Hydrogenation disproportionation desorption and recombination (HDDR) studies on direct-reduced Nd15Fe77-xB8 Gax powders

Direct-reduced Nd-Fe-B powders with additions of 1.0% and 2.0% Ga were treated by the HDDR process at temperatures between 770°C and 900°C in order to optimise the processing parameters. The microstructures of the powders were investigated by scanning electron microscopy (SEM) on rotary forged Al bonded samples. The investigation of the magnetic properties was carried out on wax-bonded samples on a vibrating sample magnetometer (VSM). It was found that the direct-reduced Nd-Fe-B powders containing Ga additions show significantly increased intrinsic coercivities of up to 895 kA/m compared to those of the ternary alloy or those of powders with Zr additions. The remanence appears to be, for both Nd₁₅Fe₇₆B₈Ga₁ and Nd₁₅ Fe₇₅B₈Ga₂, relatively constant over a processing temperature range of ~50°C with Nd₁₅Fe ₇₆B₈Ga₁ exhibiting higher values of up to 680 mT. The disproportionation time has a significant influence on the magnetic properties of the material, higher processing temperatures requiring shorter times in the disproportionated state, lower temperatures longer times. Under the conditions employed in these experiments, the HDDR treatment removes existing anisotropy in the as-received coarse grain material, thus leading to predominantly isotropic behaviour