Changes in Microstructure and Magnetic Properties during HDDR Process in the Nd-Fe-B Master Alloys with Segregation

摘要

Anisotropic magnet powders are generally prepared via the usual hydrogenation decomposition desorption recombination (HDDR) process after heat-treating raw material master alloys for a long time to homogenize them. In this study, the possibility of omitting the homogenization heat treatment, i.e., the applicability of the HDDR process to as-cast Nd-Fe-B system master alloys with segregation, was examined with the aim of reducing the process cost. The HDDR powders made from the as-cast alloy with segregation showed anisotropy, large coercive force, and good magnet characteristics. At the conclusion of the hydrogen decomposition (HD) process, areas having a fine structure composed of NdH_2 + alpha-Fe + Fe_2B and large-scale areas with alpha-Fe and Nd-rich parts measuring 30 mu m in size were observed. However, after the desorption recombination (DR) process was completed, the Nd_2Fe_(14)B phase was formed over the whole alloy. The Nd_2Fe_(14)B phase after the HDDR process had an almost homogeneous structure, which was composed of recombined crystals with a fine grain size of 300-400 nm.