BSE micrographs prior to (left) and after (right) thermocyclic exposure at 1100°C showing the microstructural degradation of NiAl diffusion-coatings
|Laufzeit:||01.07.2012 - 30.11.2017|
|Geldgeber:||MTU via BMWi|
Owing to their outstanding high temperature mechanical stability Ni-base superalloys are widely used in aerospace industry, especially as turbine blades. The formation of a protective oxide scale during service at high temperature generates a diffusion barrier and decelerates oxidation and corrosion. In order to extend the lifetime, the components are coated with suitable oxide formers by a CVD pack cementation process. This surface enrichment leads to the formation of an intermetallic region, which under service conditions acts as a reservoir for the oxide former and ensures the stable growth of the oxide. Superalloy, intermetallic coating and the protective oxide build up a composite material system, in which their respective mechanical and physical properties depend on the oxide former concentration. Under thermo-mechanical loading mechanical stresses are imposed due to the mismatch of thermal coefficients, phase transformations, oxide growth, and the mechanical stress relaxation due to creep deformation. These stresses can lead to the fracture of a component of the composite material system and eventually to the fracture of the whole system. Hence it is crucial to determine the stress state for the material system and forecast the tolerable load.
In this study a mechanical approach is developed in order to determine the oxidation lifetime for aluminized Ni-base superalloy. The mechanical model which is created at DFI will be combined with a chemical lifetime model of FZJ (Forschungszentrum Jülich) in order to comprise a global oxidation lifetime model.
To achieve this goal, rectangular beam samples of pack-aluminized Ni-base superalloy are oxidized in laboratory air under cyclic and isothermal conditions at 900, 1050, 1100, and 1150°C, and a 4-point bending test with in-situ acoustic emission measurement is performed subsequently at room temperature in order to determine the critical strain value under tensile loading for each thermo-cyclic oxidation case. Al concentration in the coating after oxidation is determined by EPMA measurement, and the Young's Modulus of the coating is measured by nanoindentation for the corresponding Al concentration. Eventually the experimental results are conveyed into a model which can be used to calculate the mechanical stress in each coating zone. This stress is correlated with the cracking of the oxide scale and therefore with the Al depletion as well as with the determination of the tolerable external load which can be applied.zurück