Some examples of thermodynamical modelisation use for a better understanding of sub-surface microstructural phenomena after high temperature oxidation of superalloys
Résumé
When superalloys are being oxidized at high temperature, oxide scales grow on their surface, involving some of their constitutive elements. Among them are aluminum and chromium - added to the alloy to form a protective stoechiometric oxide scale - and other reactive elements such as titanium or tantalum. Then, these elements diffuse from an increasing depth in the bulk towards the oxidation front. When temperature and/or time exposition are high enough, concentration gradients appear and a progressive disappearance of phases, dispersed in the matrix and containing some oxidizing elements, can occur. If we know where metallic elements from disappearing phases are going to, it is much more difficult for other constitutive elements. For instance, chromium carbides get dissociated into Cr - involved in the oxide scale growing - and carbon. The latter element can diffuse towards the oxidation front to be oxidized by air into CO2, or can go back into the matrix. Carbon, light element which is moreover present in small quantity in most superalloys (less than 2%w), is difficult to analyse by microprobe. Fortunatly, the association of metallographic examinations, microprobe analysis of heavier elements and thermodynamical modelisation can help to know where light elements are going, to better specified the oxidation phenomena or its microstructural consequences.
In the high temperature oxidation field as well, thermodynamical modelisation is able to give useful indications for a better understanding of both the occurrence of new phases and the destination of light elements. The examples of results given here are overall qualitative, but quantifications are possible by the mean of image analysis. This technique allows the determination of surface - then volume, weight and finally molar - fractions of each phases metallographically observed. Indeed, knowing all the heavy elements contents and quantities of the different phases, then comparing these data to modelisation predictions, this method would indentify the content of the last and light element such as carbon. The classical equation between chemical composition and the following microstructure is then inverted, the new unknown quantity being %C.
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