Coercivity of Hot-pressed Compacts of Nd-Fe-B-type HDDR-treated Powder

$Nd_12.5Fe_80.6B_6.4Ga_0.3Nb_0.2$ HDDR-treated powder was compacted by hot-pressing using different configurations of dies and heating rates. The die configurations were especially different in terms of the evacuation system that was used in heating for hot-pressing. The coercivity in the compacts was influenced by the evacuation system of the die and heating rate. In spite of the identical hot-pressing temperature and heating rate, coercivity was radically reduced above $600^\circC$ in the compacts prepared in the closed-type die compared to that in the compacts prepared in the open-type die. The coercivity in the compacts prepared in the closed-type die decreased with increasing heating rate and the value further increased when extreme high heating rate was employed. $Nd_12.5Fe_80.6B_6.4Ga_0.3Nb_0.2$ HDDR-treated powder contained a significant amount of residual hydrogen (approx. 1500 ppm) in the form of $Nd_2Fe₁₄BH_x$ hydride. The dramatic coercivity decrease in the compact prepared in the closed die is attributed to the disproportionation of $Nd_2Fe₁₄BH_x$ hydride. High coercivity is mainly due to the effective desorption of hydrogen or the suppression of hydrogen-related disproportionation upon hot-pressing.