High temperature fatigue and creep-fatigue behaviors in a Ni-based superalloy: Damage mechanisms and life assessment

The low cycle fatigue (LCF) and creep-fatigue behaviors of Ni-based GH4169 superalloy are investigated by uniaxial strain-controlled fully-reversed testing at 650 degrees C. Compared with LCF tests, the effects of tensile and compressive strain hold times on creep-fatigue lifetimes are experimentally explored with varying total strain ranges in the present work. In order to elucidate the damage mechanisms under complex loading waveforms, an additional series of tests with both tensile and compressive hold times are carried out at a given total strain range of 2.0%. Posterior to the cyclic tests, main-crack-failure modes and secondary cracking modes are studied via optical microscopy (OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) techniques. Main-crack failure mechanisms are examined by the fracture appearance observations. Cracking modes are explored through quantitative characterization on the distributions of secondary cracks in the longitudinal cross sections under different loading waveforms. Moreover, a generalized life model based on linear damage summation (LDS) framework and energy dissipation criterion (EDC) is elaborated to estimate the damage mechanisms of fatigue, creep and oxidation. The prediction results can well establish the correlations between the reductions of numbers of cycles to failure and the presences of different damage mechanisms under respective loading waveforms.