Deformation and cracking near a hole in an oxide-forming alloy foil subjected to thermal cycling

This work investigates the behavior of a hole on an alumina-forming alloy coupon subjected to thermal cyclic loading. The experiments, performed using a unique facility, revealed that a hole often deformed and cracked. To explain the phenomenon, the stress near a hole and the consequent deformation or cracking were analyzed and traced according to the loading cycles by theoretical approaches based on material properties of thermally grown oxide (TGO) and the substrate, including creep deformation, oxidation rate and growth strain, some of which were also measured by the facility, while others, such as the temperature-dependent material properties, were realistically assumed. The analytic solution for a two-dimensional model gave fairly good estimations of the mechanical behavior of the holes in comparison with the experimental observations, and the analytic solution was validated by numerical analysis for a three-dimensional model with TGO on the lateral surfaces. These results demonstrated that, without the cooling air flow, the subsequent thermal gradient or underlying bond-coated superalloy, deformation or cracking may occur near the cooling holes in turbine blades in gas turbine engines purely by thermal cycling, depending on the number of loading cycles.