Correlation of microstructure and magnetic softness of Si-microalloying FeNiBCuSi nanocrystalline alloy revealed by nanoindentation

Compared to the commercial soft-magnetic alloys, the high saturation magnetic flux density (B_s) and low coercivity (H_c) of post-developed novel nanocrystalline alloys tend to realize the miniaturization and lightweight of electronic products, thus attracting great attention. In this work, we designed a new FeNiBCuSi formulation with a novel atomic ratio, and the microstructure evolution and magnetic softness were investigated. Microstructure analysis revealed that the amount of Si prompted the differential chemical fluctuations of Cu element, favoring the different nucleation and growth processes of α-Fe nanocrystals. Furthermore, microstructural defects associated with chemical heterogeneities were unveiled using the Maxwell–Voigt model with two Kelvin units and one Maxwell unit based on creeping analysis by nanoindentation. The defect, with a long relaxation time in relaxation spectra, was more likely to induce the formation of crystal nuclei that ultimately evolved into the α-Fe nanocrystals. As a result, Fe₈₄Ni₂B_12.5Cu₁Si_0.5 alloy with refined uniform nanocrystalline microstructure exhibited excellent magnetic softness, including a high B_s of 1.79 T and very low H_c of 2.8 A/m. Our finding offers new insight into the influence of activated defects associated with chemical heterogeneities on the microstructures of nanocrystalline alloy with excellent magnetic softness.