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

The microstructural characteristics of a single crystalline nickel based su peralloy (AMI) tested under high temperature fatigue at 950 degreesC are re ported. For repeated fatigue (R-epsilon = 0) through a range of cycle numbe rs N with imposed total strain amplitude As', two main types of behaviour a re found depending on N and Delta epsilon (t). This allows a map of microst ructures versus the number of cycles N and Delta epsilon (t) to be construc ted. A domain called A presents anisotropic microstructures due either to a partition of the plasticity throughout the gamma channels, or to an orient ed coarsening of the gamma' precipitates of the so-called type N (rafts per pendicular to the loading axis). The domain called H shows homogeneous defo rmation microstructures. The coarsening observed in repeated fatigue is acc ounted for on the basis of a model proposed earlier in the literature by Ve ron. Alternate fatigue (R-epsilon = -1) also leads to the same type of coar sening, 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 po ssible shearing of the precipitates and to different diffusion processes. ( C) 2001 Elsevier Science BN. All rights reserved.