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

Nd-Fe-B permanent magnets have been widely used in the field of renewable energy applications with high working temperatures [1]. Since the Nd2Fe14B main phase (2:14:1 phase) exhibits a rapid decrease in anisotropy field (HA) with increasing temperature and low Curie temperature [2], heavy rare earth (HRE) elements of Tb and Dy are often added into magnets for high stability of magnetic properties at elevated temperatures. The grain boundary diffusion (GBD) process provides an exciting approach to improve the coercivity of Nd-Fe-B magnets with less consumption of HREs [3], [4]. It mainly strengthens the “weak” grain surface by forming (Nd,HRE)2Fe14B shell with high HA. However, the constantly increasing cost of HRE raw materials in recent years requires the development of more cost-effective diffusion sources such as those based on light rare earth (LRE) [3]. Different from the HREs, the LRE elements mainly modify the GB by thickening GB layers for decoupling the neighboring grains [5], [6]. As a result, the LRE-based low-melting alloys have been verified to be effective diffusion sources for coercivity enhancement [7], [8].

In addition to the magnetic properties, sufficient mechanical strength is required for Nd-Fe-B magnets to ensure their smooth installation and stable operation, especially for motor applications [9]. However, fewer slip systems exist in the tetragonal Nd2Fe14B lattice with low symmetry and complex structure, resulting in poor mechanical property, which has negative effects on the machining and working process. Therefore, it is of great significance to improve both the magnetic and mechanical properties of Nd-Fe-B magnets. Since the GBD has become a common process in the industry, its effect on the mechanical strength of magnets should be clarified. However, this issue has received less attention. Here, we demonstrated that the diffusions of Pr-Al-Cu or Al-Cu alloys can simultaneously improve the coercivity and strength of the sintered Nd-Fe-B magnets, which is quite different from the HRE ones. The present findings indicate that the GBD based on non-HRE alloys can be used to improve not only magnetic properties but also mechanical properties.