PREPARATION AND TRANSMISSION ELECTRON-MICROSCOPE INVESTIGATION OF SINTERED ND15.4FE75.7B6.7CU1.3NB0.9 MAGNETS

Sintered magnets were prepared from jet-milled and hydrogen-decrepitated jet-milled Nd-Fe-B:(Cu,Nb) powder. The HD process started after heating the cast material up to 200°C. A single-step annealing treatment at 525°C improved the magnetic properties of all samples. Magnets prepared from the jet-milled powder required a short time sintering (<60 min) to obtain optimum coercivity but had poor density. Magnets prepared from the HD powder showed a higher remanence and energy density product but a slightly smaller coercivity than the magnets prepared from the jet-milled powder. The sintered and annealed magnets contain a multiphase microstructure. Nb was found as small precipitates within the hard magnetic grains and as intergranular boride phase. The addition of Cu lowers the melting point of the liquid phase during sintering and leads to the formation of an orthorhombic NdCu phase that is partly replacing the Nd-rich intergranular phases. The intergranular δ-phase Nd 6Fe13Cu was found regularly in the annealed samples. Nd2Fe17 was not detected in the investigated samples.