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

Abstract

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 (ω) transformation occurs in such systems is hardly expected due to the high configurational entropy and sluggish diffusion. Here, we confirmed unusual ω and ω-like transformations occurred in a configurational entropy stabilized refractory bcc TiZrNbTa model HEA facilitating the formation of colony hierarchical microstructures. These chemically ordered ω and polytyped ω-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 ω superstructure, and a certain strain induced collective displacement of the (0001) atomic-layers along one of the <211¯0>ω directions can transition the traditional orthogonal ω to the non-orthogonal polytyped ω-like superstructures. Two ordered ω and/or ω-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 ω and polytyped ω-like superstructures were fully revealed by using aberration-corrected scanning transmission electron microscopy combined with first-principles calculations. These atomistic insights into the ω and polytyped ω-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.