Thermodynamics and Phase Transformations in BCC-B2 Refractory Superalloys

Since being reported by Senkov et al. in 2014, two-phase, body-centered cubic (BCC) plus ordered B2, refractory-based compositionally complex alloys (R-CCAs), also known as high-entropy alloys, have received significant research attention as potential next generation materials for extreme environments beyond Ni-based superalloys. These recent efforts are predated by a comprehensive investigation by Naka and Khan in the late 1990s searching for refractory alloy systems strengthened by intermetallic phases, or so-called “refractory superalloys” (RSAs). In their study, Naka and Khan identified the AlMoNbTaTiZr system as containing a two-phase BCC+B2 microstructure, coincidentally the same system first reported by Senkov et al. Of the outstanding issues currently facing BCC+B2 RSAs, microstructural evolution and stability is one of the more prominent. Most often, an inverted “BCC-precipitate in B2 matrix” microstructure manifests in these materials, while other times a spinodal-like microstructure is observed. This is a result of the higher-order thermodynamic phase transformation between BCC and B2, which had been largely overlooked until recently. The present work details the underlying thermodynamics governing the BCC-B2 phase transition in multicomponent alloys, specifically RSAs, and the multitude of pathways available for the BCC→BCC+B2 transformation. Microstructural evolution in BCC-B2 RSAs parallels that in binary alloys such as Fe-Al, but the added compositional complexity opens new transformation pathways and may improve B2 phase stability. Previously reported RSAs are discussed in the context of concurrent ordering and phase separation involving higher order phase transitions in multicomponent alloys, including those utilizing both the TiAl- and RuX-based (where X is Ti, Zr, or Hf) B2 phases. 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.