Determination of mechanical degradation of FeAl coatings at high temperaturesThursday (29.09.2016) 09:30 - 09:45 Part of:
The oxidation protection of Fe-base superalloys at high temperatures relies on the formation of a protective oxide scale, which generates a diffusion barrier and decelerates oxidation and corrosion under service conditions. In order to extend the lifetime, the surface of components is enriched with chemically stable oxide formers e.g. Al via various manufacturing methods such as pack cementation. This surface enrichment leads to the formation of an intermetallic FeAl region, which acts as a reservoir for the oxide former and ensures the stable growth of the oxide. The coating and the substrate form a composite material system, in which the chemical and mechanical gradients govern the lifetime of the component. Under oxidizing conditions, due to the Al-depletion by oxidation and interdiffusion, the microstructure of the coating is altered as a function of time and temperature. Hence the mechanical properties and the mechanical impact of the coating on the substrate are modified at any point of oxidation. Especially for thin-walled components, determining the mechanical contribution of the coating is crucial, in order to forecast the lifetime of the component under service conditions.
In this study, rectangular beam samples of pack-aluminized Alloy 800H are oxidized in laboratory air under thermocyclic and isothermal conditions in the range of 800 – 900°C and 4-point bending test with in-situ acoustic emission measurement is performed subsequently at room temperature in order to determine the critical strain under tensile loading for each oxidation case. Al-Concentration in the coating after oxidation is determined by EPMA measurement, and the Young’s Modulus of the coating zones is measured via Nanoindentation. With the help of the stated experimental methodology, mechanical properties of the coating zones were expressed as a function of the Al-concentration.