Characterization of the Microstructure of Gas-atomized Nd-Fe-B Alloy Particles of Non-peritectic Chemical Compounds

Abstract

Gas-atomized Nd-Fe-B droplets of non-peritectic chemical compositions Nd₁₆Fe₇₆B₈, Nd₁₇Fe_74.5B_8.5, 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 Nd₂Fe₁₇B_x dendrites plus interdendritic Nd₂Fe₁₄B grains plus intergranular Nd_1.1Fe₄B₄ and Nd-rich grains, where the metastable Nd₂Fe₁₇Bx dendrites are partially or completely decomposed into a mixture of <font face="Symbol">g</font>-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 Nd_1.1Fe₄B₄ and Nd-rich phase grains. Additionally, a tiny amount of <font face="Symbol">g</font>-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 Nd₂Fe₁₇B_x grains in individual particles.