Catalytic nucleation of a meta-stable δ phase in phase-seeded undercooled Fe₇₅Ni₂₅ melts

Undercooled Fe₇₅Ni₂₅ melts were phase-seeded by a high purity iron. It was found that above a critical undercooling, ΔT_c=135 K, a metastable δ phase (b.c.c) solidifies from the iron-seeded melts instead of a thermodynamically stable γ phase (f.c.c). While undercooling of the melt (ΔT) is below ΔT_c, solidification of the γ phase prevails in the iron-seeded melts. For the undercooled melts subjected to spontaneous nucleation, the γ phase always solidifies. After solidification, the as-solidified γ phase transforms completely to martensite; whereas the as-solidified metastable δ phase partially transforms to the γ phase, and then to martensite. The untransformed δ phase retains as α-ferrite particles in the microstructures. Based on the classical nucleation theory, catalytic nucleation of the metastable δ phase in the phase-seeded undercooled Fe₇₅Ni₂₅ melts was analyzed. It was quantitatively demonstrated that when ΔT>ΔT_c, the formation of the δ phase can be ascribed to a better catalytic effect of the iron on its nucleation than that on the nucleation of the γ phase.