Temperature-Dependent Misfit Stress in Gamma Double Prime Strengthened Ni-Base Superalloys

Misfit stresses in the γ″-strengthened Ni-base superalloy Inconel 718 were calculated from the measured constrained misfit strain using Eshelby’s inclusion method. The constrained misfit strains of the γ″ precipitates were measured using neutron diffraction at various temperatures with the aid of the stress-induced variant selection method. Eshelby tensor was calculated using the expressions for the case of anisotropic matrix given by Mura. Results show the presence of significant compressive misfit stresses in the γ″ precipitates with an anisotropic distribution, namely 3.0 GPa along the habit plane and 1.7 GPa along the plane normal direction at room temperature, and 2.0 and 1.2 GPa at 664 °C. The decrease in misfit stresses was due to the decrease in stiffness and the different coefficients of thermal expansion of the γ and γ″ phases. The average internal stresses in the γ matrix due to lattice misfit were found to be ~ 329 MPa at room temperature and ~ 186 MPa at 664 °C in tension. The possibility of relieving such high levels of misfit stresses in precipitates by loss of coherency during continued growth of precipitates is also discussed.