Vacancy-mediated complex phase selection in high entropy alloys

TL;DR

This study investigates how vacancies influence phase selection in high entropy alloys, combining experimental and computational methods to reveal vacancy ordering's role in stabilizing specific phases.

Contribution

It uncovers the role of vacancy ordering and vacancy-atom SRO in phase selection, especially in stabilizing a $ ext{bcc}$ superstructure in TiZrHfAl alloys, advancing understanding of defect-driven phase stability.

Findings

Vacancy ordering mediates $ ext{γ}$-brass phase stabilization.

Increased Al stabilizes $ ext{bcc}$ structure.

Vacancy-atom SRO suppresses atomic SRO.

Abstract

Phase selection in Ti-Zr-Hf-Al high-entropy alloys was investigated by in-situ high-energy X$-$ray diffraction, single-crystal X$-$ray diffraction, and density-functional theory based electronic-structure methods that address disorder and vacancies, predicting formation enthalpy and chemical short-range order (SRO). Samples with varying Al content were synthesized, characterized, and computationally assessed to ascertain the composition-dependent phase selection, as increased Al content often acts as a stabilizer of a body-centered-cubic structure. Equiatomic TiZrHfAl was especially interesting due to its observed bcc superstructure - a variant of $\gamma-$brass with 4 vacancies per cell (not 2 as in $\gamma-$brass). We highlight how vacancy ordering mediates selection of this variant of $\gamma-$brass, which is driven by vacancy-atom SRO that dramatically suppress all atomic SRO. As vacancies are inherent in processing refractory systems, we expect that similar discoveries await in other high entropy alloys or in revisiting older experimental data.