Metastable phase formation from undercooled melt of Nd–Fe–B alloys

The undercooled melts of the Nd–Fe–B alloy, the chemical composition of which is stoichiometric Nd2Fe14B, solidified during free fall in a 26 m drop tube. The microstructures of the as-dropped samples were examined using X-ray diffraction analyzer and a scanning electron microscope with an energy dispersive X-ray analyzer. The microstructures were classified into two types: type I is the mixture of properitectic α-Fe and peritectic Nd2Fe14B, and type II is the monochromatic metastable phase having a crystal structure similar to that of rhombohedral Nd2Fe17. The samples cooled at relatively low cooling rates were observed to be type I and the samples cooled at relatively high cooling rates were observed to be type II. The high cooling rate followed by high undercooling facilitates the formation of the metastable phase directly in the undercooled melt. The critical conditions for the formation of the metastable phases in the undercooled melt are thermodynamically and kinetically discussed.