Benefit of Ni addition in Dy-Co diffusion alloys for enhancing the coercivity and corrosion resistance of sintered Nd-Fe-B magnets

Commercial Nd-Fe-B magnets without heavy rare earths (HREs) exhibit low coercivity and poor chemical resistance. Here, the coercivity and anti-corrosion performance of sintered Nd-Fe-B magnets have been successfully improved by grain boundary diffusion of Ni alloyed Dy-Co alloy. Starting from Dy-Co binary alloy, the composition of diffusion source was optimized by modifying Dy/Co ratio and Ni substitution for Co. The coercivity of the magnet was enhanced by Dy₆₀Co₁₀Ni₃₀ diffusion with relatively low Dy content from 1362 to 1853 kA/m, which is even 104 and 91 kA/m higher than those of the Dy₈₀Co₂₀ and Dy₇₀Co₁₅Ni₁₅ diffused magnets, respectively. The formation of higher anisotropic field of (Nd,Dy)₂Fe₁₄B and deep diffusion of Dy are the main reasons for the significantly enhanced coercivity. Partial substitution of Co by Ni cannot only promote the infiltration of Dy into the magnet, but also reduce the cost and improve the performance/cost ratio of diffused magnet. Meanwhile, Dy₆₀Co₁₀Ni₃₀ diffusion also improves the corrosion resistance of the magnet by minishing the corrosion electric current density from 9.93 to 3.45 μA·cm⁻². The present results indicate that Ni in diffusion source is conducive to simultaneously improving coercivity and chemical stability.