Microstructure of shell and grain boundary phase in Nd-Fe-B sintered magnets grain boundary diffusion processed with low-melting LRE-Al-Cu (LRE = La and Pr) alloys

We report magnetic and microstructural changes in Nd-Fe-B sintered magnets after the grain boundary diffusion process (GBDP) of low-melting LRE-Al-Cu alloys [LRE (Light Rare-earth) = La and Pr]. A distinctive microstructural feature of the magnets GBD treated with La-Al-Cu (LAC) and Pr-Al-Cu (PAC) were characterized via the electron probe microanalysis and high-angle annular dark-field scanning transmission electron microscopy analysis at a depth of 50 μm from the magnet surface. The formation of a thick high-anisotropy Pr-Al-rich shell was clearly observed in the PAC-GBDP magnets, whereas there was no distinct shell formation in the LAC-GBDP magnets. La, Al, and Cu were dissolved in the Nd-rich grain boundary phase (GBP) rather than in the main phase, thereby thickening the GBP. This resulted in a difference in the coercivity gain between PAC-GBDP (+6.4 kOe) and LAC-GBDP (+3.3 kOe). The point to note here is that the remanence reduction induced by LAC-GBDP (−0.2 kG) is much smaller than that induced by PAC-GBDP (−1.2 kG) because the grain boundary diffused La-Al-Cu, which can dilute the saturation magnetization of the Nd₂Fe₁₄B (2–14-1) crystal, does not dissolve into 2–14-1. Furthermore, the squareness of demagnetization curves of the LAC-GBDP magnets (98%) was much improved than that of the PAC-GBDP magnets (86%). This is because chemically induced liquid film migration (CILFM), an undesirable grain growth phenomenon induced by shell formation, does not occur in LAC-GBDP magnets. In conclusion, the deterioration in remanence and squareness, which are serious problems in the LRE-GBDP, can be minimized by the LAC-GBDP; thus, the utilization of La in the GBDP is a promising method for obtaining a high maximum energy product of the magnets. Based on the results of this analytical work, we propose a guide for developing a cost-effective novel GBDP source that can prevent grain growth by CILFM and increase the magnetocrystalline anisotropy of the shell.