Microstructures and magnetic properties of hydrogenation disproportionation desorption recombination-processed Nd–Fe–B materials with different Nd content of 11.0 and 12.6 at.%

The hydrogenation disproportionation desorption recombination (HDDR) process was performed on the generally used alloy composition of Nd12.6Fe63.1Co17.4Zr0.1Ga0.3B6.5 and a low rare earth content alloy composition of Nd11.0Fe65.0Co17.8Zr0.1Ga0.3B5.8. A detailed evaluation was made of the relationship between the microstructure and magnetic properties of these HDDR-processed magnetic powders with respect to their different rare earth element concentrations. The HDDR-processed powders of both alloy compositions were transformed to the Nd2Fe14B phase consisting of fine recombined crystal grains of around 400–500 nm in size and maintained the anisotropic magnetic characteristic that was present before HDDR processing. However, reduction of the rare earth content drastically reduced coercivity, and the alloy composition of Nd11.0Fe65.0Co17.8Zr0.1Ga0.3B5.8 did not manifest magnetic properties. From the results of an examination of their microstructures, it was inferred that the coercivity decreased due to a decline in the concentration of the rare earth element at the grain boundaries of the fine Nd2Fe14B grains with the reduction of the rare earth content of the alloys. Accordingly, in magnetic powders obtained by the HDDR process, the nucleation type of coercivity mechanism predominates, in which rare earth-rich regions present at the grain boundaries of fine Nd2Fe14B grains play a large role in the manifestation of coercivity.