The nucleation and migration mechanisms of asymmetric {112‾3} twin boundary in hexagonal close-packed titanium

Abstract

The $\left\{11\bar{2}3\right\}$ twin, with a large twinning strain (1.89), hardly nucleated in hexagonal close-packed (HCP) titanium. This poses challenges for experimental detection, resulting in limited studies. Consequently, the twin boundary (TB) structure, nucleation, and migration mechanisms of this twin remain poorly understood. In this study, we use atomistic simulations and electron backscatter diffraction (EBSD) to elucidate these mechanisms. Contrary to classical theory, our results show that the $\left\{11\bar{2}3\right\}$ TB with mirror symmetry deteriorates during relaxation and becomes asymmetric. This breakdown is attributed to the intense repulsive interactions between the short-bonded atom pairs located at mirrored positions, with a distance smaller than half of the lattice parameter a. During TB migration, a well-defined single-layer height twinning dislocation b₁ with Burgers vectors of $1/\left(4{\Lambda }^2+6\right)\left[11\bar{2}\bar{2}\right]\pm 1/12\left[1\bar{1}00\right]$ was identified, which differs from the twinning dislocation b₂ predicted by classical theory. Meanwhile, an inverse shuffling displacement occurs along the $\left[1\bar{1}00\right]$ direction for the double-layered prismatic $\left(1\bar{1}00\right)$ planes. Notably, our research indicates that the nucleation of individual $\left\{11\bar{2}3\right\}$ twin within HCP structure is inherently challenging. Nevertheless, $\left\{11\bar{2}3\right\}$ TB can nucleate via the interactions among TBs, specifically between the $\left\{11\bar{2}1\right\}$ and $\left\{11\bar{2}2\right\}$ TB. These insights advance our understanding of the plastic deformation inherent in titanium alloys.