Investigation of γ/γ′ lattice mismatch in the polycrystalline nickel-base superalloy IN738LC: Influence of heat treatment and creep deformation

Abstract

The γ/γ′ lattice misfit in the polycrystalline nickel-base superalloy IN738LC has been determined by convergent beam electron diffraction (CBED) technique in a transmission electron microscope (TEM). The influence of heat treatment and creep deformation on the misfit was investigated. The chemical compositions of the γ and γ′ phases were analysed with the help of Energy Dispersive Spectrometry (EDS). The influence of different heat treatments on the γ/γ′ misfit can be explained in terms of the dependence of the chemical compositions of γ and γ′ phases on the heat treatments. The creep deformation causes a tetragonal distortion in γ′ precipitates and leads to considerable changes in the misfit. The tetragonality in γ′ phase and the evolution of the misfit during creep process can be understood on the basis of a dislocation model. An estimation of the local internal stress in γ′ phase generated by γ/γ′ interfacial dislocations formed during creep deformation shows it to be higher than the applied stress.