Normal and Abnormal Grain Growth in Hydrogenation-Disproportionation-Desorption-Recombination Processed Nd-Fe-B Strip-Cast Alloys

Abnormal grain growth (AGG) is a common phenomenon in liquid-phase sintering of Nd-Fe-B materials. To identify the growth mechanism and suppress AGG, we investigated the grain growth in hydrogenation-disproportionationdesorption-recombination processed Nd-Fe-B strip-cast alloys by field emission scanning electron microscopy and scanning transmission electron microscopy. The Nd2Fe14B grains grew up normally, and then AGG occurred due to the formation of faceted grains. The Nd2Fe14B grain that directly contacted with the large liquid triple junction was faceted and presented the maximum driving force (Delta g(max)) due to its largest grain size. The increase of Nd content and heating temperature induced the occurrence of abnormal Nd2Fe14B grains and shortened the incubation period of AGG, but within the incubation period, the normal grain growth (NGG) of Nd2Fe14B grains was promoted. In this work, we proposed an improved mixed control model to analyze the AGG of Nd2Fe14B grains and presented a two-step heat-treatment process to promote NGG by modifying Delta g(max) to below the critical driving (Delta g(c)). The average Nd2Fe14B grain size of 2.63 pm for NGG was obtained.