Effect of systematic pre-ageing on crack Initiation and Propagation in a coated superalloy during low cycle fatigue at 950°C

Low cycle fatigue (LCF) experiments have been performed at 950°C with R = -1 under strain control on specimens made from cast nickel-based superalloy IN 100 coated with a NiCoCrAlY oxidation protection layer. Specimens were tested after processing and systematic pre-ageing, respectively. The LCF tests were interrupted after defined number of cycles, and the specimens were investigated by microscopic and microanalytic methods for characterizing the effect of systematic pre-ageing on crack initiation and propagation. The microstructural evolution due to ageing only has been systematically investigated on separate specimens in order to achieve information on interaction between fatigue and ageing. Main observations were (1) due to pre-aging a thermally grown oxide (TGO) layer formed on the coating surface, and at the interface between substrate and coating a diffusion zone with intermetallic phases evolved, (2) fatigue cracks were initiated at the oxidized surface, predominantly at rumpling instabilities of the TGO, and (3) crack propagation was influenced by the spatial distribution and morphology of intermetallic phases in the diffusion zone between coating and substrate. The obtained results reveal complex interaction of mechanisms due to ageing and LCF loading. The observed effects on crack initiation and growth will be discussed with respect to strategies for accelerated testing by separation of pre-aging and fatigue and prospects to life time extension of components