Bidirectional Phase Transformations in Multi-Principal Element Alloys: Mechanisms, Physics, and Mechanical Property Implications.

The emergence of multi-principal element alloys (MPEAs) heralds a transformative shift in the design of high-performance alloys. Their ingrained chemical complexities endow them with exceptional mechanical and functional properties, along with unparalleled microscopic plastic mechanisms, sparking widespread research interest within and beyond the metallurgy community. In this overview, a unique yet prevalent mechanistic process in the renowned FeMnCoCrNi-based MPEAs is focused on: the dynamic bidirectional phase transformation involving the forward transformation from a face-centered-cubic (FCC) matrix into a hexagonal-close-packed (HCP) phase and the reverse HCP-to-FCC transformation. The light is shed on the fundamental physical mechanisms and atomistic pathways of this intriguing dual-phase transformation. The paramount material parameter of intrinsic negative stacking fault energy in MPEAs and the crucial external factors c, furnishing thermodynamic, and kinetic impetus to trigger bidirectional transformation-induced plasticity (B-TRIP) mechanisms， are thorougly devled into. Furthermore, the profound significance of the distinct B-TRIP behavior in shaping mechanical properties and creating specialized microstructures c to harness superior material characteristics is underscored. Additionally, critical insights are offered into key challenges and future striving directions for comprehensively advancing the B-TRIP mechanism and the mechanistic design of next-generation high-performing MPEAs.