HRTEM studies of grain boundaries in die-upset Nd–Fe–Co–Ga–B magnets

Highly textured magnets of MQU-F material (Nd 13.6 Fe 73.6 Ga 0.6 Co 6.6 B 5.6 ) were produced by die-upsetting. A remanence B r =1.39 T, a coercivity μ 0j H c =1.18 T and an energy product (BH) max =370 kJ m −3 were obtained when using a relatively high degree of deformation (height reduction: 80%). Differential scanning calorimetry measurements reveal, beside an increase of the Curie temperature of the main phase, a substantial decrease of the melting point of the intergranular phase (660 °C) in comparison to that of the ternary alloy MQP-A (680 °C). The microstructure of the hot-deformed MQU-F material, especially of the intergranular phase, was investigated by high-resolution transmission electron microscopy and energy dispersive X-ray spectroscopy (EDX) in the scanning transmission electron microscopic mode with a subnanometer probe. A very homogeneous distribution of the intergranular phase was observed. The Nd 2 Fe 14 B crystals are surrounded by an amorphous Nd-rich phase with a thickness of 0.4 to 2 nm. EDX analysis shows that the grain boundaries are enriched in Nd and Ga. Also, a small amount of Co was detected.