Understanding the Enhancement in Hard Magnetic Properties of Rapidly Solidified (Nd, Pr)-Fe-B Melt-spun Ribbons

Abstract

The coercivity of RE2Fe14B-type permanent magnet is strongly influenced by the microstructural features such as grain boundary phases as well as grain size. Micromagnetic simulations are powerful tools for revealing the correlation between microstructure and the evolution of hard magnetic properties, including the effects of grain size and grain boundary phases. We have combined micromagnetic simulations and experiments to elucidate the role of excess RE (Nd/Pr) in determining the resulting hard magnetic properties of Nd-Pr-Fe-B melt-spun ribbons. Also, the effect of non-magnetic grain refining refractory carbide (TiC) on both the microstructure and magnetic hardening was studied. For the simulated polycrystalline microstructure of Nd-Pr-Fe-B magnets (Fig. 1(a)) with a mean grain size of 40 nm, the coercivity increases with decreasing inter-grain magnetic exchange interaction Fig. 1(b). The increasing trend in magnetic coercivity of measured melt-spun (Nd, Pr)–Fe–B alloys is ascribed to weak inter-grain exchange coupling due to non-magnetic RE-rich grain boundary phase (Fig. 1(c)) and/or TiC at grain boundary (Fig. 1(d)). This work provides useful information on the role of non-magnetic grain boundary phase to improve the coercivity in Nd-Pr-Fe-B magnets. Combined with experimental and modeling results, we will discuss the mechanism responsible for the enhancements in coercivity and the suitability of the alloys for high performance permanent magnet development. This research was supported by the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office.