Improving Oxidation Resistance by Addition of Boron in FeNi-Based Superalloy

This study investigates the effect of trace boron addition on the oxidation resistance of FeNi-based superalloys based on Incoloy 706. Although boron is known to improve high-temperature mechanical properties, its role in oxidation remains controversial because it can either enhance or degrade oxide-scale stability depending on its distribution. To clarify this effect, boron-free, 0.1 at% B-added, and 0.2 at% B-added alloys were fabricated and oxidized at 1023 K under isothermal conditions. In the boron-free alloy, a thin Cr- and Al-rich oxide layer initially formed but rapidly thickened after 50 h, developing into a porous Fe2O3/Cr2O3 scale exceeding 5 μm after 100 h. In contrast, the boron-containing alloys maintained thin and uniform spinel-rich Fe–Cr mixed oxide scales of approximately 400 nm after 100 h, with no significant porosity. TGA/DSC analysis showed gradual weight gain without exothermic peaks in B0.1 and B0.2, whereas B0 exhibited accelerated oxidation after approximately 75 h. Glow discharge analysis of B0.1 revealed boron segregation at the oxide/matrix interface. These results suggest that interfacial boron segregation is associated with improved oxide-scale stability and may contribute to enhanced oxidation behavior.