Study of the effect of a repeated HDDR treatment on the coercivity of two rare-earth-transition metal hard magnetic materials

In an attempt to achieve a more complete and more homogeneous recombination in the hydrogenation, disproportionation, desorption and recombination (HDDR) of Sm2Fe17-type and Nd-Fe-B-type permanent magnet alloys, thus achieving high coercivity, an HDDR treatment was applied repeatedly to those alloys (the HDDR-treated Sm2Fe17-type alloy was nitrogenated afterwards). The effect of this treatment on the coercivity was investigated. The hydrogenation and disproportionation characteristics of the previously HDDR-treated alloys were also examined. It was found that for the Sm2Fe17-type alloy the hydrogenation temperature was slightly raised as the number of HDDR cycles increased, while the disproportionation temperature was significantly decreased. For the Nd-Fe-B-type alloy both the hydrogenation and the disproportionation temperatures decreased with the number of HDDR cycles. Repeated HDDR increases the coercivity of both Sm2Fe17Nx-type and Nd-Fe-B-type materials. The increase is greater for the Sm2Fe17Nx-type material.