Enhancing coercivity and mechanical strength of sintered Nd-Fe-B magnets by heavy rare earth-free grain boundary diffusion

Abstract

In addition to magnetic properties, mechanical strength is also an important factor in the stability of Nd-Fe-B magnets during service. Here, we proposed a strategy to simultaneously enhance magnetic and mechanical properties by grain boundary diffusion (GBD) without using heavy rare earths (HRE). For a 4.5 mm thick sintered magnet, the coercivity of Pr70Al20Cu10 and Al75Cu25 (at.%) diffused magnets can enhance from 1000 to 1346 and 1089 kA/m, respectively, due to the formation of continuous grain boundary (GB) layers for magnetic decoupling, respectively. The increased compressive strength and surface hardness of the magnet mainly result from the enhanced connection between 2:14:1 grains. Since the diffusion of HRE-free alloys improves GB phase by both increasing the wettability of GB phase and reducing the defects at the interface of 2:14:1 phase, more energy is required for the crack propagation which results in an increase in mechanical properties. Furthermore, this work generalizes the difference between the HRE and non-HRE diffusion sources in enhancement mechanisms of coercivity and mechanical strength. It is thus believed that GBD can be used to improve not only magnetic properties but also the other properties of magnets.