Influence of desorption recombination temperatures on microstructure and coercivity of HDDR-processed anisotropic Nd-Fe-B magnet powders

In the hydrogenation decomposition desorption recombination(HDDR)processed Nd Fe Co B Al Ga Zr magnet powders, influence of the desorption recombination(DR)temperatures on the microstructures was investigated by high resolution scanning electron microscopy and scanning transmission electron microscopy. By applying the DR treatment under high vacuum(DR Step 2)at relative low temperatures about 973 K, we can obtain the excellent texture in which the average size of Nd₂Fe₁₄B grains is refined to about 240 nm and the grain boundary(GB)phase is uniformly dispersed around the Nd₂Fe₁₄B grains. In the GB phase, furthermore, the Nd composition increases and the(Fe, Co)composition decreases drastically with decreasing the DR temperatures, and then Nd rich GB phase with more than 65 at.%Nd can be formed at 973 K. At 973 K, however, the region with unformed GB phase is remained about 10% of the specimen, which leads to deterioration of the coercivity. It was found that the gently DR treatment is very effective not only for increasing the coercivity but also for improving the squareness in demagnetization curve.