Influence of annealing on the magnetic properties and microstructure of NdFeB based magnets

Abstract

The effect of heat treatment on the magnetic properties and microstructure of (Nd, Dy)–(Fe, Co)–B based sintered magnets with small additions of alloy elements are investigated. It is found that the magnetic properties of these magnets are almost unchanged by heat treatment at 900°C (T1), which is useful for relieving stresses. The intrinsic coercivity is strongly dependent on the two-step post heat treatment, especially, on the second step post heat treatment temperature (T2). Optimum intrinsic coercivities for these magnets are obtained when the second step heat treatment is carried out slightly below their corresponding eutectic temperatures. The morphology and distribution of the phases found in these magnets depend also on heat treatment. The structure of the grain boundaries is remarkably improved.