Shear-activated phase transformations of diborides via machine-learning potential molecular dynamics

Abstract

The layered character of transition metal diborides (TMB₂:s)—with three structure polymorphs representing different stackings of the metallic sublattice—evokes the possibility of activating phase-transformation plasticity via mechanical shear strain. This is critical to overcome the most severe limitation of TMB₂:s: their brittleness. To understand finite-temperature mechanical response of the $\alpha$, $\omega$, and $\gamma$ polymorphs at the atomic scale, we train machine-learning interatomic potentials (MLIPs) for TMB${}_2$:s, TM $=$ (Ti, Ta, W, Re). Validation against ab initio data set supports the MLIPs’ capability to predict structural and elastic properties, as well as shear-induced slipping and phase transformations. Nanoscale molecular dynamics simulations ($>10^4$ atoms; $\approx 5^3{\mathrm{nm}}^3$) allow evaluating theoretical shear strengths attainable in single-crystal TMB${}_2$:s and their temperature evolution from 300 up to 1200 K. Quantitative structural analysis via angular and bond-order Steinhardt parameter descriptors shows that $\left(0001\right)\left[\overline{1}2\overline{1}0\right]$ and $\left(0001\right)\left[10\overline{1}0\right]$ shearing activates transformations between the (energetically) metastable and the preferred phase of TiB${}_2$, TaB${}_2$, and WB${}_2$. These transformations can be promoted by additional tensile or compressive strain along the [0001] axis. The preferred phase of ReB${}_2$ shows negative thermal expansion and an unprecedented shear-induced plasticity mechanism: metallic/boron layer interpenetration and uniform lattice rotation.