Microstructural transitions of metastable phase in undercooled Fe–7.5 at% Ni alloy

Microstructure characterization and transition of undercooled Fe–7.5 at% Ni melt were investigated, adopting melt fluxing and cyclical overheating. For the samples subjected to low undercoolings, single-phase microstructure results, whereas many second-phase particles, which are deemed as the so-called dendrite core originating from the primary solidification of the metastable δ phase, were observed in the as-solidified samples after high undercoolings. Applying the classical nucleation theory, the competitive nucleation between δ and γ phases was studied, thus giving calculations consistent with the observed experiment results. In order to understand the forming mechanism of the dendrite core, a schematic diagram incorporating solidification and solid-state transformation was given. On this basis the microstructural transition of the metastable phase was declared.