Effects of Pr in Tb-Pr-M diffusion source on grain boundary diffusion behavior of sintered Nd-Fe-B magnets

Abstract

To identify the effects of Pr in the grain boundary diffusion (GBD) source on sintered Nd-Fe-B magnet, Tb₇₀M₃₀(M=Co, Al) and Tb₃₅Pr₃₅M₃₀ alloys were prepared as the diffusion sources. These two types GBD samples were made with the same Tb weight gain and heat treatment for 2 h and 8 h, respectively. The distribution of element Tb and the coercivity were jointly investigated. The micro-morphology and distribution of rare earth elements show that the presence of Pr significantly boosts the diffusion efficiency of Tb. With the assistant of Pr, after a short time heat treatment, the excessive aggregation of Tb near the coated surface is prevented, allowing Tb to diffuse more deeply into the magnet. This leads to a greater coercivity enhancement of the entire magnet and higher local coercivity at different depth from the coating surface. However, when the diffusion time is extended, Pr induces much thinner shell and lower Tb concentration in the grain shells near the surface, and promotes a more homogeneous distribution of Tb in the magnet diffused with Tb₃₅Pr₃₅M₃₀. Moreover, a more uniform distribution of local coercivity, characterized by a reduction of local coercivity enhancement at the surface and a substantial increase of local coercivity enhancement inside of the magnet, was detected in the magnet diffused Tb₃₅Pr₃₅M₃₀. This contradiction results in the smaller increment of coercivity of the entire magnet than that of the magnet diffused with Tb₇₀M₃₀. The study reveals the importance of balancing these two main effects caused by the addition of Pr in the diffusion source under various diffusion conditions, which is helpful for the industry to develop the high-efficiency grain boundary diffusion technology.