Low cycle fatigue of a nickel based superalloy at high temperature: deformation microstructures

The microstructural characteristics of a single crystalline nickel based superalloy (AM1) tested under high temperature fatigue at 950°C are reported. For repeated fatigue (Rε=0) through a range of cycle numbers N with imposed total strain amplitude Δεt, two main types of behaviour are found depending on N and Δεt. This allows a map of microstructures versus the number of cycles N and Δεt to be constructed. A domain called A presents anisotropic microstructures due either to a partition of the plasticity throughout the γ channels, or to an oriented coarsening of the γ′ precipitates of the so-called type N (rafts perpendicular to the loading axis). The domain called H shows homogeneous deformation microstructures. The coarsening observed in repeated fatigue is accounted for on the basis of a model proposed earlier in the literature by Véron. Alternate fatigue (Rε=−1) also leads to the same type of coarsening, even for a very low number of cycles (N=113). The microstructures observed are ascribed to internal stress effects and are related to the possible shearing of the precipitates and to different diffusion processes.