Hydrogen absorption and desorption characteristics of high coercivity NdDyFeCoNbCuB sintered magnet. I. Low temperature hydrogen decrepitation treatments

Research highlights: NdDyFeCoNbCuB magnets were hydrogenated/dehydrogenated at low temperature under moderate hydrogen pressure conditions. → Decrepitation of NdDyFeCoNbCuB magnets was studied versus time, hydrogen pressure and heating rate conditions. → Hydrogen treatment at high temperature enables to control better the size of the hard magnetic particles. → Well hydrogenated / dehydrogenated NdDyFeCoNbCuB powders allow to develop good coercivity and excellent induction levels. - Abstract: Hydrogen absorption/adsorption properties of high coercivity NdDyFeCoNbCuB sintered magnets were determined. Hydrogenation kinetics were analyzed using both differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Hydrogenation of the Nd-rich intergranular phase results in a rather broad and large peak at ∼100 ± 50 oC, then the tetragonal main phase (Φ phase) reacts readily close to 195 oC. The disproportionation process of the whole magnet initiates at T ∼ 500 oC, then accelerates in the vicinity of 600 oC and finally ends at T ∼ 780 oC. Furthermore, the first hydrogenation reaction, which is associated with the hydrogen diffusion in the bulk via the intergranular Nd-rich phase, was seen to proceed quite differently depending on the heating rate, or the applied plateau temperature. While hydrogen absorption at 50 oC is rather slow, it results in higher hydrogen uptake than at 150 oC, though there it happens much faster. Three modes of hydrogenation of sintered magnets are discussed in terms of practical operability. Using the optimized hydrogen decrepitation/desorption annealing route leads to a demonstration that the anisotropic NdDyFeCoNbCuB powders obtained by the HD/D technique have recovered most of the magnetic performance initially displayed in the bulk magnets.