Effect of carbide precipitation on Coffin-Manson relationship of a polycrystalline nickel-based superalloy

Abstract

This study explores the dual-slope Coffin-Manson (C-M) behavior of a polycrystalline nickel-based superalloy, EA, used in turbine engine applications with an emphasis on discerning the micro-mechanisms responsible for it. The motivation for distinguishing the micro-mechanisms responsible for bi-linear C-M behavior stemmed from the earlier evolved comprehensions that state that the fatigue life estimation based on the extrapolation of any single line gives inaccurate results. Transmission electron microscopy (TEM) investigations of fatigue fractured specimens at low strain amplitudes, Delta epsilon/2, revealed that dislocations are homogeneously distributed in the gamma-channels and occasionally form networks at gamma/gamma ' interface. Whereas deformation is heterogeneous at high Delta epsilon/2 owing to the complex dislocation reactions. Cr23C6 carbides are precipitated during the high Delta epsilon/2 fatigue tests, which act as obstacles for dislocations. The deformation heterogeneity resulting from the dislocation-gamma ' precipitate interactions and the dislocation-M23C6 carbide interactions accounts for the dual-slope C-M behavior.