High-temperature magnetic performance and corrosion resistance of hot-deformed and sintered Nd-Fe-B magnets with similar room-temperature magnetic properties

Abstract

Both hot-deformed (HD) and sintered Nd-Fe-B magnet have huge prospect in the application of permanent magnet servomotor, but its magnetic performance is often influenced by increased working temperature and corrosion environment. Hereby, the high-temperature magnetic performance and corrosion resistance of HD magnets and sintered magnets with similar room-temperature magnetic properties were investigated. The melt-spun Nd-Fe-B powders of MQU-F and MQU-G have been used to prepare the HD magnets without (HDF) and with (HDG) heavy rare earth of Dy element. The sintered magnets of 45H grade (S45H) and 35UH grade (S35UH) were used as reference magnets to evaluate the high-temperature magnetic performance and corrosion resistance of HDF and HDG, respectively. The results show that both HDF and HDG have superior temperature coefficient of coercivity (β), irreversible flux loss (hirr) and corrosion resistance to the same-grade sintered magnets. The analysis on microstructure and element composition reveals that the difference in β and hirr between HD and sintered magnets is mainly resulted from grain size and Pr content. The nanograins in HD magnets are much smaller than the micrograins in sintered magnets. Consequently, |β| and hirr reduces significantly with the decrease of grain size. Meanwhile, the Pr-contained sintered magnets of S45H and S35UH consist of both Pr2Fe14B and Nd2Fe14B phases. As the magneto-crystalline anisotropy of Pr2Fe14B decreases faster than that of Nd2Fe14B with the increased temperature. |β| and hirr of the HDF and HDG are better than those of the same-grade sintered magnets. In addition, the difference in corrosion resistance is primarily caused by the grain boundary (GB) width, the total rare earth (TRE) content and Co content. The corrosion on Nd-Fe-B magnet initiates from and propagates along GB and the boundary of flake particles at the surface of HD and sintered magnets. The narrower GB width, lower TRE content and higher Dy content all play important role in better corrosion resistance of HD magnets.