Effects of Tensile Stress on the High-temperature Oxidation of an Fe–38Ni–13Co–4.7Nb–1.5Ti–0.4Si Superalloy in Air

The oxidation behavior of an Fe–38Ni13Co–4.7Nb–1.5Ti–0.4Si superalloy (Incoloy 909 type alloy) under tensile stress conditions has been investigated in the temperature range of 1100 to 1300 K in air. Optical microscopy, X-ray diffraction measurements, and electron probe microanalyses on the oxide scales formed during oxidation indicate that the scales consist of an external scale and an internal scale which has an intergranular scale (above 540 ks at 1100 K and above 1200 K) and an intragranular scale. The oxide phases in each scale are identified as (α-Fe2O3 and CoO·Fe2O3) and (FeO·Nb2O5 and α-Fe2O3), respectively. An effect of tensile stress on the compositions of the scales is not observed. Although the effects of the tensile stress on growths of the external scale and the intragranular scale are small, the growths of the thickness and the width of the intergranular scale are remarkably promoted as the tensile stress increases. The stress accelerated grain boundary oxidation in this alloy is confirmed distinctly.