Effects of microstructure in high temperature fatigue: Lifetime to crack initiation of a single crystal superalloy in high temperature low cycle fatigue
Résumé
Single crystal superalloys are now the best alloys to make blades and vanes in aero-engines and gas turbines due to their temperature capability, their creep and fatigue resistance. Their composition and microstructure are optimised by heat treatments but low cycle fatigue resistance is mostly controlled by the initiation and early growth of micro-cracks at casting pores. An enriched engineering damage model is proposed to describe micro-crack growth from pores with a process zone concept. Damage equations use summation of contributions on all slip systems as the constitutive model. Oxidation can play a significant role that can be described through embrittled material ahead of micro-crack tip. To investigate the behaviour under small scale yielding at finer scale, experiments were carried out using sharp notches. Early growth of cracks in the notch vicinity was studied at two temperatures 650 °C and 950 °C. While at the lower temperature the notch can be analysed as a crack, anomalous crack growth rates are observed at the higher temperature. Using finite element computations and a damage model, this behaviour is attributed to local viscoplastic strain concentrations, and oxidation effects. Effects of strain concentration are attenuated since only a small volume of material is highly strained at the notch vicinity.