Characterization of the microstructure of gas-atomized Nd-Fe-B alloy particles of non-peritectic chemical compositions

Gas-atomized Nd-Fe-B droplets of non-peritectic chemical compositions Nd16Fe76B8, Nd17Fe74.5B8.5, and Nd18Fe73B9 were undercooled and solidified in an 8 m drop tube filled with helium. The microstructure of the received particles was investigated by means of scanning electron microscopy and powder X-ray diffractometry. It was found that in each alloy composition, in excess of 71% of the particle population shows a microstructure comprising primary Nd2Fe17Bx dendrites plus interdendritic Nd2Fe14B grains plus intergranular Nd1.1Fe4B4 and Nd-rich grains, where the metastable Nd2Fe17Bx dendrites are partially or completely decomposed into a mixture of α-Fe plus Nd2Fe14B depending on particle size. There is often a certain amount of Nd2Fe14B dendrites in these particles, resulting in a dual microstructure. On the other hand, the rest of the particle population shows a microstructure consisting of primary Nd2Fe14B grains, either columnar or dendritic, plus intergranular Nd1.1Fe4B4 and Nd-rich phase grains. Additionally, a tiny amount of α-Fe dendrites is formed in some of the particles independent of the type of the microstructure. They are exclusively distributed near the particle surface, and are surrounded either by Nd2Fe14B grains or by Nd2Fe17Bx grains in individual particles.