Brittle to ductile behavior transition of β-NiAlPt coated Ni-based single crystal superalloy at high temperature
Résumé
In service, turbine blades undergo mechanical stresses at elevated temperatures and severe atmospheres (oxidation and corrosion). To meet these specifications, Ni-based single-crystal superalloys were generally overlaid with Thermal Barrier Coating (TBC). TBC systems insure a reliable design for these cooled structural components during several thousands of hours of flights. TBC systems are layered materials constituted of a thermally insulated zirconia, which is deposited on top of a bond coating. Bond coatings are commonly made of NiAlPt or MCrAlY alloys. They was thought to prevent the superalloy from oxidation/corrosion, to lower thermal expansion mismatch between the superalloy and the zirconia and to avoid premature crack initiation due to a sufficiently high brittle to ductile transition temperature. However, such layered materials evolve in service due to the combined effect of the oxidation, the interdiffusion and the mechanical stresses at high temperatures. These evolutions as rumpling/ratcheting, edge delamination, phase transformations, voids formation, crack initiation are deleterious for the lifetime of turbine blades. Furthermore, the properties of the TBC system, i.e. the thermal expansion and the mechanical properties, might also change during blade life and so reduce reliability of design. Therefore, improvements in the prediction of the mechanical behavior and the lifetime of turbine blades require the understanding and the quantification of these evolutions. Currently required experimental data are missing to complete the modeling of the overall behavior of the TBC systems. Following study aims to contribute to fulfill a complete experimental database related to the intrinsic properties of each layer at different ageing times.
This study deals with the local characterization of the mechanical and physical behaviors of a β-NiAlPt coated AM1 superalloy up to 1100°C. Free-standing microtensile specimens were extracted from the substrate, the interdiffusion zone and the bond coating in order to assess the thermal expansion and the mechanical intrinsic behavior of the specific layers. Tensile experiments have been carried out in a temperature range of 700 to 1100°C. A database on mechanical and thermal expansion properties has been set up to describe the gradient of properties existing in such a system at operating temperatures.