Effect of hot workability parameters on the magnetic properties of nano-crystalline Nd-Fe-B magnets

Abstract

The anisotropic rate earth magnets have been produced by hot deformation by hot consolidated, melt-spun ribbons of Nd-Fe-B alloys. A systematic study has been presented in this paper for the evolution of microstructure under the two process variable, the temperature and strain rate, with an attempt to establish a structure property correlation. The hot deformation temperature and the true strain rate have been varied in the range of 973K to 1173K and of 1x10+1 to 1X10.3 sed, respectively, at a constant true strain of 0.67. The deformed magnets have been characterized for the micro structural evolution (using SEM), induced magnetic anisotropy and the texture development. The microstructure analysis has revealed the grain growth within the ribbons from the initial size of 20 to 50 nano-meter (nm), to the final size of 50 to 800 nm, depending upon the processing parameters of deformation, with abnormal grain growth at the ribbon interface in some of the cases. The microstructure, where ribbon interface has been retained with the restricted grain growth, have optimum magnetic properties with remanence (Br 1.24 Tesla, coercive force (Hc) 640 kAm-1 and energy product (BHmax)> 1270 kJm-3 from the initial values of 0.74 Tela, 488 kAm., and 88 kJm-1, respectively. The sample, whose ribbons interface have completely diffused, have given rise to the abnormal grain growth, resulting into inferior magnetic properties. The microstructural features could qualitatively establish the texture development as no uniform grain orientation could be revealed, but the crystallographic orientation could be well established by the X-ray diffraction, where the reflection peak from basal plane (006) of maximum intensity has been observed.