Effect of surface Nd-rich phase and oxygen content of melt-spun flakes on formation of coarse grains in hot-pressed Nd-Fe-B magnet

Nd-Fe-B hot-pressed (HP) magnet prepared from melt-spun MQU-F flakes features coarse grains (CG) with the average size of both 200 nm (CGS) and 700 nm (CGL) at flake boundary. The grain growth at the flake boundary of Nd2Fe14B/α-Fe composite HP magnet before and after diffusion of low-melting-point Pr82Cu18 phase was investigated, revealing the indispensable role of surface RE-rich phase of melt-spun flakes in the formation of CG in HP magnet. The dominant role of surface oxygen content of melt-spun flakes in the formation of CGL has been clarified with etching method. The HP magnet prepared from the etched flakes with dramatically decreased oxygen content exhibits the CG regions merely with homogeneous equiaxed CGS at flake boundary. Consequently, the coercivity (μ0Hc) shows significant increase while remanent magnetization (μ0Mr) inappreciable change. Further investigation with sieving method reveals the elimination of CGL via removal of the fine Nd-Fe-B flakes smaller than 54 μm due to their much higher oxygen content, confirming the dominant role of oxygen content in the formation of CGL. The quantitative analysis on the magnetic properties of the above HP magnets reveals the monotonic increase of coercivity (μ0Hc) and negligible change of remanent magnetization (μ0Mr) with decreased oxygen contents of Nd-Fe-B flakes. The maximum value of coercivity (μ0Hc) increases from 2.26 to 2.47 T as the oxygen content decreases from 0.1692 wt% to 0.079 wt%.