Undercooled Fe75Ni25 melts were phase-seeded by a high purity iron.It was found that above a critical undercooling,△Te=135 K,a metastable δ phase (b.c.c) solidifies from the ironseeded 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 assolidified 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 △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.