Process technology,properties,and microstructure of hot rolled rare earth–transition metal–boron permanent magnets

Abstract

Permanent magnets were produced by rolling ingots with the three compositions: Nd₁₇ Fe_76·5 B₅ Cu_1·5, Nd_17·1 Fe₇₄ Nb₁ B_6·7 Cu_1·2, and Pr₁₇ Fe_76·5 B₅ Cu_1·5. Rolling temperatures were 750, 850, and 950°C, and the specimens were subsequently heat treated. The microstructure was very homogeneous and the grain size exhibited an inverse correlation with rolling temperature. A fine microstructure corresponded to high coercivity levels, and the highest value of this magnetic property was achieved by the praesodymium containing magnet. High temperature heat treatment was deleterious to the coercivity of the neodymium containing magnets but the opposite was observed for the praesodymium magnets. Although, as a rule, the heat treatment did not affect the remanence, this magnetic property increased noticeably for the praesodymium magnet. As for the magnetic alignment or texture, X-ray diffraction showed that it was increased by hot rolling compared with the as cast ingot. The degree of magnetic texture was increased by deformation but was also favoured by a previously existing highly developed and well oriented lath microstructure.