A New Type of Compositionally Complex M5Si3 Silicides: Cation Ordering and Unexpected Phase Stability

TL;DR

This paper reports the synthesis and characterization of a new class of compositionally complex M5Si3 silicides with cation ordering and unexpected phase stability, expanding high-entropy ceramic materials.

Contribution

It introduces a novel type of compositionally complex silicides with cation ordering and demonstrates their stable hexagonal phase at high temperatures.

Findings

Discovery of a new hexagonal gamma phase in VCrNbTaW5Si3

Cation ordering reduces configurational entropy in these silicides

Potential for combining with high-entropy alloys for advanced composites

Abstract

A new type of compositionally complex (medium- or high-entropy) M5Si3 silicides is synthesized. Both (V1/5Cr1/5Nb1/5Ta1/5W1/5)5Si3 and (Ti1/5Zr1/5Nb1/5Mo1/5Hf1/5)5Si3 form single-phase homogenous solid solutions. Notably, (V1/5Cr1/5Nb1/5Ta1/5W1/5)5Si3 forms the hexagonal gamma (D88) phase, while all its five constituent binary silicides, V5Si3, Cr5Si3, Nb5Si3, Ta5Si3, and W5Si3, are stable in the tetragonal alpha (D8l) or beta (D8m) phases. Annealing at 1600C demonstrates that this hexagonal gamma phase is stable. Comparison of the experimental and calculated X-ray diffraction patterns, Rietveld refinements, and analysis of aberration-corrected scanning transmission electron microscopy high-angle annular dark-field images suggest cation ordering, which reduces the configurational entropy. This work expands the field of high-entropy and compositional complex ceramics by not only discovering a new compositional complex silicide phase but also demonstrating the cation ordering and unusual phase stability. These compositionally complex silicides can be combined with refractory high-entropy alloys to make the high-entropy counterparts to the Nb-silicide and Mo-Si-B composites.