Effect of Cu/Al powder mixing on Dy diffusion in Nd-Fe-B sintered magnets treated with a grain boundary diffusion process

Abstract

<p>We investigate the microstructural and magnetic property changes of DyH<sub>2</sub>, Cu + DyH<sub>2</sub>, and Al + DyH<sub>2</sub> diffusion-treated NdFeB sintered magnets with the post annealing (PA) temperature. The coercivity of all the diffusiontreated magnets increases with increasing heat treatment temperature except at 910°C, where it decreases slightly. Moreover, at 880°C, the coercivity increases by 3.8 kOe in Cu and 4.7 kOe in Al-mixed DyH<sub>2</sub>-coated magnets, whereas this increase is relatively low (3.0 kOe) in the magnet coated with only DyH<sub>2</sub>. Both Cu and Al have an almost similar effect on the coercivity improvement, particularly over the heat treatment temperature range of 790-880°C. The diffusivity and diffusion depth of Dy increases in those magnets that are treated with Cu or Al-mixed DyH<sub>2</sub>, mainly because of the comparatively easy diffusion path provided by Cu and Al owing to their solubility in the Nd-rich grain boundary phase. The formation of a highly anisotropic (Nd, Dy)<sub>2</sub>Fe<sub>14</sub>B phase layer, which acts as the shell in the core-shell-type structure so as to prevent the reverse domain movement, is the cause of enhanced coercivity of diffusion-treated Nd-Fe-B magnets.</p>