Structural instabilities and mechanical properties of U$_2$Mo from first principles calculations

TL;DR

This study uses first principles calculations to analyze the structural stability, elastic properties, phonon dispersion, and ideal strengths of U$_2$Mo, revealing its mechanical instability and shear-dominated failure mode.

Contribution

It provides the first comprehensive first principles analysis of U$_2$Mo's structural and mechanical properties, correcting previous assumptions about its stability.

Findings

U$_2$Mo's $I4/mmm$ structure is mechanically and dynamically unstable.

The ideal shear strength (~8.1 GPa) is much lower than tensile strength (18-28 GPa).

U$_2$Mo is more likely to fail by shear than by tension.

Abstract

We perform detailed first principles calculations of the structural parameters at zero pressure and high pressure, the elastic properties, phonon dispersion relation, and ideal strengths of U$_2$Mo with $C11_b$ structure. In contrast to previous theoretical studies, we show that this $I4/mmm$ structure is indeed a mechanically and dynamically unstable phase, which is confirmed by the negative elastic constant $C_{66}$ as well as the imaginary phonon modes observed along the $\Sigma_1$-N-P line. The calculations of ideal strengths for U$_2$Mo are performed along [100], [001], and [110] directions for tension and on (001)[010] and (010)[100] slip systems for shear load. The ideal shear strength is about 8.1 GPa, much smaller than tension of 18-28 GPa, which indicates that the ductile U$_2$Mo alloy will fail by shear rather than by tension.