Beneficial effects of Cr addition on the nanocrystalline Si and B modified Co-Zr permanent magnetic alloys

Abstract

Co-Zr alloys with Si and B modifications show potential for rare earth-free permanent magnets. In this work, Cr was employed for improving the hard magnetic properties of melt-spun Co78-xZr16CrxSi3B3 (x = 0, 1, 2, 3, and 4) alloys. Their phase constitution, microstructure, and properties have been investigated. The results show that Cr addition can suppress the formation of the soft magnetic FCC-Co phase. The coercivity increased almost linearly with increasing Cr addition, accompanied by decreasing remanence. A high coercivity of 7.7 kOe was obtained in the alloy with 4 at.% Cr substitution for Co, manifesting a 67% coercivity enhancement from 4.6 kOe of the Cr-free alloy. Cr atoms were found to partially substitute Co atoms in the Co-Zr-Cr-Si-B alloys. The size of nanograin decreased with Cr addition alloys, partly due to the precipitation of the secondary Cr phase. After Cr addition, the enhanced positive peak in Henkel plots and the closed recoil loop indicate the improved exchange coupling and consistent magnetic reversal. The alloys with x = 0 and 4 exhibited much lower absolute values of coercivity temperature coefficient, 0.183%/℃ and 0.251%/℃, respectively, and one or two orders of magnitude smaller corrosion current density than that of melt-spun Nd-Fe-B alloys, demonstrating that Co-Zr based alloys have not only excellent thermal stability but also good corrosion resistance. In addition, Cr substitution can further reduce the corrosion rate of Co-Zr alloys in the 3.5 wt% NaCl solution.