Ultra-high annealing stability via the synergistic homogenization mechanism of Cu-P clusters in Fe-P-based nanocrystalline alloy

•A novel microalloying approach is proposed to design the Fe-P based soft magnetic nanocrystalline alloys with ultra-high annealing stability.•The immiscible element, Cu, are introduced into the Fe-P based nanocrystalline alloys, and the carrier effect of P on Cu is utilized to spontaneously form Cu-P clusters.•P acts as heterogeneous element "as-spun binder" and "thermal dispersant".•The decomposition of Cu-P clusters and the residual P-rich clusters cooperate to homogenize the α-Fe grains. Herein, a novel Fe-P-C-B-Cu nanocrystalline alloy with 0.6 at.% Cu is reported, which has extremely high thermal stability and excellent soft magnetic properties. We exploited the tendency of P element to bond with metal elements and observed that Cu enhances its distribution uniformity in the matrix by spontaneously forming Cu-P clusters. Notably, P can disperse immiscible elements during annealing process, meaning that the decomposition of Cu-P clusters precipitated nano fcc-Cu phase. This, in turn, facilitates the heterogeneous nucleation of α-Fe and further improve the grain refinement of α-Fe. Additionally, the residual P-rich clusters significantly hinder the coarsening of α-Fe grains, leading to the formation of stable, uniform and fine crystal microstructure. This work will provide new insights into the design, manufacture and commercialization of Fe-based soft magnetic materials with wide annealing temperature windows and ultra-high thermal stability.