Pressure-dependent microstructure evolution of Fe-based amorphous alloy powders via high-pressure gas atomization

The pressure-dependent microstructure evolution of the gas-atomized Fe-based amorphous alloy powders at gas pressures in the range from 5 MPa to 8 MPa has been studied experimentally and theoretically. The results showed that the amorphous fraction and nucleation mode of particles were sensitive to the gas pressure. Specifically, at 7 MPa, the particles presented a fully amorphous structure with the maximal size class of 45–75 µm, and preferred to nucleate heterogeneously within the sphere rather than on the surface. These interesting phenomena were associated with the difference of cooling rate within droplets, reflected by the gas-liquid relative velocity at the cooling and solidification stage in the atomization process. It was also indicated that the amorphous phase underwent three stages of reactions upon devitrification with the precipitations of {Fe, Ni} and Fe2P, α-Fe and M23B6, respectively. Finally, glass-forming features of powders at different pressures have been elucidated based upon a kinetics analysis of the cooling rates of droplets and the phase selection competition. This work provides beneficial guidance for the devisal of metallic glass powder with different microstructures via gas atomization.