Coupled thermomechanical high cycle fatigue in a single crystal Ni-base superalloy

Abstract

Thermomechanical fatigue (TMF) is the life-limiting damage phenomenon occurring in gas turbine components. Laboratory TMF tests are traditionally used to simulate the operating conditions inside of the turbine. However, typical LCF laboratory TMF test conditions do not account for the superposition of a HCF waveform which occurs during operation. This work studies the effects of the superposition of a HCF waveform during a high temperature hold occurring in both in-phase and out-of-phase bithermal fatigue TMF tests performed on a single crystal Ni-base superalloy. Results indicate that an atypical inelastic deformation–environmental degradation interaction is caused by the modification of the waveform due primarily to the change in the mean strain during HCF loading. The effect on life was found to be severe under certain conditions. Analysis of the deformation response and the extent of environmental degradation, in the form of γ′ depletion, are presented. An existing physics-based life model (Amaro, R.L., Antolovich, S.D., Neu, R.W. Mechanism-based life model for out-of-phase thermomechanical fatigue in single crystal Ni-base superalloys. Fatigue Fracture Eng Mater Struct; 2012) [1], which explicitly accounts for inelastic–environmental interactions, is used to provide insight to the likely dominant deformation modes.