Measurement of the effective γ/γ′ lattice mismatch during high temperature creep of Ni-based single crystal superalloy

The high temperature effective (constrained) gamma/gamma' lattice mismatch of a Ni-based single crystal superalloy has been measured in a wide temperature range [930 degrees C-1125 degrees C] using three different techniques: in-situ X-ray diffraction under synchrotron radiation, and post-mortem gamma/gamma' interfacial dislocation network mesh size measurements using Transmission Electron Microscopy (TEM) or high resolution Scanning Electron Microscopy (SEM). It is shown that all three techniques are complementary considering precision and spatial distribution of lattice mismatches in the analyzed volume and that they provide a very similar average value of the gamma/gamma' lattice mismatch. Since isothermal creep experiments were performed under variable applied stress, it is deduced that the post-mortem lattice misfit measurements using both TEM and SEM are representative of the very last loading step of the experiments, even if dislocation networks are out of an equilibrium configuration. Contrary to the unconstrained gamma/gamma' lattice mismatch, it is shown that the effective lattice mismatch is a function of the temperature, applied stress and accumulated plastic strain, i.e. of the thermomechanical histories of the samples. (C) 2012 Published by Elsevier Inc.