A new type of compositionally complex M5Si3 silicides: Cation ordering and unexpected phase stability

A new type of compositionally complex (medium- or high-entropy) M5Si3 silicides is synthesized. Both (V1/5Cr1/5Nb1/5Ta1/5W1/5)(5)Si-3 and (Ti1/5Zr1/5Nb1/5Mo1/5Hf1/5)(5)Si-3 form single-phase homogenous solid solutions. Notably, (V1/5Cr1/5Nb1/5Ta1/5W1/5)(5)Si-3 forms the hexagonal gamma (D8(8)) phase, while all its five constituent binary silicides, V5Si3, Cr5Si3, Nb5Si3, Ta5Si3, and W5Si3, are stable in the tetragonal alpha (D8(l)) or beta (D8(m)) phases. Annealing at 1600 degrees C 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 compositionally complex silicide phase but also demonstrating 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. (C) 2022 The Author(s). Published by Elsevier Ltd on behalf of Acta Materialia Inc.

Automated summary

A new type of compositionally complex silicides has been synthesized, demonstrating stable hexagonal gamma phase and cation ordering. This work expands the field of high entropy ceramics and shows potential for high-entropy counterparts to Nb-silicide and Mo-Si-B composites.