Designing a thermodynamically stable and intrinsically ductile refractory alloy

TL;DR

This study uses density functional theory to identify alloy compositions with low unstable stacking fault energy and high surface energy, leading to intrinsically ductile and thermodynamically stable refractory alloys.

Contribution

It demonstrates that positive enthalpy interactions in BCC alloys reduce $ ext{γ}_{usfe}$, enhancing intrinsic ductility, which challenges traditional reliance on elements like Re.

Findings

Positive enthalpy reduces $ ext{γ}_{usfe}$ and improves ductility.

Equiatomic alloys show maximum enthalpy, indicating strong repulsive interactions.

Alloys with repulsive interactions are intrinsically ductile.

Abstract

Developing ductile refractory BCC alloys has remained a challenge. The intrinsic ductility (D) of an alloy is the ratio of surface energy ($\gamma_s$) and unstable stacking fault energy ($\gamma_{usfe}$). Lowering the valence electron concentration has been shown to improve the intrinsic ductility of refractory alloys. However, Re has been widely used to ductilize W, contrary to the low valency criteria suggested in the literature. Here we use density functional theory to calculate the enthalpy of formation, $\gamma_{usfe}$ and $\gamma_s$ of Group IV, V, VI elements and their 25 equiatomic binary alloys in BCC crystal structure. We found that positive enthalpy leads to a considerable reduction in $\gamma_{usfe}$ compared to composition averaged value, resulting in improved intrinsic ductility. Enthalpy is maximum at the equiatomic concentrations indicating the highly repulsive interaction between the alloy constituents and vicer-versa. We found that the repulsive interaction between the alloy constituents leads to a reduction in $\gamma_{usfe}$, making alloys intrinsically ductile.