Influence of phase shift and strain path on the thermomechanical fatigue behavior of CMSX-4 specimens

In this paper, the fatigue and lifetime effects resulting from thermomechanical loadings with different phase shifts between temperature and mechanical strain and with different strain paths are investigated for the single crystal nickel-based superalloy CMSX-4. In-phase tests, experiments with a phase shift of −20° between mechanical strain and temperature as well as tests with a non-proportional ratio of mechanical strain and temperature were carried out. It could be shown that the induced stress alone does not allow a lifetime estimation. For all the tests carried out at the same mechanical strain range but different phase relations the induced stresses were approximately the same but the resulting lifetime varied. It turned out that the time interval in which high temperature and high mechanically induced tensile stresses are acting simultaneously is a major aspect for the damage development. For cracks connected to the surface the crack propagation is supported by an oxide induced γ′-dissolving, whereas mechanically induced γ′-dissolving may arise in the region between approaching internal cracks.