EFFECT OF PRE-AGEING ON THE LOW CYCLE FATIGUE BEHAVIOUR OF A COATED NICKEL-BASED SUPERALLOY

Isothermal low cycle fatigue (LCF) experiments under strain control have been performed on specimens made from nickel-based superalloy with a thermal barrier coating (TBC) system. Specimens were tested in as-processed and in pre-aged condition in order to achieve infor-mation on interaction between fatigue and ageing. The damage features and their evolution were investigated by microscopic and microanalytic methods. Cracks were initiated in most cases at the interface between the metallic oxidation protection layer of the TBC system and the adjacent thermally grown oxide. The observed crack growth was assisted by severe oxidation once the crack reached the nickel-based substrate. Ageing of specimens before the LCF test changed the crack feature and crack growth rate drasti-cally; pre-aging at 950°C for 250 h in air resulted in crack deviation at the diffusion zone be-tween bond coat and substrate with a significant slow down of the crack propagation into the substrate, while extended pre-aging for 500 h did not cause expressed crack deviation but compared to the as coated condition a deceleration of crack propagation. Microstructural analyses showed that formation of a chain of Cr-rich intermetallic precipitates caused the crack deviation after 250 h pre-ageing, while after 500 h a wide meshed network of precipi-tates formed, which rather slowed down the crack propagation than causing crack deviation. The obtained results are set into context with lifetime assessment methods.