Effects of Strain and Stain Rate on Microstructure and Magnetic Properties of Nd-Fe-B Magnets During Die-Upsetting Process

The effect of strain and stain rate on microstructure and the magnetic properties of Nd-Fe-B magnets during hot-deformation process have been studied with commercial melt-spun flakes with a composition of Nd13.6Fe73.6Co6.6Ga0.6B5.6. The compacts produced by hot pressing at 700 degrees C under 100 MPa in vacuum were subjected to die upsetting at 700 degrees C with different deformation conditions of strain rate 0.1-0.001 s(-1) and height reduction 40%-75%. The remanence and coercivity tended to increase and decrease with increasing strain, respectively. At below the strain of 0.5, the remanence increased more quickly with decreasing the strain rate, but the tendency was gradually reversed with increasing the strain. This was because the prolonged deformation induced the squeeze-out of Nd-rich phase from grain boundaries and the formation of coarse Nd-rich phase. In addition, the sample subjected to the prolonged deformation had quite thin and discontinuous grain boundary phase, which resulted in the decrease of the coercivity.