Abstract
Alloys forming external alumina scales can be used at very high temperature because of the excellent ability of alumina to protect against further reaction with the environment. Traces, ~0.1 at. %, of the so-called reactive elements (REs) are added into alumina- (and chromia-) forming alloys in order to enhance their oxidation performance. In this talk, the formation mechanism of protective alumina scales on several FeCrAl and FeNiCrAl alloys for high temperature application will be first discussed. As demostrated in our paper [1], the interaction between water and REs faciliates the formation of the "protective" alumina scales via the formation of a nanocrystalline alumina layer with yttrium-decorated grain boundaries at the early stages of oxidation. Using state-of-the-art analytical tools such as STEM-EDXS-EELS and NanoSIMS, it has also been revealed that the water/REs interplay leads to the formation of hydride- and hydroxide-nanodomains in the alumina scale. In this context, findings of our latest research on the failure modes of alumina scales will be also presented with the emphasis on the key role of RE particle size distribution and the alloy grain orientation in the protective charactrestics of the oxide scales. Controlled high-temperature oxidation exposures in conjunction with quasi-in-situ experiments revealed that failure of the protective oxide scale occurs at specific, predictable (deterministic) sites on the alloy surface. The new undrestaning that will be discussed in this talk are expected to open up new avenues of research on the development of the next generation of high temperature alloys with superiour properties.