Microstructure evolution and strengthening mechanism of FeCo-1.5V0.5Nb0.4 W soft magnetic alloy rolled strip with high yield strength and low coercivity

Abstract

The low strength of soft magnetic alloy limits the ability to operate high speed rotors in aero-generators. Strengthening of soft magnetic alloys remains a challenge, as most methods that enhance strength lead to the deterioration of magnetic properties. In this work, a rolled strip of FeCo-1.5V0.5Nb0.4W (wt.%) alloy with high yield strength and low coercivity was developed. We demonstrated that minor additions of Nb and W has a little effect on the magnetic properties. Due to the microalloying has a negligible effect on magnetocrystalline anisotropy, and low content of the secondary phase. Additionally, solute atoms are enriched at the grain boundaries, which reduces the grain boundary energy and promotes the nucleation and growth of Laves-Co2Nb phase at the grain boundary by grain boundary diffusion. These lead to almost stagnation of grain growth during isothermal annealing at 1123 K for 3 h and stabilization at ∼6 μm. Therefore, based on the reduction of dislocation density by high-temperature annealing to restore the magnetic properties of the alloy, fine-grain strengthening coupled with solid solution strengthening, and a certain amount of Orowan bypass effect and dislocation strengthening contribution, provide for the high strength of the alloy. The yield strength of the rolled strip reached 742 MPa under the conditions of no significant decrease in the saturation magnetization of 2.39 T and an extremely low coercivity of 152 A/m. These results are of great significance for understanding the relationship between the strengthening mechanisms of metallic materials and further developing high-performance FeCo-based soft magnetic alloys.