Hot deformation of nanocrystalline Nd-Fe-B alloys

Commercial melt-spun powders MQP-A and MQP-B were hot compacted in vacuum. Thereafter (i) die-upsetting experiments were performed under an argon atmosphere at 500 to 800 °C with strain rates of 10−4 to 10−1 s−1 or (ii) radially textured ring magnets were made by backward extrusion. The remanence achieved for die-upset MPQ-A samples is Br = 1.3 T (compared to 0.8 T obtained after hot-compaction). In the case of ring magnets typical values of Br, μ0Hcj and (BH)max are 1.25 T, 1.1 T and 280 kJ m−3, respectively. The deformation and formation-of-texture processes can be explained by the model of solution-precipitation creep. The main driving forces of the deformation processes are the chemical potentials of the atomic species in the liquid phase or at the surfaces of the crystallites. The activation energies of these processes are 280 and 400 kJ mol−1 for MQP-A and MQP-B samples, respectively. The strain rate was found to be proportional to σ3 and d−1 where σ is the flow stress and d is the mean grain size of the sample. These values indicate an interface-controlled solution-precipitation process. The liquid grain-boundary phase does not seem to be required for deformation and texturing. However its presence is necessary for a crack-free deformation at high strain rates.