Microstructural Transitions in Commercial Fe-Ni–Based Soft-Magnetic Alloys Quenched from Undercooled Liquid Droplets

摘要

Liquid droplets of three commercial Fe-Ni–based alloys were undercooled to different temperatures by electromagnetic levitation and quenched. Discontinuous changes in the grain size and microstructure as a function of melt undercooling (ΔT) were observed in all three alloys. At ΔT < 80 K, fine, equiaxed grains with a substructure consisting of spherical elements were observed. In the range 80 K < ΔT < 155 K, the microstructure consisted of colonies of columnar dendrites. The colonies were two orders of magnitude larger than the equiaxed grains reported at lower ΔT. At ΔT > 155 K, the equiaxed grains with a substructure of spherical elements returned. The critical undercoolings at which the microstructural transitions occur (ΔT * 1 = 80 K and ΔT * 2 = 155 K) show good agreement with those for similar binary systems. The results were successfully described by calculating the time needed for breakup of the primary dendrites as a function of ΔT and comparing this to the time during which the sample temperature is above the solidus due to recalescence.