Metastable phase and nanotwin's formation in undercooled equiatomic solid solution medium entropy alloys

The metastability of undercooled melts plays a critical role in driving non-equilibrium solidification in equiatomic solid solution medium-entropy alloys (MEAs), yet remains largely unexplored. In this study, we uncover dual recalescence phenomena in undercooled CrFeNi, revealing the formation of both a metastable tetragonal phase and a stable FCC phase. While it is established that CrFeNi forms a single-phase FCC structure above 1273K and a dual-phase FCC/BCC structure at lower temperatures, our work provides the first evidence of a complex tetragonal phase emerging during solidification. This metastable phase, characterized by a high density of dislocations, stacking faults, nanotwins, and nanoscale structures, was subjected to cyclic nanoindentation testing, providing insights into its mechanical properties. These findings offer a deeper understanding of the metastable phases and structural transformations in single-phase FCC equiatomic MEAs, shedding light on the mechanisms underpinning the development of nanotwinned structures in undercooled conditions.