Properties of α-Fe/Nd₂Fe₁₄B-type Nd-Fe-Co-V-B system bulk exchange-spring magnets prepared by spark plasma sintering

We have made a detailed examination of the microstructure, crystallization process, volume ratio of hard/soft phases, and magnetic properties of Nd_xFe_85-xCo₈V₁B₆ (x = 7-11) bulk exchange-spring magnets having a hard Nd₂Fe₁₄B phase and a soft α-Fe phase. The bulk exchange-spring magnets were made by applying the spark plasma sintering (SPS) method to rapidly quenched ribbons containing amorphous parts. The SPS method is a type of solid compression sintering and can be performed at lower temperatures with a shorter hold time. The obtained bulk compacts showed excellent Isotropic magnetic properties of (BH)_max greater than 130 kJ/m³, equal to those of the rapidly quenched ribbons. In the densification process by SPS, crystallization of compounds close to Fe₃B was followed by nearly simultaneous crystallization of the hard Nd₂Fe₁₄B phase and the soft α-Fe phase. The magnetization state of the hard phase must be considered in order to quantify the hard/soft phase ratio. The phase ratio calculated for a saturation state of the hard phase agreed well with the predicted value. The bulk magnets densified without any crystal grain growth between the grains of the starting ribbon powders, and a grain boundary region of about 150-200 nm in thickness formed between the ribbon powders. There was no composition segregation in the grain boundary region, and its mechanical strength was equal to that of the interior of the powder grains. Fine crystal grains smaller than 50 nm occurred in the interior of the starting powders, and the grain size decreased with increasing compressive pressure during the sintering process.