Enhancement in hard magnetic properties of nanocrystalline (Ce,Y)–Fe–Si–B alloys due to microstructure evolution caused by chemical heterogeneity

Abstract

To release the pressure on the supply of critical rare earths (REs), much effort has been made to substitute Nd and Pr by more highly abundant La, Ce, and Y elements in Nd–Fe–B magnets. The chemical heterogeneity reported in multi-main-phase sintered magnets is a promising solution to suppress the magnetic dilution of La-, Ce-, or Y-added Nd–Fe–B magnets. Here, we investigated the nanocrystalline (Ce_1−xY_x)₁₇Fe₇₅Si₃B₆ (x = 0–0.6) alloys prepared by melt spinning and observed an unusual chemical heterogeneity in the (Ce_0.5Y_0.5)₁₇Fe₇₅Si₃B₆ alloy. Here, the Y segregation in RE₂Fe₁₄B (2 : 14 : 1) phase and Ce segregation in REFe₂ (1 : 2) phase were demonstrated due to the significantly different Ce and Y diffusion rates in these two phases. As a result, an extremely high coercivity H_c of 432 kA m⁻¹ and a greatly enhanced maximum energy product (BH)_max of 7.1 MGOe with Curie temperature T_c of 547 K were obtained. In the alloys with x = 0.1–0.4, H_c decreased with Y substitution but the remanence J_r, (BH)_max and T_c did not increase significantly despite the higher magnetization M_s and T_c of the Y₂Fe₁₄B phase than those of Ce₂Fe₁₄B. The reason for this could be attributed to the formation of a large amount of 1 : 2 phase and the insignificant Y and Ce segregations. For the alloy with x = 0.6, though a further increase of J_r was obtained, (BH)_max value of 7.2 MGOe was similar to that of the alloy with x = 0.5 because H_c was significantly reduced. The unique microstructural evolution was responsible for the notable change in magnetic properties. The nano-level chemical heterogeneity manifested by the present work offers a potential approach to improve the cost performance and understand the physical mechanism of free-RE magnets.