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Influence of the microstructure fineness on the high temperature oxidation during heating and on the oxide spallation during cooling for carbides-strengthened cast super alloys

Patrice Berthod, Aur�?�?�?�?�?�?�?©lie Chiaravalle, Steven Raude

The heating parts and cooling parts of thermogravimetry curves obtained for nickel-based and cobalt-based superalloys, solidified either slowly or more rapidly, were studied in order to look for a possible influence of the microstructure fineness on the first stage of oxidation (at heating) and the resistance of the external oxide isothermally formed against spallation (at cooling). Compared with a fine microstructure, a coarse microstructure obtained by a slower solidification tends to enhance oxidation at heating, with a lower temperature of oxidation start and finally a higher mass gain at the end of heating. A coarse microstructure also favours spallation, which may be due to the greater quantity of oxide to which the coarse microstructure led because of a slightly faster isothermal oxidation compared to the fine microstructure. A high temperature of isothermal oxidation, and the presence of other very oxidable elements such as tantalum, also tend to enhance spallation at cooling by either increasing the quantity of oxide present before cooling, or by reducing the adherence of external chromia on the alloy surface.