Crystallization, Corrosion, and Magnetic Properties of Soft Magnetic Nanocomposite Co_75.4Fe_2.3Mn_2.3Si₂Nb₄B₁₄ Alloy for Elevated Temperature Operation

The Co-based nanocomposite alloys exhibit promising magnetic properties suitable for high-temperature applications. However, their metastable microstructure, especially the intergranular amorphous phase, raises concerns regarding their stability in elevated temperature conditions. In this study, we investigated the case of the Co_75.4Fe_2.3Mn_2.3Si₂Nb₄B₁₄ alloy. Initially, we measured activation energies under non-isothermal annealing conditions using both the Kissinger and Ozawa methods, and then further measured local activation energies by the Ozawa method. The thermal stability of the amorphous phase below the secondary crystallization temperature was explored using isothermal DSC tests, and represented using a time-temperature-transformation (TTT) diagram. Subsequently, we conducted experimental examinations to explore the microstructure evolution and air oxidation behavior at 500°C for up to 20 days. During the 20-day annealing process, abnormal grain growth of secondary crystallization was observed, leading to a deterioration in the soft magnetic properties of the material. In the 20-day oxidized sample, we found that larger crystalline grains were concentrated at the center, while the surface retained nanosized crystalline grains embedded in the residual amorphous phase. This stabilized surface area offers excellent corrosion resistance in the long-term exposure of the nanocrystalline Co_75.4Fe_2.3Mn_2.3Si₂Nb₄B₁₄ alloy.