Texture formation in hot deformed rapidly -quenched neodymium iron boron permanent magnets

Lin Li, University of Pennsylvania

Abstract

An unusual texture formation and deformation behavior in an intermetallic compound system where dislocations play no part has been understood and explained on the basis of preferential grain growth by liquid phase diffusion. NdFeB rare earth permanent magnetic materials, based on the intermetallic compound Nd$\sb2$Fe$\sb{14}$B, have the highest energy product (BH)m, of all permanent magnets, up to 50 MGOe. Isotropic magnets with (BH)m about 10-14 MGOe can be made by hot pressing rapidly-quenched NdFeB ribbons into bulk at about 700$\sp\circ$C. However, if the isotropic bulk material is compressed about 60% in thickness at about 750$\sp\circ$C, the magnet has much higher (BH)m, up to 30-40 MGOe. Clearly, there is a strong crystallographic texture formed during this hot deformation. Preliminary TEM microstructure studies showed no dislocations in the samples either before and after the hot deformation. The question therefore arises: how can the material be deformed and how can the strong texture developed without dislocations present? To address this unusual problem, we systematically studied the deformation behavior of rapidly-quenched NdFeB from General Motors Corporation, using SEM, TEM, EDAX, AES, and HRTEM to extensively characterize the microstructure, especially the Nd-rich grain boundary phase. X-ray diffraction and magnetic measurements were also used for texture determination. We find that the mechanism of texture formation is the anisotropic growth of favored oriented Nd$\sb2$Fe$\sb{14}$B grains under uniaxial pressure, via liquid phase transport in the Nd-rich grain boundary phase at high temperature. During hot deformation, a cluster of 25-30 small randomly oriented grains converts into a single large aligned platelet grain by the liquid diffusion noted above. Therefore, the hot deformation and texture formation take place in this material without the presence or motion of dislocations. We have established a semi-quantitative model based on liquid diffusion estimates and microstructural evidence, which is consistent with experimental results from various research groups.

Subject Area

Materials science

Recommended Citation

Li, Lin, "Texture formation in hot deformed rapidly -quenched neodymium iron boron permanent magnets" (1992). Dissertations available from ProQuest. AAI9227707.
https://repository.upenn.edu/dissertations/AAI9227707

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