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

TL;DR

This study investigates the microstructural evolution of a nickel-based superalloy under high temperature fatigue, revealing distinct microstructure domains and coarsening mechanisms influenced by internal stresses and cyclic loading conditions.

Contribution

It provides a detailed microstructural map of a superalloy during high temperature fatigue, linking microstructure types to fatigue cycle number and strain amplitude, and discusses coarsening mechanisms.

Findings

Identification of anisotropic and homogeneous microstructure domains.

Coarsening occurs even at low cycle numbers under alternate fatigue.

Microstructural changes are driven by internal stresses and diffusion processes.

Abstract

The microstructural characteristics of a single crystalline nickel based superalloy (AM1) tested under high temperature fatigue at 950$^\circ$C are reported. For repeated fatigue (R$_\varepsilon$=0) through a range of cycle numbers N with imposed total strain amplitude $\Delta\varepsilon^t$, two main types of behaviour are found depending on N and $\Delta\varepsilon^t$. This allows a map of microstructures versus the number of cycles N and $\Delta\varepsilon^t$ to be constructed. A domain called A presents anisotropic microstructures due either to a partition of the plasticity throughout the k channels, or to an oriented coarsening of the k% 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 V\'eron. Alternate fatigue (R$_\varepsilon$ = -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.