Microstructure evolution in undercooled Fe-7.5 at% Ni alloys

The microstructure evolution of undercooled Fe-7.5 at% Ni melt was investigated by fluxing and cyclic superheating. Within the undercooling, ΔT, range achieved, two kinds of grain refinement and one grain coarsening occur and different as-solidified morphologies result. The first grain refinement, occurring if ΔT<56 K, can be ascribed to the breakup of dendrite-skeleton owing to remelting, whereas the second grain refinement was caused by recrystallization induced by the stress originating from the rapid solidification contraction with ΔT>153 K. With further increase of ΔT beyond 287 K, however, grain coarsening occurs as a result of the formation of metastable phase δ and the solid-state grain coalescence.