On the Thermodynamics and Phase Transformation Pathways in BCC-B2 Refractory Compositionally Complex Superalloys

Refractory-based superalloys (RSAs) containing a disordered body-centered cubic (BCC) plus ordered B2 phase two-phase microstructure offer potential for future high-temperature applications, but several issues currently hinder their further development. Among them, the observed inverted “BCC precipitates in B2 matrix” and co-continuous spinodal decomposition-like microstructures are a result of concurrent long-range atomic ordering and phase separation involving the thermodynamically higher-order BCC-B2 phase transition. The nature of the BCC-B2 phase transition, the development of a BCC + B2 field in phase diagrams, and the possible phase transformation pathways from BCC → BCC + B2 are not well understood in multicomponent systems, leading to misinterpretation of experimental observations, hindering the scientific understanding required to control such transformations and design new materials. The present work details the underlying thermodynamics governing the BCC-B2 phase transition and various transformation pathways available for the BCC → BCC + B2 transformation. It is shown that BCC-B2 microstructure evolution in RSAs parallels that in binary alloys such as Fe–Al in many ways, and proper physics-based models can enable computational thermodynamics-guided discovery of new compositions where “B2 precipitates in BCC matrix” microstructures are possible. The previously reported RSAs are explained using a modeling framework constructed by combining modern computational thermodynamics with classic phase transformations theory, and new alloys are predicted for previously uninvestigated systems. The compositional complexity offered by RSAs is also found to create more opportunity for developing BCC + B2 microstructures compared to binary alloys. The ideas and discussion herein offer insight into the thermodynamics of microstructure development in RSAs and provide tools and guidance for future research in this promising class of materials.