Nonlinear deformation and elasticity of BCC refractory metals and alloys

TL;DR

This paper investigates the nonlinear elastic behavior and structural instabilities of BCC refractory metals and alloys under strain, revealing discontinuous phase transitions driven by electronic and lattice changes.

Contribution

It provides a detailed analysis of strain-induced instabilities in refractory metals, including eigenvalue classification and electronic structure insights, advancing understanding of their nonlinear elasticity.

Findings

Identification of discontinuous lattice instabilities

Eigenvalue analysis of Wallace tensor reveals instability modes

Electronic structure changes drive structural transformations

Abstract

Application of isotropic pressure or uniaxial strain alters the elastic properties of materials; sufficiently large strains can drive structural transformations. Linear elasticity describes stability against infinitesimal strains, while nonlinear elasticity describes the response to finite deformations. It was previously shown that uniaxial strain along [100] drives refractory metals and alloys towards mechanical instabilities. These include an extensional instability, and a symmetry-breaking orthorhombic distortion caused by a Jahn-Teller-Peierls instability that splays the cubic lattice vectors. Here, we analyze these transitions in depth. Eigenvalues and eigenvectors of the Wallace tensor identify and classify linear instabilities in the presence of strain. We show that both instabilities are discontinuous, leading to discrete jumps in the lattice parameters. We provide physical intuition for the instabilities by analyzing the changes in first principles energy, stress, bond lengths and angles upon application of strain. Electronic band structure calculations show differential occupation of bonding and anti-bonding orbitals, driven by the changing bond lengths and leading to the structural transformations. Strain thresholds for these instabilities depend on the valence electron count.