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Undercooled Fe75Ni25 melts were phase-seeded by a high purity iron. It was found that above a critical undercooling, △Tc 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 (△T) is below △Tc, 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 a-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 △T>△Tc, 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.
Undercoolled Fe75Ni25 melts were phase-seeded by a high purity iron. It was found that above a critical undercooling, ΔTc 135 K, a metastable δ phase (bcc) solidifies from the iron-seeded melts instead of a thermodynamically stable γ phase (fcc While undercooling of the melt (ΔT) is below ΔTc, solidification of the γphase prevails in the iron-seeded melts. For the undercoolled melts subjected to spontaneous nucleation, the γ phase always solidifies. After solidification, the as- solidified γ phase transforms completely to martensite; and the as-solidified metastable δ phase partially transforms to the γ phase, and then to martensite. The Untransformed δ phase retains as a-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 when when ΔT> ΔTc, the formation of the δ phase can be ascribed t o a better catalytic effect of the iron on its nucleation than that on the nucleation of the γ phase.