Development of solidification microstructures in atomized Fe-Ni alloy droplets

Inert-gas atomization, using nitrogen as the atomizing medium, has been used to process a series of Fe-Ni alloys. The weight distributions of the powders were bi-modal with relative maxima in the 38 and 60–90 μm ranges. Droplet cooling rates were calculated and the final solidification microstructures were investigated by optical microscopy, X-ray diffraction and transmission electron microscopy (TEM). The smallest droplets were predominantly dendritic with the largest droplets being a mixture of dendritic and grain-refined. For additions of 2 at.% Ni, a number of droplets were found to show solid-state transformations, having a characteristic martensitic microstructure. The primary phase to nucleate was the equilibrium b.c.c. δ-Fe phase and the transformation path was determined to be δ-Fe->-γ-Fe,Ni→α-Fe. TEM examinations of the transformed microstructure showed an interlocked structure with a number of different orientations. In these particles, some duplex b.c.c.-c.c.p. structures were also present. For higher Ni contents, the metastable b.c.c. δ-Fe phase nucleated in preference to the equilibrium γ-Fe,Ni phase.