Inverse grain-size effect on the formation of nano-lamellar structures and FCC-to-HCP phase transformation in FeCoCrNiMo0.2 high-entropy alloy at cryogenic temperature

In conventional alloys, deformation twins and phase transformations preferentially nucleate within coarse grains. In contrast, deformation-induced nano-twinning and phase transformation in the present work exhibit an inverse grain-size dependence. Notably, the moderately fine-grained CRA50 alloy undergoes FCC-to-HCP phase transformation under tensile loading at 77 K. The resulting transformation-induced plasticity (TRIP) markedly enhances the strain-hardening ability of the alloy. The finest-grained CRA80 alloy develops a high density of nano-lamellar substructures, which differ from conventional discrete deformation nanotwins and isolated stacking faults (SFs). These unique substructures consist of densely packed and orderly arranged deformation nanotwins or SFs. Such nano-lamellar structures trigger the dynamic Hall-Petch effect and improve the strength-ductility synergy together with the strain-hardening ability. Neither FCC-to-HCP transformation nor nano-twin lamellar substructures are observed in the coarse-grained CRA30 alloy. Furthermore, no FCC-to-HCP transformation is detected in CRA80. This finding elucidates a competitive relationship between FCC-to-HCP transformation and the formation of nano-lamellar substructures at fine grain sizes, where twin activation proceeds more readily than phase transformation. The present results provide useful insights into the cryogenic tensile deformation mechanisms of FCC-structured high-entropy alloys with different grain sizes.