Effects of Al/Cu co-doping on crystal structure and chemical composition of Nd-rich phases in Nd-Fe-B sintered magnet

We investigated the effects of Al/Cu co-doping on the magnetic and microstructural properties of a Nd-Fe-B sintered magnet. In particular, the structural and compositional changes of Cu-dissolved Nd-rich (Cu-rich) triple junction phase (TJP) and grain boundary phase (GBP) experienced upon Al/Cu co-doping were intensively analyzed. Al was selectively dissolved into the Cu^low-rich phases (B phase, 3–15 at.% Cu) other than the Cu^high-rich phases (A phase, 20–45 at.% Cu). The B phase with an h-Nd₂O₃ structure was transformed to the C phase (Al-dissolved Cu^low-rich phases) with a c-Nd₂O₃ structure upon Al dissolution. In addition, the continuity of C GBP was improved and the Fe content of C GBP was reduced (25 → 5 at.% Fe). As a result, the coercivity (H_c) of the Al/Cu co-doped magnet was higher than that of a Cu-doped magnet. The critical role played by Al in improving the magnetic properties and microstructure of the Al/Cu co-doped magnet was elucidated. The detailed mechanism for the H_c enhancement of the Al/Cu co-doped magnet was discussed based on the results.