On dysprosium utilisation in multi-main-phase Nd–Dy–Fe–B magnets with core–shell microstructures

The development of high-performance Nd–Dy–Fe–B magnets that minimise the consumption of scarce rare earth (RE) element Dy has been a major pursuit for both industry and academia. Here, we designed an alloy microstructure comprising of a uniform Dy-lean core–Dy-rich shell in a series of multi-main-phase (MMP) Nd–Dy–Fe–B magnets. Assessed by the increment of the coercivity and remanent magnetisation per unit weight percentage of the Dy addition (Δµ0Hc/ΔDy and Δµ0Mr/ΔDy), the resulting MMP Dy1 and Dy3 magnets with an overall Dy level of 1 and 3 wt.% possessed respective Δµ0Hc/ΔDy values of 0.5 and 0.3 T/wt.%, and identical Δµ0Mr/ΔDy values of -0.01 T/wt.%. The resulting MMP Dy3 magnet exhibited a high coercivity (2.4 T), an excellent thermal stability of coercivity (|β| = 0.531%/°C), a high squareness factor (> 95%), with a little diminishment in the remanent magnetisation (1.35 T) and maximum energy product (43.6 MGOe). These properties are superior to the currently available sintered Nd–Dy–Fe–B magnets which utilise higher levels of Dy of 5 wt.%. The Dy-lean core–Dy-rich shell microstructure and the non-ferromagnetic low-Fe RE-rich grain boundary phase jointly contribute to the synergistic magnetic performance. Most importantly, on the basis of multi-scale microstructural characterisation data, the formation and the absence of the Dy-lean core–Dy-rich shell microstructure is rationalised via solid-state-diffusion and solution reprecipitation during liquid-phase sintering. The present work deepens our understanding of constructing uniform Dy-lean core–Dy-rich shell microstructure, and delights a more sustainable utilisation of Dy in high-performance permanent magnets.