Changes of microstructure and magnetic properties of Nd–Fe–B sintered magnets by doping Al–Cu

The microstructural and magnetic properties of Al100−xCux (15at%≤x≤45 at%) doped Nd–Fe–B magnets were studied. The distribution and alloying effects of Cu or Al on the intergranular microstructure were investigated by thermodynamic analysis, differential scanning calorimetery and microscopy techniques. It was observed that when the Cu content of Al100xCux exceeds to 25 at%, the (Pr, Nd)Cu and CuAl2 phases form in these magnets. The formation of (Pr, Nd)Cu phase depends on the negative formation enthalpy of (Pr, Nd)Cu and the exclusive distribution of Cu in the intergranular regions. The eutectic reaction between (Pr, Nd)Cu phase and (Pr, Nd) occurs at 480 °C, which forms the liquid phase that dissolves the (Pr, Nd)2Fe14B surface irregularities and thus increases the quantities of (Pr, Nd)-rich phase at the grain boundaries. These changes benefit the grain boundary microstructure, especially the distribution of (Pr, Nd)-rich phase, which effectively improves the intrinsic coercivity iHc due to the decreases of exchange coupling between the (Pr, Nd)2Fe14B grains.