Pre-orientation sintering: an alternative process to prepare high-performance anisotropic Nd-Fe-B HDDR magnets

Hydrogen-disproportionation-desorption-recombination (HDDR) Nd-Fe-B magnetic powders are promising to prepare bulk anisotropic magnets, but high magnetic performance has not been achieved due to the absence of an Nd-rich phase in the powder and the low degree of orientation of the bulk magnets. In this study, an alternative process of pre-orientation sintering via magnetic alignment followed by spark plasma sintering was proposed to prepare the precursor of hot-deformation (HD) magnets, and a high maximum energy product of 294 kJ m⁻³ was achieved in the HD magnet with a relatively low height reduction of 35%, then an improved coercivity of 1107 kA m⁻¹ could be obtained followed by a grain boundary diffusion of Pr₄₀Tb₃₀Al₂₀Cu₁₀. Microstructure analysis indicates that pre-orientation of HDDR powders facilitates grain rotation and alignment during the HD process, thereby reducing the minimum deformation ratio. It helps to obtain the deformed grains with lower shape anisotropy and smaller grain size, enabling a good compatibility of magnetic and mechanical behaviors. In-situ Lorentzian transmission electron microscopy results show that the magnetic domains have been strongly fixed by the thick intergranular RE-rich phase and the fully Tb-diffused grains, which contributes to the improved coercivity after grain boundary diffusion. This study provides a guiding significance for the preparation of the anisotropic Nd-Fe-B HDDR magnets with optimized performance.