Influence of stress state on the kinetics of γ-channel widening during high temperature and low stress creep of the single crystal superalloy CMSX-4

The present study investigates the kinetics of the widening of γ-channels in the γ/γ′-microstructure of the single crystal superalloy CMSX-4 during high temperature and low stress creep using scanning and transmission electron microscopy (TEM) in combination with quantitative image analysis. γ-channel widening is an important process because it affects dislocation plasticity in the γ-channels which is one key element of the overall creep process in single crystal super alloys. We compare the kinetics of rafting in 〈001〉-tension and in {011} 〈011̄〉-shear creep testing at a constant maximum principal stress σ1=50 MPa at 1080 °C. γ-channel widths follow log-normal-distributions. After subtracting effects due to the change of morphology associated with rafting, the increase of the mean value of γ-channel widths is governed by a parabolic rate law and there is no significant difference between uniaxial 〈001〉-tension and {011} 〈011̄〉-shear creep testing. This suggests that γ-channel widening is controlled by a multiatom diffusion process through the channels and that for multiaxial loading, the normal stress components perpendicular to the γ/γ′-interfaces are important.