Hydrogen disproportionation phase diagram and magnetic properties for Nd15Fe79B6 alloy

Transformation-temperature-hydrogen pressure phase diagram was constructed for a Nd15Fe79B6 alloy in order to estimate appropriate conditions for hydrogenation, disproportionation, desorption and recombination reaction (the HDDR). Optimised recombination time (the highest coercivity) was found to be 10 min. for 5 g samples processed at 740 °C. Several HDDR processes were carried out at 30 kPa of hydrogen pressure at various temperatures. No correlation between magnetic propertiec and a direction of measurement was observed for the samples processed at 740 °C. Remanence anisotropy was induced along an alignment direction when the temperature of the HDDR process was increased up to 800 °C and 850 °C for <100 μm and 100–160 μm particles, respectively. Simultaneously, a small drop in coercivity was observed in the direction of alignment for <100 μm particles, but no for 100–160 μm particles. Furthermore, probably an ordered phase was found by TEM microstructure analysis in the bulk sample disproportionated at 850 °C under 150 kPa of hydrogen. Grains with antiphase domains were observed and corresponding electron diffraction patterns were resolved, likely indicating superlattice structures.

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