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

发布时间:2024-01-15 13:35浏览次数:times


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.


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.

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