Oxidation of nickel superalloys for aero-engine applications

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Nickel superalloys are used in aero-engines due to their outstanding mechanical strength up to high temperatures. Thermally grown oxides can weaken the alloys, however, when carefully designed, these oxides are the alloy's first line of defence against corrosion. Oxides can protect alloys by passivating the surface and preventing further reactions. For this, oxides need to be continuous, non-porous and adhere well to the substrate. This leaves plenty of scope for the optimization of alloy composition to improve the oxide scale. The aim of your PhD project is to optimize the composition to create a strong oxide through targeted alloying additions. In particular, the effect of trace amounts of Si have been shown to promote the formation of a continuous, protective layer of alumina on the surface. This layer of alumina is more protective than the chromia or nickel-based spinels that normally form. However, there is an optimal amount of silicon beyond which the effect will be deleterious both for oxidation and for the precipitation of embrittling phases within the alloy, causing microstructural instability. Silicon in the oxide scale has been shown to interact directly with tantalum, though no systematic study on the topic has ever been performed and the mechanisms are not well understood