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 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 350°C to 450°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.

Effect of cyclic sintering process for NdFeB magnet on microstructure and magnetic properties

“…For example, the addition of Dy, Co and Al improved the coercivity and thermal stability, but significantly decreased the Br and (BH) max [3,5]. Studies have also shown that post-sintering heat treatment can greatly enhance the coercivity of sintered Nd-Fe-B magnets [6][7][8].…”