Abstract
Gas-atomized Nd–Fe–B droplets of non-peritectic chemical compositions Nd 16Fe 76B 8, Nd 17Fe 74.5B 8.5, and Nd 18Fe 73B 9 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 Nd 2Fe 17B x dendrites plus interdendritic Nd 2Fe 14B grains plus intergranular Nd 1.1Fe 4B 4 and Nd-rich grains, where the metastable Nd 2Fe 17B x dendrites are partially or completely decomposed into a mixture of α-Fe plus Nd 2Fe 14B depending on particle size. There is often a certain amount of Nd 2Fe 14B 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 Nd 2Fe 14B grains, either columnar or dendritic, plus intergranular Nd 1.1Fe 4B 4 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 Nd 2Fe 14B grains or by Nd 2Fe 17B x grains in individual particles.
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