Twins In Mg Research Articles

An atomistic study of Y segregation at a {101¯1}–{101¯2} double twin in Mg

Segregation at a triple junction of grain boundaries has not been explained much because the structure of a triple junction is very complicated. The present paper describes Monte Carlo simulations by which Y segregation was investigated at a triple junction of a {101¯1}–{101¯2} double twin in Mg. Y atoms segregated at the extension sites in the {101¯1} and {101¯2} twin boundaries. However, they were not necessarily more segregated at the triple junction of the double twin, although the free volumes at the extension sites of the triple junction were larger on average than those of the other boundaries. Thus, the Y segregation behavior at the triple junction cannot be explained only by the free volume. The anisotropic factor of the atomic Voronoi polyhedron was developed to explain the Y segregation behavior at the triple junction. In addition, the shortest interatomic distance and coordination number affected Y segregation at the triple junction. Also, segregation at the triple junction strongly depended on the Y concentration, which resulted from variations in the local atomic configuration. Thus, the Y segregation behavior at the triple junction was complicated, in contrast to those at twin boundaries, even when the size effect was predominant.

Impact of solute elements on detwinning in magnesium and its alloys

By combining finite element analysis, compressive tests, reverse tensile tests, in-situ electron-backscattering diffraction, and high-resolution transmission electron microscopy, we investigate systematically twinning and detwinning behaviors of {101¯2} twins in pre-compressed pure Mg and AZ31 and AZ91 alloys. We find that the solute elements Al and Zn and the resultant precipitates disrupt the synchronous motion of atoms in twinning boundaries (TBs) during twin formation, yielding a large amount of incoherent TBs and low-angle boundaries from the pre-existing TBs after detwinning. The detwinning in pure Mg and its alloys is found to be linked to both twin boundary (TB) mobility and the interaction of their boundaries with dislocations. At a low strain level (0–1%), comparable detwinning activities in both pure Mg and its alloys are observed due to the selective detwinning in grains with the highest SF, while at high strain level (1–4%), the contribution of detwinning in Mg alloy is higher. This is because a small twin size in Mg alloys leads to strong interactions between basal slip and TBs, and also because in pure Mg, twin size is much larger and its tip is pinned by grain boundaries. In addition, the secondary twins are also found to be formed remarkably in the interior of the primary twins in Mg alloys with a high solute-element concentration owing to the low mobility of incoherent TB and the variation of local stress state by precipitates. The ultimate strength of reverse tensile test is found to increase more substantially in the pre-compressed Mg alloys in comparison to pure Mg, especially in the AZ91 alloy, which is attributed to the hardening effect caused by the formation of many low-angle and secondary TBs and also for the reduced Schmid factor for the basal slip in Mg alloys during detwinning.

Mgの二重双晶に及ぼすY偏析の影響の原子シミュレーション

Mgの二重双晶に及ぼすY偏析の影響の原子シミュレーション

WITHDRAWN: Effect of grain neighborhood on deformation twinning in Mg–8.5 wt%Al alloy

WITHDRAWN: Effect of grain neighborhood on deformation twinning in Mg–8.5 wt%Al alloy

Twinning in magnesium under dynamic loading

Twinning is an important mode of deformation in magnesium (Mg) and its alloys at high strain rates. Twinning in this material leads to important effects such as mechanical anisotropy, texture evolution, tension-compression asymmetry, and sometimes non-Schmid effects. Extension twins in Mg can accommodate significant plastic deformation as they grow, and thus twinning affects the overall rate of plastic deformation. We use an experimental approach to study the deformation twinning mechanism under dynamic loading. We perform normal plate impact recovery experiments (with microsecond pulse durations) on pure polycrystalline Mg specimens. Estimates of average TB velocity under the known impact stress are obtained by characterization of twin sizes and aspect ratios developed within the target during the loading pulse. The measured average TB velocities in our experiments are of the order of several m s −1 . These velocities are several orders of magnitude higher than those so far measured in Mg under quasi-static loading conditions. Electron back-scattered diffraction (EBSD) is then used to characterize the nature of the twins and the microstructural evolution. Detailed crystallographic analysis of the twins enables us to understand twin nucleation and growth of twin variants under dynamic loading.

Numerical study of the stress state of a deformation twin in magnesium

We present here a numerical study of the distribution of the local stress state associated with deformation twinning in Mg, both inside the twinned domain and in its immediate neighborhood, due to the accommodation of the twinning transformation shear. A full-field elastoviscoplastic formulation based on fast Fourier transformation is modified to include the shear transformation strain associated with deformation twinning. We have performed two types of twinning transformation simulations with: (i) the twin completely embedded inside a single crystal and (ii) the twin front terminating at a grain boundary. We show that: (a) the resulting stress distribution is more strongly determined by the shear transformation than by the intragranular character of the twin or the orientation of the neighboring grain; (b) the resolved shear stress on the twin plane along the twin direction is inhomogeneous along the twin–parent interface; and (c) there are substantial differences in the average values of the shear stress in the twin and in the parent grain that contains the twin. We discuss the effect of these local stresses on twin propagation and growth, and the implications of our findings for the modeling of deformation twinning.

Analysis of Twin in Mg Alloys Using Electron Backscatter Diffraction Technique

Electron backscatter diffraction (EBSD) is widely used for quantitative microstructural analysis of the crystallographic nature of variety of materials such as metals, minerals, and ceramics. EBSD can provide a wide range of information on materials including grain size, grain orientation, texture, and phase identity. In the case of metallic alloys, EBSD now has become an essential technique to analyze the texture, particularly when severe deformation is applied to the alloys. In addition, EBSD can be one of the very useful tools in identification of twin, particularly in Mg alloys. In Mg alloys different type of twin can occur depending on the c/a ratio and stacking fault energy on the twinning plane. Such an occurrence of different type of twin can be most effectively analyzed using EBSD technique. In this article, the recent development of Mg alloys and occurrence of twin in Mg are reviewed. Then, recently published example for identification of tension and compression twins in AZ31 and ZX31 is introduced to explain how EBSD can be used for identification of twin in Mg.

The influence of a secondary twin on the detwinning deformation of a primary twin in Mg–3Al–1Zn alloy

In the present paper, the influence of a {101¯2}–{101¯2} secondary twin on detwinning of a {101¯2} primary twin is systematically investigated by compression along the normal direction (ND) of pre-strained samples at room temperature. Samples containing both a {101¯2} primary twin and a {101¯2}–{101¯2} secondary twin are prepared by pre-compression along the transverse direction (TD) and subsequent re-compression along the rolling direction (RD) of a hot-rolled AZ31 Mg alloy plate at room temperature. Our results show that the introduction of a {101¯2}–{101¯2} secondary twin into the primary twin can enhance the compression yield strength along the ND. During compression along the ND, the {101¯2}–{101¯2} secondary twin mainly deforms by {101¯2} twinning, forming a {101¯2}–{101¯2}–{101¯2} ternary twin, while detwinning of the {101−2} primary twin also takes place. When a {101¯2}–{101¯2} secondary twin happens to totally separate a {101¯2} primary twin from the matrix, it can serve as a barrier to impede detwinning of this {101¯2} primary twin. The {101¯2} twinning in a {101¯2}–{101¯2} secondary twin can induce a preferred distribution of prismatic planes in twinned regions, which is closely related to the activation of preferential twin variants.

Grain size effect on deformation twinning in Mg–Al–Zn alloy

Specimens of Mg–3Al–1Zn alloy with a wide range of grain size distribution were compressed along different directions. The compressed microstructure was examined to clarify the grain size effect on deformation twinning in magnesium alloys. Small strains were used to reveal the twinning behaviour. The results show that the grain size affects the formation of deformation twins in an Mg–Al–Zn alloy. The reason for a different result being previously reported is given. This study also reports the different deformation microstructures in specimens compressed along different directions.

Analysis of dissociation of 〈c〉 and 〈c + a〉 dislocations to nucleate twins in Mg

A mechanism for twin nucleation in Mg is studied in which edge 〈c〉 and mixed 〈c + a〉 lattice dislocations dissociate into a stable twin, having at least the minimum 6-layer thickness formed by three glissile twinning dislocations, plus a residual stair rod dislocation. Continuum dislocation theory is used to compute the energy of the initial and final states of the proposed dissociation process, using the twin boundary energy computed by density functional theory. For the 〈c〉 dislocation, the proposed dissociation is energetically favorable. An alternative dissociation path into partials on two -type pyramidal planes is possible, as seen in an atomistic analysis, and the continuum analysis predicts this alternative path to be more favorable than the twin process. For the 〈c + a〉 dislocation, the continuum model also predicts that dissociation into the twinned structure is energetically favorable for 6-layer and thicker twins. In both 〈c〉 and 〈c + a〉 cases, the equilibrium twin length is predicted to increase with increasing applied resolved shear stress and grow unstably beyond a critical stress. Atomistic simulations of these processes are then performed. For 〈c〉, a twinned structure is stable under zero loading but with higher energy than the alternative dissociation on two planes. Under a positive applied strain of 4%, resolved on the twin plane, the twinning structure grows while under a negative applied strain of −3%, it reverts back to the alternative low-energy dissociated configuration on the pyramidal planes. For the mixed 〈c + a〉 dislocation, the atomistic models predict that the dissociation into twinning dislocations does not occur spontaneously at zero applied strain but there is a stable twinned region at finite applied loads. These results demonstrate that dislocation-assisted mechanisms for twinning in Mg, initiating from lattice dislocations with large Burgers vectors, are physically feasible, and therefore twin nucleation from grain boundaries is not necessarily the dominant mechanism of twinning in Mg.

Energetic Analysis of Deformation Twins and Twinning Dislocations in Magnesium

Twin boundaries and twinning dislocations for (10\bar{1}2) and (10\bar{1}1) twins in Mg are investigated, using the generalized-embedded-atom-method interatomic potential. The twin boundary energy for the (10\bar{1}2) twin is found to be larger than that for the (10\bar{1}1) twin. On the other hand, both the dislocation energy and the Peierls barrier of the twinning dislocation are low for the (10\bar{1}2) twin. This implies that the (10\bar{1}2) twin is capable of having a winding morphology and can grow easily.

Prediction of internal stresses during growth of first- and second-generation twins in Mg and Mg alloys

The present work is dedicated to the development of a mean-field continuum mechanics method capable of predicting internal stresses in parent and twin phases during first- and second-generation twinning. For that purpose, a generalized Tanaka–Mori scheme in heterogeneous elastic media with plastic incompatibilities is developed. The work is applied to the case of first- and second-generation twinning in hexagonal close packed magnesium. In the case of first-generation twinning, the model is capable of predicting the trends in the development of back-stresses within the twin domain. A parametric study is performed to explain the roles of grain and twin shape and of relative volume twin volume fractions on the magnitude and directions of the back-stresses. In addition, applying the methodology to the case of second-generation twinning allows identification, in exact agreement with experimental observations, of the most likely second-generation twin variants to grow in a primary twin domain.

Variant selection during secondary twinning in Mg–3%Al

Secondary twin formation during the tensile deformation of Mg–3.4%Al–0.33%Mn was studied by means of the electron backscatter diffraction technique. This was employed to identify the particular variants that formed in each grain. For this purpose, the variants were characterized with respect to the orientation of the parent grain rather than that of its host primary twin. This approach led to a regrouping of the 36 possible variants into four sets, namely S A, S B, S C and S D, consisting of variants that are geometrically equivalent. A statistical analysis revealed that the observed secondary twins were almost entirely of S A and S D type (misorientations of 37.5° and 69.9°, respectively). The former variant is shown to require the least accommodation strain within the parent grain and to have the greatest potential for growth. Formation of the S D variant, in contrast, can be attributed to its being favored by the highest resolved shear stresses, i.e., it obeys Schmid’s law.

An atomic and probabilistic perspective on twin nucleation in Mg

We discuss the nucleation of deformation twins in Mg from a fundamental perspective. Atomistic simulations reveal twinning mechanisms and suggest that twin nucleation most likely occurs at grain boundaries (GBs). We observe twin nucleation from symmetrical tilt grain boundaries using molecular dynamics and reveal that the nucleation pathway depends on the tilt angle and the GB defect state. In particular, twin nucleation is preferred at GBs with low misorientation angles, in agreement with electron back-scattering diffraction (EBSD) analyses. A probabilistic description of twin nucleation is then proposed with the aim of linking atomic-scale information with meso-scale EBSD statistical analyses.

Nucleation of a [formula omitted] twin in hexagonal close-packed crystals

We have studied the atomic structures of the nucleus of a [ 1 0 1 ¯ 1 ] ( 1 ¯ 0 1 2 ) twin in Mg by atomistic simulations using density function theory and an empirical potential. The twinning mechanism for ( 1 ¯ 0 1 2 ) twins is described. The results show that the nucleus consists of one partial dislocation with a Burgers vector of - 50 / 107 [ 1 0 1 ¯ 1 ] together with multiple twinning dislocations (TDs) with a Burgers vector of 1 / 15 [ 1 0 1 ¯ 1 ] . The minimum, stable nucleus involves eight TDs and one partial dislocation, corresponding to a thickness of 17 crystallographic planes.

Atomic Shuffling Dominated Mechanism for Deformation Twinning in Magnesium

Deformation twinning is often mediated by partial dislocation activities at the twin boundary. Using molecular dynamics simulations, we have uncovered a new mechanism for the most commonly observed {1012}<101 1> deformation twinning in Mg and other hexagonal close-packed metals. Here the twin growth involves no definable dislocations at the twin boundary, and the twin orientational relationship can be established by local atomic shuffling, directly constructing the twin lattice from the parent lattice.

A potential for simulating the atomic assembly of cubic AB compounds

Stillinger–Weber interatomic potentials can be used to study the crystal growth of AB compounds with the zinc-blende (B3) structure, but have been unable to be used for other cubic structured compounds. Here we extend a recently modified Stillinger–Weber potential for cubic elements so that it is suited for studying the growth of cubic compounds with the B1 and B2 structures. We also parameterize the potential for the Mg–O system. The potential is shown to accurately model the lattice constants and the cohesive energies of the fcc Mg, fcc O, and the B1 MgO structures. It also correctly ensures that the equilibrium phase of each of these materials possesses the lowest cohesive energy with respect to all their other phases. The potential correctly predicts crystalline growth during molecular dynamics simulations of the vapor deposition of fcc Mg and B1 MgO thin films. The results also reveal the formation of various defects in the films, including islands and twins in Mg, interstitials and disrupted regions on the MgO growth surface, and local un-oxidized regions at the MgO/Mg interface during the oxidation of Mg. The simulation approach enables the study of atomic assembly processes controlling the formation of these defects.