Fe-based amorphous alloys are widely used in electronic devices such as high-frequency transformers and choke cores due to their low coercivity, low loss, and high saturation magnetic induction intensity. However, these alloys have a relatively low crystallization temperature and are prone to oxidation, which limits their applications in high-temperature environments. The addition of copper and niobium elements can suppress the growth of crystal nuclei and improve thermal stability. However, the influences on the alloy's high-temperature oxidation resistance and structural evolution are still unclear. In this work, static air oxidation is used to investigate the microstructure evolution of Fe73.5Si13.5B9Cu1Nb3 amorphous alloy after high-temperature oxidation and its influence on magnetic properties. Besides, long-time oxidation, say, 3000 hours or longer at 500 ℃, is generally hard to perform in the laboratory. Thus, the Van’t Hoff’s rule is used to evaluate outcomes under the condition of the long-time and relatively low-temperature oxidation through using rapid high-temperature oxidation. Based on Van’t Hoff’s rule, the oxidation at 650 ℃ for 5 min will show similar or more severe oxidation effects on the microstructure of Fe73.5Si13.5B9Cu1Nb3 alloy after oxidation at 500 ℃ for 2730 h. The microstructure evolution reveals that silicon and niobium in this alloy will quickly diffuse toward the sample surface during oxidation at 650 ℃, and these two elements will form a dense layer to impede oxygen diffusion. Meanwhile, an α-Fe(Si) phase, mainly composed of iron elements, will be generated in the alloy, with its grain size slowly increasing in the oxidation process. Thermodynamic analysis indicates that the segregation of silicon and niobium can preserve the thermodynamic stability of the alloy system during oxidation and suppress the formation of intermetallic compounds during crystallization. The magnetic hysteresis loop results show that the coercivity of Fe73.5Si13.5B9Cu1Nb3 alloy after 5-min oxidation at 650 ℃ will stay at approximately 0.3 Oe, suggesting that the Fe73.5Si13.5B9Cu1Nb3 alloy may be a candidate for operating at 500 ℃ for more than 2700 h. Subsequently, its coercivity gradually increases to 61 Oe as the oxidation time rises to 0.5 h, while its saturation magnetic induction intensity remains unchanged (~140 emu/g).
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