Origin of Poor Thermal Stability of HDDR-Treated Nd-Fe-B-Type Material

Thermal stability of the Nd_12.5Fe_80.6B_6.4Ga_0.3Nb_0.2 hydrogenation, disproportionation, desorption and recombination (HDDR)-treated powder was studied, and it was correlated to the residual hydrogen in the powder. Presence of the residual hydrogen in the HDDR-treated powder was investigated by X-ray diffraction (XRD), thermopiezic analyser (TPA) and hydrogen analysis. The HDDR-treated material contained a significant amount of hydrogen, and it influenced the thermal stability of the material. Coercivity of the HDDR-treated powder was markedly reduced by heating above 600 ^°C, and it was related to the residual hydrogen. The residual hydrogen existed in the form of Nd₂Fe₁₄BHx hydride, and it was disproportionated into α-Fe, Fe₂B and NdH₂ phases at an elevated temperature. The coercivity reduction was attributed to the magnetically soft α-Fe and Fe₂B phases formed by the disproportionation.