plate impact experiment is modeled and simulated, with help of the finite element. A thermodynamically consistent system of equations is formulated in the frame of large deformation to consider material anisotropy, rate dependence, and multi-physics including nonlinear elasticity, dislocation based plastic slip, deformation twinning, and phase transformation. A novel kinematics is proposed to consider phase transformation in the α twin variants and also to consider the dislocation based slip in all of components of parent material, primary twins and the ω phase. The thermodynamically consistent driving forces for plastic slip, twinning, and phase transformation are developed based upon the second law of thermodynamics. For nonlinear elasticity, the stiffness matrix is dependent on the volume fractions of all components, and the volumetric part of Cauchy stress is obtained from the nonlinear equation of state. Dislocation based plastic slip in multi-variant and multiphase heterogeneous materials is developed and interactions of dislocations among all slip modes are taken into account. A mechanism for dislocation density evolution during twinning and phase transformation is proposed. The shock loading along the [0001] and [101¯1] directions of single crystal high purity Ti is investigated computationally. Very good correspondence between simulation and experiment is obtained, which includes pole figures, volume fractions of components, free surface velocity, peak pressure, and phase transformation pressure. Multiple experimental phenomena are interpreted based upon the progression of dislocation slip, deformation twinning, and phase transformation. In compression with the [101¯1] crystal, a higher volume fraction in the primary twins but a lower secondary twin volume fraction in the [0001] crystal in experiment was observed. The higher propensity for phase transformation occurs in the [0001] crystal is reproduced. In addition to material texture, distributions of temperature, stresses, and plastic strains dependent on the impact loading directions are revealed.