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Effect of a high hafniumcontent on the high temperature oxidation of chromium-rich cast alloys. Part 3:Oxidation at heating ofNi-based alloys

Patrice Berthod, Elodie Conrath

Hafnium is added to high temperature alloys to improve their resistance against hot oxidation. Despite that this element is often present only in small quantities (around 1wt.%), it plays an efficient role. It is possible to add it in much higher quantities, for example in carbon-containing alloys inwhichHf can be trapped in carbides. It is therefore interesting to see whether its beneficial effect on high temperature oxidation is stronger or remains the same or, on the contrary, or whether too much hafniumbecomes detrimental for oxidation resistance. Here, cast chromium-rich nickel-based alloys containing particularly high Hf quantities were subjected to oxidation at 1000 and 1100°C in dry air for 46 hours, similarly to what was earlier done for similar alloys but at 1200°C. The heating parts of the obtained thermogravimetric curveswere exploited after correction fromair buoyancy variation. The temperatures of oxidation start, the total mass gain during heating, the instantaneous linear constant versus temperature and the transient oxidation linear constant were determined and analysed versus the contents of the alloys in carbon and in hafnium. This led to interesting observations, notably concerning the oxidation start temperature and its dependence on the chemical composition of these nickel-based alloys.