Study of the mechanisms of initial oxidation and of the interaction with reactive elements in the halogen effect for Ni-base alloys

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Ni-base superalloys with a relatively low amount of Al (2-5 wt.%) are often used in high temperature technology, e. g. turbine blades in stationary gas turbines and aircraft engines. Due to their low Al-amount these alloys are unable to form a protective alumina scale by themselves. Therefore Al-rich coatings are applied to obtain a protective alumina scale. By using the halogen effect a dense protective alumina scale can be achieved even from the alloy itself. Fluorine is used because of giving the best results. The adherent alumina scale shows a good oxidation protection under isothermal conditions and for long oxidation times. However during cyclic oxidation the protective alumina scale partly spalls, probably due to a mismatch in CTE of the alloy and the top scale. The Reactive Element (RE) effect reduces the oxide growth rate, changes the structure of the top scale and increases the adherence of the oxide scale. Whereas in the present project phase the influence of the halogen effect has been studied, the proposed renewal phase will focus on the mechanism by combining both effects, i. e. halogen effect and Reactive Element effect. The investigations will include short and long oxidation times. Starting with isothermal oxidations to understand the behaviour of the halogen and the reactive elements the emphasis will concern with oxidation under cyclic oxidation conditions. A Ni-based model alloy and 1st and 2nd generation Ni-based superalloys are chosen for the studies. Three methods of RE-addition are compared: The reactive elements Y and Hf are doped on the surface via ion implantation and magnetron sputtering, whereas a volume addition is performed by melting. As result an optimized data base for the combined halogen and RE-effect will be obtained to improve the adherence of the formed protective alumina scale.