Effects of Y solutes on the activation of the {112̄1}〈1̄1̄26〉 deformation twinning in MgY solid solution alloys

Abstract

The $\left\{11\overline{2}1\right\}〈\overline{1}\overline{1}26〉$ deformation twin is common in nominally pure HCP Ti, Zr, Re, Co, but not in Mg. Rare-earth (RE) solutes such as Y and Gd at low atomic concentrations, however, can activate this twin mode and make it competitive to the regular $\left\{10\overline{1}2\right\}〈\overline{1}011〉$ extension twin in Mg. Previously models have not explicitly revealed the role of RE solutes on twinning and the RE-effects remain poorly understood. Using a reduced-constraint (RC) slip path, DFT calculations and two newly-developed interatomic potentials for MgY and MgAl, we reveal the physical origin of the RE solute effects in activating the $\left\{11\overline{2}1\right\}〈\overline{1}\overline{1}26〉$ twin in MgY alloys with random and short range order solute states. The RC-path shows that localized slip in the $〈\overline{1}\overline{1}26〉$ twinning direction simultaneously drives shear and $〈1\overline{1}00〉$-direction shuffle displacements on the corrugated $\left\{11\overline{2}1\right\}$ twinning plane. The shuffle displacements are essential to reverse the asymmetry of $\left\{1\overline{1}00\right\}$ atom-planes to satisfy the twin symmetry. However, such shuffle displacements are insufficient in pure Mg or MgAl, leaving twin nucleation to the multi-layer slip path with high energy barriers. Y-solutes are able to enhance the slip-driven shuffling to levels sufficient to reverse the $\left\{1\overline{1}00\right\}$ atom-planes asymmetry, leading to spontaneous nucleation of multi-layer twin embryo with reduced energy barriers lower than that for the competing $\left\{10\overline{1}2\right\}〈\overline{1}011〉$ extension twin in Mg. The RC-slip path and $\left\{11\overline{2}1\right\}〈\overline{1}\overline{1}26〉$ twin nucleation is directly demonstrated in atomistic simulations of a blunted cracks under mode-II loading. This slip driven, solute-modulated twin nucleation mechanism is expected to be general across all HCP metals and alloys.