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

Abstract

We investigated the mechanisms of coercive force enhancement on Nd11Dy4Fe76.5TM2.5B6 (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 Nd2Fe14B phases induced penetration of the Nd-rich phase into the Nd2Fe14B grain boundary through capillary force and compress pressure by CHT. Also, dislocations in the Nd2Fe14B 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.