Nucleation and growth of {101¯1} compression twin in Mg single crystals

Abstract

The $\left\{10\overline{1}1\right\}$ compression twin is more commonly observed under a-axis tension rather than c-axis compression in hexagonal close-packed (hcp) Mg single crystals both in experiments and atomistic simulations. In this work, molecular dynamics simulations are applied to investigate the nucleation and growth of $\left\{10\overline{1}1\right\}$ twin in Mg single crystal, which is crucial for a comprehensive understanding of the plasticity of hcp metals. A $\left\{10\overline{1}1\right\}$ twin embryo with 4-layer height is found to be naturally nucleated at the free surface via 〈a〉 dislocation slip in Mg single crystal. The nucleation of the $\left\{10\overline{1}1\right\}$ twin embryo involves the cross-slip of the nucleated prismatic 〈a〉 dislocation onto pyramidal $\left\{10\overline{1}1\right\}$ plane, which is achieved by slip of 〈a〉 dislocation and shear along the $〈10\overline{1}2〉$ direction. Basal stacking faults (SFs) forms in twin embryo and grows with the migration of $\left\{10\overline{1}1\right\}$ twin boundary. The formation of basal SFs in $\left\{10\overline{1}1\right\}$ twin embryo can greatly reduce the required atomic shuffling in $〈1\overline{2}10〉$ direction, thus facilitate the development of a $\left\{10\overline{1}1\right\}$ twin embryo. The basal SFs inside $\left\{10\overline{1}1\right\}$ twin are anomalous SFs without dislocation activities involved, thus the $\left\{10\overline{1}1\right\}$ twinning shear is not influenced by the existence of basal SFs, which is consistent with the irrational shear of 2-layer twinning dislocation.