Mechanistic understanding of the chemical failure of FeCrAl-RE alloys in oxidising environments

Chemical failure in high temperature alloys, such as Fe-20Cr-5Al with reactive element (RE) additions, occurs when the protective alumina scale is unable to be maintained, leading to catastrophic breakaway oxidation. Voluminous oxides of iron and chromium are then produced and any remaining metal is rapidly consumed by oxidation. The onset of chemical failure is controlled by a large number of factors including alloy composition, microstructure, specimen thickness, oxide scale growth and cracking, thermal history and stress levels within the scale. In this study the oxidation of commercial and specially prepared model alloys has been investigated at temperatures up to 1300degreesC in laboratory air. Two distinct types of failure have been identified. Firstly, in thin samples (similar to200mum), Intrinsic Chemical Failure (InCF) can occur, where the growth of the protective scale depletes the A1 level in the alloy to such an extent that chromia formed by alumina reduction is thermodynamically favourable. The samples adopt a greenish tinge as chromia is formed either below or within the existing alumina scale and this is followed by classical breakaway oxidation. The second type of failure is mechanically induced by cracking and/or spalling of the oxide scale. If this occurs when the aluminium concentration in the alloy is below a critical level C-NOSH (NOSH = NO-Self-Healing), then the protective alumina scale cannot reform and the alloy goes into breakaway oxidation via a Mechanically Induced Chemical Failure (MICF) mechanism. Repeated spallation and regrowth of the oxide scale during the early stages of exposure lead to more rapid degradation, since the aluminium level is quickly reduced below the C-NOSH value. This paper illustrates that the colour changes of the oxide, as oxidation progresses, form a useful guide to the degradation mechanism for a given alloy. This colour change is correlated with the instantaneous parabolic rate constant from thermogravimetric studies and demonstrates that the instantaneous rate constant changes from a parameter that decreases with time to one that increases with time at this transition, precisely pinpointing the onset of breakaway. Results from mechanically weak model alloys have been contrasted with much stronger oxide dispersion strengthened commercial alloys such as PM2000.