The solidification thermal history of gas-atomized droplets plays a decisive role in their solidification microstructure. In this work, the size-dependent microstructure and solidification thermal history of gas-atomized droplets of high-carbon martensitic stainless steel M390 are investigated experimentally and theoretically. Three primary solidification microstructures, including dendrites, equiaxed cellular grains, and flower-like dendritic fragments, are formed within the powder as a function of particle size. A transformation from dendrites to a mixed structure, and ultimately to predominantly flower-like fragments, occurs during the quasi-isothermal stage via thermally induced dendrite fragmentation. This mechanism necessitates adequate recalescence temperature and duration. Larger particles exhibit minimal δ-Fe, while smaller particles contain more, attributable to their higher cooling rates and undercooling. As the powder size increases, the extent of thermally induced dendritic fragmentation, spheroidization, and coarsening recorded in the microstructure becomes more pronounced, which is a key factor contributing to the increase in grain size and number. Furthermore, a mathematical model for the solidification of individual spherical droplets during gas atomization is proposed based on the application of Newton's heat transfer equation and classical nucleation theory. The thermal-physical conditions governing the microstructural transformation of M390 alloy powder during rapid solidification are discussed and predicted by the comparison of results of mathematical modeling and microstructural observation.
Liu et al. (Sun,) studied this question.