Crack formation and microstructure-sensitive propagation in low cycle fatigue of a polycrystalline nickel-based superalloy with different heat treatments

Gas turbine engine materials demand high performance of fatigue life under alternative loading. In this paper, crack initiation and propagation in the low cycle fatigue (LCF) of a polycrystalline nickel based superalloys was studied via experimental and numerical methods. LCF experiments were conducted with the specimens subjected to different heat treatment regimes, including of standard heat treatment (SHT), heat isostatic pressing (HIP), and combined heat treatment (HIP + SHT). Simulations based on finite element method (FEM) were implemented with the 3D digital model obtained from synchrotron radiation X-ray computerized tomography (CT) to investigate the effect of microporosity. Results indicated that microporosity was a dominant factor of fatigue crack and the parameters of microporosity including of porosity, effective diameter, and distribution co-contributed to affect fatigue life. The delta stack and carbide were observed to be fatigue crack sites. The HIP specimen had long fatigue cycle life since casting pore and acicular delta were eliminated, while further SHT can reintroduce delta into matrix resulting in high strength and low fatigue life. LCF crack behaviors in HIP + SHT superalloy depended on surface quality, stress concentration, and the quantity of carbide.