Microstrain evolution during creep of a high volume fraction superalloy

The creep of superalloys containing a high volume fraction of γ′ phase is significantly influenced by initial misfit and by the evolution of internal stresses. An in situ neutron diffraction technique was used to monitor elastic microstrains in a polycrystalline superalloy, CM247 LC. The misfit was nearly zero at room temperature and it increased to −0.17% at 900 °C. These values are rationalized in terms of thermal mismatch using an eigenstrain formulation and a simple formula is derived to relate the thermal mismatch to the misfit strain. During creep at 425 MPa at 900 °C, the material exhibited primarily tertiary behavior. For grains with [0 0 1] axis close to the loading direction, the elastic microstrain in the loading direction increased with creep time for the γ′ phase, whereas the opposite occurred for the γ phase. These results are explained in terms of constrained deformation in the narrow γ channels and by an interface dislocation buildup. TEM analysis of the crept microstructure provides evidence of the interface dislocation network.