Abstract: The grain refinement mechanism for rapid solidification of undercooled melts is still an open problem even after 60 years of on-going studies. In this work, rapid solidification of undercooled Ni and equi-atomic FeCoNiPd melts was studied and spontaneous grain refinement was found at both low and high undercooling. After a detailed electron backscattered diffraction analysis, subgrain-induced grain orientation scattering and splitting were found to occur along with the transition from coarse dendrites to fine equiaxed grains at low and high undercooling, respectively, indicating that subgrains play an important role during the formation of fine equiaxed grains. On this basis, a compromise mechanism of subgrain-assisted spontaneous grain refinement was proposed. Because the dendrite re-melting induced thermo-mechanical process and fluid flow induced dendrite deformation occur simultaneously during the post-recalescence stage, stress accumulation would be maximum at both low and high undercooling, thus inducing dynamic recrystallization, during which the formation and rotation of subgrains make the grain orientations scattering and even splitting. Furthermore, the grain/subgrain size of undercooled FeCoNiPd ascribing to its unique co-segregation behavior keeps almost invariable from low to high undercooling, indicating that the co-segregation strategy would be effective to inhibit grain growth after rapid solidification and would be useful in practice.

Key words: Rapid solidification, Undercooling, Spontaneous grain refinement, Grain growth, High entropy alloy