Tailoring Ni, Nb in commercial FeCo-V soft magnetic alloys to promote strength-coercivity synergy

Abstract

High yield strength is crucial for soft magnetic materials (SMMs) applied for high-speed power generators to maximize rotation speed and load. Traditional SMMs are therefore developed with “unclean” microstructures, namely, to increase obstacles like solid solution atoms, grain boundary, or second-phase particle-to-pin dislocation motion and further increase the yield strength. This alloy design strategy poses a significant challenge for coercivity, yet SMMs serving in generators must reduce energy consumption owing to hysteresis losses. This work presents a comprehensive investigation of composition and annealing process optimization to overcome the trade-off between coercivity and strength. Nb and Ni elements addition introduces solid solute atoms and precipitation. Solid solute atoms promote yield strength through solid solution strengthening and coercivity through forming internal stress to reverse domain orientation. Hard magnetic NbCo₃ second-phase particle has a higher anisotropy constant K₁ than the matrix, significantly hinders magnetic domain wall motion while has little effect on dislocation movement, resulting larger increment in coercivity than yield strength. The small-sized precipitations through annealing were considered. Proper annealing process can also obtain γ-fiber texture, low dislocation, and high grain boundary density to elevate yield strength and mining coercivity. The developed FeCo-V-Ni-0.3Nb alloy demonstrates impressive high yield strengths of 764 MPa and low coercivity of 198 A m⁻¹. Compared to commercial HiperCo50 HS alloy, the present alloy exhibits ∼29% yield strength increase and ∼60% coercivity decrease. We also proposed a model $H_{\text{cp}}=\frac{{\delta }_{\text{particle}}{V}_{\mathrm{p}}}{1+\text{exp}\left({\delta }_{\text{matrix}}-AaD\sqrt[3]{\pi /6V_{\mathrm{p}}}\right)}$ to estimate coercivity increment for magnetic second-phase particles.