Isoelectronic Substitutions and Aluminium Alloying in the Ta-Nb-Hf-Zr-Ti High-Entropy Alloy Superconductor

TL;DR

This study investigates how isoelectronic substitutions and aluminium alloying affect the superconducting properties of Ta-Nb-Hf-Zr-Ti high-entropy alloys, revealing that elemental composition significantly influences the superconducting transition temperature.

Contribution

It provides new insights into the impact of specific elemental substitutions and aluminium alloying on the superconductivity of high-entropy alloys, highlighting the importance of composition over electron count.

Findings

Isoelectronic replacements can lower Tc by over 60%.

Aluminium alloying alters the crystal structure and suppresses superconductivity at high concentrations.

Superconductivity is sensitive to elemental make-up and structural stability.

Abstract

High-entropy alloys (HEAs) are a new class of materials constructed from multiple principal elements statistically arranged on simple crystallographic lattices. Due to the large amount of disorder present, they are excellent model systems for investigating the properties of materials intermediate between crystalline and amorphous states. Here we report the effects of systematic isoelectronic replacements, using Mo-Y, Mo-Sc, and Cr-Sc mixtures, for the valence electron count 4 and 5 elements in the BCC Ta-Nb-Zr-Hf-Ti high entropy alloy (HEA) superconductor. We find that the superconducting transition temperature Tc strongly depends on the elemental make-up of the alloy, and not exclusively its electron count. The replacement of niobium or tantalum by an isoelectronic mixture lowers the transition temperature by more than 60 %, while the isoelectronic replacement of hafnium, zirconium, or titanium has a limited impact on Tc. We further explore the alloying of aluminium into the nearly optimal electron count [TaNb]0.67(ZrHfTi)0.33 HEA superconductor. The electron count dependence of the superconducting Tc for (HEA)Alx is found to be more crystalline-like than for the [TaNb]1-x(ZrHfTi)x HEA solid solution. For an aluminum content of x = 0.4 the high-entropy stabilization of the simple BCC lattice breaks down. This material crystallizes in the tetragonal beta-uranium structure type and superconductivity is not observed above 1.8 K.