Metallurgy, superconductivity, and hardness of a new high-entropy alloy superconductor Ti-Hf-Nb-Ta-Re

TL;DR

This study discovers new Ti-Hf-Nb-Ta-Re high-entropy alloy superconductors with phase segregation, analyzes their superconducting properties, and finds a systematic decrease in critical temperature with increasing hardness and an asymmetric VEC dependence.

Contribution

It reports the first exploration of VEC range 4.6-5.0 in bcc HEA superconductors, revealing phase segregation and the relationship between hardness, VEC, and superconducting temperature.

Findings

Superconducting T_c ranges from 3.25 K to 4.38 K.

Phase segregation into two bcc phases occurs with increasing VEC.

T_c decreases systematically with increasing hardness beyond 350 HV.

Abstract

We explored quinary body-centered cubic (bcc) high-entropy alloy (HEA) superconductors with valence electron concentrations (VECs) ranging from 4.6 to 5.0, a domain that has received limited attention in prior research. Our search has led to the discovery of new bcc Ti-Hf-Nb-Ta-Re superconducting alloys, which exhibit an interesting phenomenon of phase segregation into two bcc phases with slightly different chemical compositions, as the VEC increases. The enthalpy of the formation of each binary compound explains the phase segregation. All the alloys investigated were categorized as type-II superconductors, with superconducting critical temperatures ($T_\mathrm{c}$) ranging from 3.25 K to 4.38 K. We measured the Vickers microhardness, which positively correlated with the Debye temperature, and compared it with the hardness values of other bcc HEA superconductors. Our results indicate that $T_\mathrm{c}$ systematically decreases with an increase in hardness beyond a threshold of approximately 350 HV. Additionally, we plotted $T_\mathrm{c}$ vs. VEC for representative quinary bcc HEAs. The plot revealed the asymmetric VEC dependence. The correlation between the hardness and $T_\mathrm{c}$, as well as the asymmetric dependence of $T_\mathrm{c}$ on VEC can be attributed to the simultaneous effects of the electronic density of states at the Fermi level and electron-phonon coupling under the uncertainty principle, especially in the higher VEC region.