Andriy Ostapovets

Twinning disconnections and basal–prismatic twin boundary in magnesium

Recent experiments have revealed that the twin boundaries in hexagonal materials are decorated by basal?prismatic (BP) facets. The objective of this paper is to investigate the mechanism that leads to the formation of these BP interfaces using atomistic simulations and to carry out their stability analysis under an externally applied shear. We demonstrate that the stability of the BP interface is a consequence of large capillary forces that are caused by differences of the energies of a perfect twin boundary and the BP interface. The latter is formed by the glide of twinning disconnections and therefore agrees with the theory of admissible interfacial defects. This observation implies that the glide of twinning disconnections is a primary mechanism of the twin boundary migration.

Double twinning in magnesium

It is demonstrated that metalworking processes performed at different temperatures can lead to the - double twinning. This twinning mode has been observed during direct extrusion of the coarse-grained Mg-0.3at.%Al alloy at 433 K and analysed in detail on room-temperature rolled magnesium single crystal with the c-axis parallel to transverse direction and the a-axis parallel to rolling direction. The - double twins originated during initial stage of the formation on coarse-grained and single-crystalline structure.

Characterization of the matrix–twin interface of a (101̄2) twin during growth

A model of twin growth in magnesium is presented together with the analysis of defects responsible for this growth. The twin interface is represented by and basal-prismatic facets. Disclinations are situated in the facet junctions creating dipoles superimposed on basal-prismatic and conjugate twin facets. The migration of facets is mediated by the conservative motion of interfacial disconnections. The interfaces contain twinning disconnections. The facet junctions serve as sources and sinks for these defects. Two types of disconnections ( and ) were observed in basal-prismatic boundary. The dipoles of disconnections were nucleated in the vicinity of existing defects of this type. Interaction of this dipole with existing leads to the creation of a disconnection, which is later absorbed in the facet junction. The nucleation of dipoles was not observed. In twin embryo growth, the basal-prismatic segments remain coherent with a fixed length, while the twin segments grow indefinitely.

Deformation due to migration of faceted twin boundaries in magnesium and cobalt

Recent experimental observations show that twin boundaries in hexagonal close-packed (hcp) metals are frequently faceted. The objective of this paper is to investigate the influence of this faceting on the strain produced by twinning. We show that basal–prismatic (BP) facets are terminated by opposite disclinations and the migration of these facets along a straight twin boundary produces ordinary twinning shear. On the other hand, joining conjugate twins gives rise to BP facets terminated on the parent twin boundaries by identical disclinations. In this case, the strain produced by the migration of BP facets is an average between the strains produced by the individual conjugate twins. These theoretical studies are complemented by two EBSD measurements on cobalt that is closely related to magnesium. The misorientation profiles measured across two conjugate twin boundaries yield a misfit of approx. 7° consistent with the theoretical prediction that the corner of a twin embryo is terminated by two identical disclinations, each accommodating a misfit of 3.7°.

Visco-plastic self-consistent modelling of a grain boundary misorientation distribution after equal-channel angular pressing in an AZ31 magnesium alloy

This study applies a visco-plastic self-consistent (VPSC) model to an AZ31 alloy processed by equal-channel angular pressing. The study focuses on the possibility of reproducing a grain misorientation distribution and the distribution of coincident site lattice boundaries in the model framework. Co-rotation and a magnesium misorientation scheme are employed together with the conventional VPSC model to improve its predictions. The results of the model are then compared with experimental data.

On basal-prismatic twinning interfaces in magnesium

The existence of basal-prismatic interfaces and their roles in twinning of hexagonal materials have recently attracted appreciable attention of scientific community. In this paper, we utilize molecular statics to investigate the formation of basal-prismatic facets in the twin boundary of magnesium. This interface is shown to be the consequence of a collective motion and interaction of twinning disconnections. By analyzing volume deformations caused by the migration of a single basal-prismatic interface, we show that the passage of this interface distorts the material equivalently to twinning shear.

Investigation of twin–twin interaction in deformed magnesium alloy

Abstract Using transmission electron microscopy, we characterised the structures of the boundary caused by the interactions between different twin variants that share the same zone axis in a deformed magnesium alloy. We found that the twin–twin boundaries can adopt the habit planes that are parallel to the (0 0 0 2) basal plane or the prismatic plane or the twinning plane of the interacting twins. To investigate the formation mechanism of various twin–twin boundaries, we also performed atomic simulations. The results indicate that the formation of a twin–twin boundary may be related to the reaction of twinning disconnections that glide on the basal-prismatic planes of the interacting twins.

On the relationship between and conjugate twins and double extension twins in rolled pure Mg

Abstract The paper presents a new type of twin-like objects observed in rolled pure magnesium. They have and habit planes and their misorientations to the matrix are close to 56° and 63° about axis, respectively. The ad hoc performed theoretical analysis and atomic simulations allow to interpret the objects as double twins formed by the simultaneous action of two twinning shears with completely re-twinned volume of primary twin. The observed inclinations from the ideal misorientations for such double twins can be explained by the compliance of the strain invariant condition in the twin boundary. It seems plausible that, once the double twin is formed, its twin boundaries are hard to move by glide of twinning disconnections. If so, these twins represent obstacles for the motion of crystal dislocations increasing the hardness of the metal.

Texture evolution in oriented magnesium single crystals processed by equal channel angular pressing

Magnesium single crystals with different initial orientations were processed by single-pass equal channel angular pressing (ECAP) at 503 K. The texture in the crystals after ECAP was investigated by electron backscatter diffraction and compared to texture simulations based on a visco-plastic self-consistent model. The results show that orientation of [0001] and ⟨10 0⟩ axes in single crystals with respect to ECAP die geometry significantly influences activation of various deformation modes and resulting texture evolution. Depending on initial orientation, {10 2} twinning represents an important deformation mechanism at the early stages of forming because it causes reorientation of the matrix to orientations suitable for slip activity. Simulations indicate that, in addition to ⟨a⟩ basal slip, there is an activity of ⟨c + a⟩ pyramidal slip in all crystals. The ⟨a⟩ non-basal slip systems (⟨a⟩ prismatic and ⟨a⟩ pyramidal) were active in crystals containing structure components with [0001] axis parallel to the transverse direction of the ECAP die. Occurrence of this component in resulting texture caused better deformation behavior without crack formation.

Planar Defects on (112) in BCC Crystals

The parameters of exponential many-body Finnis-Sinclair potentials corresponding to qualitatively different crystal lattice stability were selected and their behaviour was studied. Furthermore, a model with pairwise Lennard-Jones potential was also considered. The attention was paid to the stability of different crystal structures and the properties of simple interfaces such as stacking faults and twin boundaries were investigated.