Abstract
<TEX>$Nd_{12.5}Fe_{80.6}B_{6.4}Ga_{0.3}Nb_{0.2}$</TEX> 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 <TEX>$600^{\circ}C$</TEX> 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. <TEX>$Nd_{12.5}Fe_{80.6}B_{6.4}Ga_{0.3}Nb_{0.2}$</TEX> HDDR-treated powder contained a significant amount of residual hydrogen (approx. 1500 ppm) in the form of <TEX>$Nd_2Fe_{14}BH_x$</TEX> hydride. The dramatic coercivity decrease in the compact prepared in the closed die is attributed to the disproportionation of <TEX>$Nd_2Fe_{14}BH_x$</TEX> hydride. High coercivity is mainly due to the effective desorption of hydrogen or the suppression of hydrogen-related disproportionation upon hot-pressing.
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