Polytypic omega/omega-like transformation in a refractory high-entropy alloy

Understanding the atomistic details on microstructures and phase transformation mechanisms is essential to tailor the performances of materials. Refractory body-centered cubic (bcc) high-entropy alloys (HEAs) are considered as promising candidates for future applications in the high-temperature, superelasticity, and superconductivity fields. The classical omega (omega) transformation occurs in such systems is hardly expected due to the high configurational entropy and sluggish diffusion. Here, we confirmed unusual omega and omega-like transformations occurred in a configurational entropy stabilized refractory bcc TiZrNbTa model HEA facilitating the formation of colony hierarchical microstructures. These chemically ordered w and poly-typed omega-like superstructures nucleate from one of two phase-separated bcc products. Self-adapted atomic shuffling can convert the simple bcc lattice to the non-close-packed hexagonal omega superstructure, and a certain strain induced collective displacement of the (0 0 01) atomic-layers along one of the < 2 1 over bar1 0 > omega directions can transition the traditional orthogonal omega to the non-orthogonal polytyped w-like superstructures. Two ordered omega and/or omega-like superstructures with Ti, Zr, Nb, and Ta atoms occupied specific sites have thus been confirmed. Moreover, the chemistry, orientations, and interface relationships of the omega and polytyped omega-like superstructures were fully revealed by using aberration-corrected scanning transmission electron microscopy combined with first-principles calculations. These atomistic insights into the w and polytyped omega-like superstructures transformations would provide theoretical guidance for the design of advanced refractory HEAs, especially for designing alloys with specific properties based on the control of transformed microstructures and interfacial modification. (C) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

Understanding the atomistic details of phase transformations in high-entropy alloys is crucial for tailoring materials with desired properties. This study confirmed the occurrence of unusual omega and omega-like transformations in a refractory high-entropy alloy, leading to the formation of hierarchical microstructures. The atomic-level mechanisms of these transformations were revealed using advanced microscopy techniques and first-principles calculations. The insights gained from this study provide theoretical guidance for designing advanced high-entropy alloys with specific properties.