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

The microstructural characteristics of a single crystalline nickel based superalloy (AMI) tested under high temperature fatigue at 950 degreesC are reported. For repeated fatigue (R-epsilon = 0) through a range of cycle numbers N with imposed total strain amplitude As', two main types of behaviour are found depending on N and Delta epsilon (t). This allows a map of microstructures versus the number of cycles N and Delta epsilon (t) to be constructed. A domain called A presents anisotropic microstructures due either to a partition of the plasticity throughout the gamma channels, or to an oriented coarsening of the gamma' 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 Veron. Alternate fatigue (R-epsilon = -1) also leads to the same type of coarsening, even for a very low number of cycles (N= 113). The micro structures observed are ascribed to internal stress effects and are related to the possible shearing of the precipitates and to different diffusion processes. (C) 2001 Elsevier Science BN. All rights reserved.