Interaction Between Oxidation and Stress at High Temperatures on Scale Growth of Fe-Cr-Al Based Alloy

Abstract

The need for environmentally friendly and energy efficient high temperature components that can operate under mechanical and/or thermal stress has prompted interest in the development of Fe-Cr-Al based alloys. These alloys have been widely investigated, because of their ability to form a protective layer of a - Al203, which is able to withstand further oxidation degradation. However, despite their superior oxidation qualities, alpha-alumina scales are highly susceptible to mechanical damage when subjected to aggressive environments. The origins of such failure can be attributed to the generation and relaxation of stresses during the scaling process. As such, this study has experimentally investigated the interaction between oxidation and stress on Fe-Cr-Al based alloy, Kanthal Al. Oxidation experiments of Kanthal Al were conducted in two parts. First, the alloy's scaling process at rest with respect to intrinsic growth stress and oxide morphology was examined. Second, external stress was applied during oxidation to obtain a comprehensive understanding of its effect on scale growth with comparison to experiments conducted without stress. The formation of compact α - Al203 scales was accompanied by compressive growth stresses on the order of 1 GPa. Prolonged oxidation decreased growth stresses resulting in increased scale porosity. Maximum scale porosity occurred under oxidation at 1300°C. Consequently, the protectiveness of the scale was heavily degraded, as indicated by scale morphology, implying that in-service operation at this temperature or above would be detrimental. Applied tensile stress showed a significant decrease in the development of intrinsic growth stress, suggesting a strong interdependency between scale growth stress and creep deformation at high temperature. There was no measurable change in the scale growth rate, as compared to experiments conducted at rest. Possible explanations include insufficient tensile load and/or drastic increase in spallation/rehealing, both of which simultaneously influences the lifetime of a material under aggressive operational conditions.