Anomalous Twinning Research Articles

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.

Formation of anomalous twinning and its effect on texture development in a cold-rolled Mg-Zn-Ca alloy sheet

An Mg-6Zn-0.3Ca (mass%) alloy sheet with a strong basal texture feature was cold-rolled using a small pass reduction, and the microstructure development during the annealing was tracked by quasi-in-situ electron backscattered electron analysis. Anomalous double twinning with the (0001) pole being aligned to the transverse direction was formed after the cold-rolling. This twinning became the nucleation sites for statically recrystallized grains during the annealing, and the annealed sheet formed weak texture features with transverse-direction split (0001) poles. Static recrystallization also occurred at grain boundaries, and the new grains have a similar texture feature as the parent grains. Therefore, the basal-textured components were still preserved even after the cold-rolling and annealing. • An Mg-Zn-Ca alloy sheet showed texture weakening by cold-rolling and annealing. • Anomalous twinning was formed in a cold-rolled Mg-Zn-Ca alloy sheet. • Anomalous twinning led the formation of TD-split statically recrystallized grains. • Grain boundaries became the nucleation sites of grains with strong basal texture.

Identification and analysis of anomalous {[formula omitted]} twins in an extruded Mg-2wt%Y alloy sheet during uniaxial tensile loading

The deformation behaviors of a hot-extruded Mg-2wt%Y alloy sheet with high activity for basal slip was studied under uniaxial tensile loading along the extrusion direction at room temperature. Using electron backscatter diffraction and crystal plasticity finite element method (CPFEM) simulation, this work proposed the direct evidence for the activation and the role of anomalous {101‾2} twinning: (1) In the Mg alloy with high activity for basal slip, anomalous {101‾2} twins were indeed identified during tensile loading rather than unloading. The anomalous twins had the highest or the third highest negative Schmid factors of {101‾2} twinning, while the normal twins had positive Schmid factors. (2) During tensile loading, the anomalous twins were more easily activated than normal twins. As the true strain increased, the amounts of anomalous twins increased faster than the amounts of normal twins. (3) The normal twins induced positive shear strain along the loading direction while the anomalous twins induced negative shear strain along the loading direction. The role of anomalous twinning was accommodating the plastic compatibility and improving the ductility.

Tensile creep anisotropy of a Mg-2Y alloy extruded sheet with a splitted texture

Mg-RE based alloys usually exhibit better creep resistance than traditional RE-free Mg alloys. Creep anisotropy can be still found in Mg-RE alloys but barely investigated. Thus, in the present study, tensile creep anisotropy and creep mechanisms were studied in a dilute Mg-2Y alloy extruded sheet. It is found that tensile creep property shows not only obvious stress and temperature dependences but also strong anisotropy. Varied stress exponent n is obtained under different conditions. Under σ < 40 MPa at T = 493 K, n = 2 is shown and grain boundary sliding is the main creep mechanism. Under σ < 40 MPa at T = 523 and 553 K, n = 3–4 is obtained and dislocation motion gradually dominates the creep. Under σ > 40 MPa at all temperatures, n > 4 is seen and intense dislocation slip becomes the predominant deformation mode. Besides, steady-state strain rate ε˙s and n are always higher along ED than TD. This creep anisotropy is caused by different dislocation slip types. Basal <a> slip plays the most important role in the creep anisotropy under σ < 40 MPa and non-basal slip contributes to the anisotropy under σ > 40 MPa. Heavy basal slip and pyramidal <c + a> slip are the main mechanism during the creep along ED while cross-slip of <a> dislocations plays the dominant role along TD. Furthermore, {101‾2} anomalous twins are more likely to be found in samples crept along ED with a smaller Schmid factor for twinning. Anomalous twinning and pyramidal <c + a> slip contribute to the creep fracture of ED sample. Unlikely, cross-slip composed by basal and prismatic <a> dislocations induces the fracture of TD sample.

Anomalous twinning in AZ 31 magnesium alloy during electrically assisted forming

Abstract Electroplasticity has been explained by increased glide due to electron dislocation interactions and thermal effects. However, current pulse treatment of AZ 31 at 0.7 kA mm−1 within the elastic regime caused significant twinning, occurring in thin bands in the interior of grains. This is inconsistent with a purely dislocation based explanation of electroplasticity, which predicts a suppression of twinning as a strain accommodation mechanism. The presence of unexpected twinning offers insights on the relative contributions of dislocation mediated mechanisms and contributions from Joule heating to the overall effect.

Anomalous Twinning as the Macroscopic Deformation Mechanism for AZ31 Magnesium Alloy

In order to study the plastic deformation mechanism of AZ31 magnesium alloy, in situ texture measurement during uniaxial tensile deformation is conducted by using neutron diffraction. The specimen is prepared from a rolled sheet so that the deformation axis is parallel to the rolling direction. By increasing strain, the alignment of &lt;10-10&gt; along the tensile axis is strengthened, which is due to the activation of the prism slip system. The basal pole concentration at the prior sheet normal direction is slightly decreased by the deformation and the new texture component is formed at the transvers direction. This can be understood by activation of the {10-12} tension twinning. These results indicate that the tension twinning plays an important role even when the tensile deformation is applied parallel to the basal plane.

On the role of anomalous twinning in the plasticity of magnesium

Specially oriented Mg single crystals were deformed at ambient and elevated temperatures in channel-die compression with the c-axis inclined at 45° to the compression direction and 〈101¯0〉 parallel to the lateral constraint direction. The specimens deformed by basal slip, entailing a rotation of the c-axis towards the compression direction. At room temperature, starting already during early stages of deformation, macroscopic bands comprising a mesh of anomalous {101¯2} extension twins formed parallel to the constraint direction. These twins were characterized by a negative Schmid factor and produced a strain opposite to the imposed deformation. At the final true strain of −1, a significant volume fraction of the matrix was consumed by twins, causing a reorientation of the c-axis farther away from the compression direction, and hence delaying the formation of a hard basal texture component. An analysis of the displacement gradient tensors for the six possible extension twin variants showed that the selected twin variants, of which the bands were composed, involved a lattice rotation opposite to the one caused by basal slip. Less twinning was observed at elevated temperatures with the macroscopic bands disappearing at 370 °C. The obtained results are discussed with respect to deformation heterogeneity and the role of anomalous twinning for the deformation behavior of magnesium.

Mechanism of formation of shear bands upon cold deformation of a commercial Fe-3% Si alloy

A model of the formation of shear bands upon cold deformation of {111}〈112〉 crystals of a commercial Fe-3%Si alloy is proposed. The model suggests a two-stage mechanism for the formation of a shear band and its fine structure. At the first stage, there occurs an anomalous twinning in the {114}〈221〉 system; at the second stage, an almost complete secondary twinning of the band in two standard {112}〈111〉 systems occurs. As a result of these transformations, the shear band consists of regions with an almost cube-on-edge orientation {11 11 1}〈1 1 22〉 and an “octahedral” {111}〈112〉 orientation, which is symmetrical with respect to the initial {111}〈112〉 orientation. A crystallographic mechanism responsible for an anomalous twinning in the {114}〈221〉 system by the slip of (8a/18)〈221〉 partial dislocations is proposed. It is supposed that the shear bands observed consist of groups of different numbers of shear microbands.

Origin of the Anomalous {10\\bar12} Twinning during Tensile Deformation of Mg Alloy Sheet

In order to understand the origin of the anomalous twinning of the {10\\bar12} type, rolled sheets of AZ31 Mg alloy were deformed at room temperature in tension along the rolling direction. An excellent correlation was found between {10\\bar12} twinning tendency and basal dislocation slip activity. Calculation of strain tensor indicated that the diagonal strain components associated with the localized basal slip can be canceled completely by the {10\\bar12} twinning. The results led to the conclusion that anomalous {10\\bar12} twins were formed to accommodate the strain incompatibility caused by localized basal dislocation slip.

Two-headed, two-necked conjoined twin calf with partial duplication of thoracoabdominal structures: role of blastocyst hatching.

Gross anatomical analysis of a derodidymic monosomic stillborn male calf from an embryo transfer recipient was carried out. Two normal heads were present on two necks which were fused at the shoulders. Although the ribs were abnormal in shape and number, there was one trunk and four legs. The vertebrae were double and partially fused from the thoracic region to the sacrum, which was singular and normal. In the thoracic region there was a single vertebral canal that contained two incompletely fused spinal cords which shared common meninges. In the lumbar region there was a spina bifida. The medial neck musculature was variously fused. Two esophagi entered one enlarged rumen and the liver and gallbladder had extra lobes. Two sets of lungs occupied three pleural cavities and also part of the pericardial cavity. Two hearts were joined by a common cavernous venous sinous. The vasculature cranial to the heart reflected the doubling. The vasculature caudally was singular. The calf was uniscrotal and ipsilaterally cryptorchid. Our results are consistent with the hypothesis that atypical "hatching," that is, emergence of the blastocyst from the zona pellucida, may cause anomalous twinning.

Anatomy of cultured mouse cerebellum. II. Organotypic migration of granule cells demonstrated by silver impregnantion of normal and mutant cultures.

AbstractThe architecture of presumptive cortex in whole mount Holmes impregnations of cerebellar cortex was reassessed in order to accomplish accurate identification of the granule cell neurons now known to be present there and to seek evidence concerning possible granule cell migration in vitro. The granule cells, although their processes failed to stain by Holmes' method, were recognized as a layer of naked nuclei of characteristic size, shape, and close packing. The gross foliar and sulcal architecture of cortex remained very immature, as it was when explanted. However, a substantial minority of cultures developed microscopic organotypic lamination characteristic of mature cortex, with molecular, granular, and Purkinje cell layers. All organotypically laminated areas showed anomalous twinning of the granular layer. In the reeler mutation of the mouse, the granule cell migration fails, producing an abnormal laminar pattern in adult cerebellum. Cultures of reeler cerebellum, although well developed in other respects, never showed any trace of cortical lamination, proving that the lamination seen in normal cultures must be produced by the migration which fails in reeler but succeeds in normal mice. This is reinforced by the twinning of the granular layer: any reasonable explanation requires organotypic migration of some of all granule cells. The preferred explanation involves specific geometrical deformation of the developing explant by angular rotation of presumptive cortex.