Influence of Mo and Al co-addition on the crystallization behavior and soft magnetic properties of FeBSiCuNb alloys

The synergistic co-addition of trace Mo and Al provides an effective pathway to regulate nanocrystallization in Fe-based soft magnetic alloys through concurrent thermodynamic and kinetic control. In this work, the crystallization behavior, microstructural evolution, and soft magnetic properties of Fe 80.5 B 12 Si 4 Cu 1.5 Nb 1 Mo x Al 1- x ( x = 0.2, 0.4, 0.6 & 0.8 at.%) alloys were systematically investigated. Differential scanning calorimetry reveals a typical two-stage crystallization behavior, in which the primary crystallization peak originates from the overlap of pre-existing nucleus growth and newly activated nucleation processes. The Mo/Al ratio is found to strongly affect the thermal stability of the amorphous precursor and the nanocrystallization temperature window. Al addition promotes the formation of high-density pre-existing nuclei, while Mo effectively suppresses grain coarsening by increasing the kinetic barrier for atomic diffusion during annealing. As a result, an optimized nanocrystalline α-Fe(Si) structure is obtained after annealing. Among all the investigated alloys, the one with x = 0.6 exhibits a refined nanocrystalline microstructure with an average grain size of approximately 20.9 nm, achieving a favorable combination of high saturation magnetic flux density (1.671 T) and low coercivity (12.45 A/m). These results demonstrate that balancing thermodynamically promoted nucleation and kinetically inhibited growth via Mo/Al co-addition is an effective strategy for developing high-performance Fe-rich nanocrystalline soft magnetic alloys.