Improved coercivity and squreness in bulk hot-deformed magnets by two-step grain boundary diffusion process

Abstract

A novel method is introduced to develop high coercivity with improved squareness in bulk hot-deformed magnets with a thickness of 5.6 mm. A low-melting-point Tb20Dy10Nd40Cu30 eutectic diffusion source was infiltrated into the thick magnets through grain boundaries as the first-step followed by the eutectic diffusion process using Nd70Cu30. The first step lead to the formation of RE-rich intergranular phase as well as HRE-rich shell surrounding platelet shaped Nd2Fe14B grains while the 2nd step transferred HRE from the surface to the center of the magnet through grain boundaries. After the two-step diffusion process, the squareness factor of the demagnetization curve was improved from 0.83 to 0.91 with the enhancement of coercivity from 2.38 T to 2.43 T. TEM analysis showed that the improved squareness was due to improved uniformity in the microstructure from the surface to the center of the magnet, in particular the formation of RE-rich grain boundary phase. This uniform microstructure reduced the coercivity gradient from the surface to the center of the bulk magnet. This two-step difffusion process can be further exploited as a cost-effective method for coercivity enhancement. A Coercivity of 2.5 T with remanent magnetization of 1.32 T was achieved using only 0.45 wt.% of Dy, which is ≈10 % of that used in the conventional Dy-alloyed sintered magnets with a comparable coercivity. Excellent temperature coefficient of coercivity of –0.41 %/oC was realized in our magnet. This work paves a way toward development of bulk high-coercivity Dy-lean Nd-Fe-B magnets.