A metastable phase crystallized from undercooled NdFeCoZrGaB alloy droplets

Phase formation in rapidly solidified particles of a Nd13Fe68.4Co11Zr0.1Ga0.5B7 alloy was investigated using the drop tube technique in order to gain further understanding of the metastable solidification behavior of undercooled Nd2Fe14B-based alloy melts. The results show that primary iron formation prevails in larger particles (360–1200 μm), as expected for a small undercooling achieved prior to solidification. However, a new metastable phase with the TbCu7-type structure was found to have crystallized directly from the undercooled liquid phase in smaller particles (80–360 μm). Based on the result of compositional analysis, it was assumed that the phase may have the chemical formula Nd2(Fe,Co)17Bx, where x=0–1. The reasons for the metastable formation of the phase are discussed by taking into account both thermodynamic and kinetic arguments which can explain phase selection in undercooled melts.