Effects of thermal stress induced by cyclic heat treatment on sintered (Nd, Dy)-Fe-B magnets

We investigated the mechanisms of coercive force enhancement on Nd(11)Dy(4)Fe(76.5)TM(2.5)B(6) (TM = Co, Cu, Al) induced by a number of cyclic heat treatments (CHT) at temperatures from 350A degrees C to 450A degrees C. The difference in thermal expansion coefficients between Nd-rich and Nd(2)Fe(14)B phases induced penetration of the Nd-rich phase into the Nd(2)Fe(14)B grain boundary through capillary force and compress pressure by CHT. Also, dislocations in the Nd(2)Fe(14)B grain were formed by thermal stresses from thermal expansion and contraction, which caused the domain wall pinning effect. Consequently, coercivity was enhanced from 2,303 kA/m before CHT to 2,480 kA/m after CHT, and a remanence of 1.18 T was maintained after two cycles.This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials, funded by the Ministry of Knowledge Economy, Republic of Korea