Roles of Cu in Fe-based soft magnetic nanocrystalline alloys with high Fe content

Cu is an essential element that significantly influences the glass-forming ability (GFA), structure, and magnetic properties of Fe-based nanocrystalline alloys. In this work, the effects of Cu on the GFA, saturation magnetic flux density (B_s), coercivity (H_c), and structure of high-Fe-content nanocrystalline alloys, along with the role of annealing heating rate (HR), have been systematically studied. The influence of Cu addition on B_s is revealed: Increment in Cu content induces a competitive mechanism of increasing the volume fraction of α-Fe while decreasing the mass fraction of Fe, causing the initial rise and the eventual decline of B_s. By utilizing the Fe₈₃B_10-xC₃Si₃P₁Cu_x (x = 0.25–1.5 at.%) alloys as the starting point, asynchronous property changes induced by Cu addition in Fe-metalloids-Cu type nanocrystalline alloys, along with the corresponding correlations between Cu content and properties, have been revealed. We further demonstrate that the effect of Cu on crystallization behavior, heating rate requirement, and corresponding coercivity is universal and quantifiable among various Fe-metalloids-Cu type nanocrystalline alloys. By using correlations revealed in this work, Fe₈₅B₈C₄Si₁P₁Cu₁ nanocrystalline alloy with ultrahigh B_s of 1.90 T was successfully developed, proving the potential of our findings to facilitate the design and development of high-B_s Fe-based nanocrystalline alloys.