Hot-working behavior of cast Pr-Fe-B magnets

The hot working is done at a low strain rate (hot pressing) and a high strain rate (hot rolling). Magnetic alignment induced by the hot working is found to be closely related to the microstructure of the cast ingots and the direction of principal stress. The appropriate structure is a columnar structure. The c-axis of the Pr₂Fe₁₄B phase lies in the plane perpendicular to the growth direction of the dendrites. The principal stress during working should be given perpendicular to the growth direction. During hot pressing carried out in an open die, the following phenomena were observed: (1) crystal alignment along the c-axis, (2) pressing out of a liquid Pr-rich phase, (3) grain refinement, and (4) diffusing of the BCC-Fe phase. The addition of elements such as Cu, Ag, Au, or Pd lowers the melting point of the Pr-rich phase and encourages these phenomena. This results in a (BH)_max value of more than 30 MGOe in the starting composition of Pr₁₇Fe_76.5 B₅M_1.5 (M=Cu, Ag, Au, or Pd). The above results were obtained by hot-pressing with a strain rate between 10^-3 and 10^-4/s. High magnetic properties, i.e., B_r=11.6 kG, i_Hc=10.7 kOe, and ( BH)_max=30.2 MGOe, were obtained in the hot-rolled Pr ₁₅Fe₇₈B_5.5Cu_1.5 magnet