Hot deformation of nanocrystalline Nd-Fe-B alloys

Die-upset experiments on hot compacted commercial melt-spun powders MQP-A and MQP-B were carried out under an argon atmosphere at 500 to 800°C with strain rates of 10^-4 to 10^-1 s^-1. The kinetic analysis of the strain-rate stress relations yields the following dependences for the strain rate: (i) on flow stress, a power law with a stress exponent of 3, (ii) on temperature, a thermal activated process with activation energies of 280 and 400 kJ/mol for the MQP-A and -B samples, and (iii) on mean grain size, a reciprocal proportionality for MQP-A. These results indicate an interface-controlled solution precipitation process as the microstructural mechanism of hot deformation and texturing. Hereby the grain-boundary phase seems to be not required to be in the liquid state for deformation. Its presence is necessary for a crack-free deformation at high strain rates. In the case of radially textured ring magnets made by backward extrusion an area reduction of about 65% is necessary for optimization of the needed press stresses as well as the magnetic properties. Typical values are B_r=1.25 T, μ₀H _cj=1.1 T and (BH)_max=280 kJ/m³