Embrittlement of gamma titanium aluminides by high temperature oxidation: mechanisms and abatement measures

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The primary objective of the proposal is to tune the surface of gamma TiAl in such a way that thermal barrier coatings can be applied to TiAl components without impairing the mechanical properties of the base material or even with improving them compared to the uncoated state. For this purpose, aluminide (= aluminum diffusion coating) and cathode-sputtered TiAl layers are applied on top of the titanium aluminide alloys followed by halogenation, so that a protective aluminum oxide top layer is formed during the subsequent heat treatment, which acts as a diffusion barrier to prevent further oxygen induced embrittlement. To reach this objective, the influence of the thermal loading during the individual process steps such as pre-oxidation and application of the thermal barrier coating as well as a subsequent high temperature ageing on the microstructure and properties in the subsurface zone have to be fundamentally investigated. Based on this, a mechanistic understanding of degradation and suitable strategies for suppressing subsurface embrittlement are developed. Based on the results of the first two-year project phase, the coating parameters, including composition and thickness of the barrier layers, are to be optimized in the final third year. The evaluation of the lifetime of the coating systems under high temperature stress as well as the completion of the investigations of the mechanical properties of selected variants are in the focus. First promising fatigue measurements show that magnetron sputtered layers with halogenation reduce oxygen embrittlement due to the Al enrichment. The investigations carried out so far have shown that a high proportion of the beta(o)-phase in the starting structure can be classified as particularly critical with regard to embrittlement due to the oxygen-induced conversion into alpha2. Therefore, in the second phase of the project, additional investigations are to be carried out on a Ti-48Al-2Cr-2Nb alloy (available at the partners), since this alloy has a low content of beta(o)-phase. After a profound analysis of the embrittlement mechanism and the effect of the coating systems, selected specimens will be mechanically tested to verify the improved properties of the low beta phase alloy with the protection system.