Microstructure evolution mechanisms in nickel-based single crystal superalloys under multiaxial stress state

Thin-walled plate specimens with square and triangular penetration pattern close-packed film cooling holes are used to study the evolution of the γ/γ′-microstructure of nickel-based single crystal superalloys during [001] orientation tensile creep. For both types of penetration patterns, the creep tests were performed at ligament efficiencies of 0.4 and 0.8 at 980 °C and a net section stress of 300 MPa. In contrast, the creep experiment of specimens with square penetration pattern film cooling holes at a ligament efficiency of 0.8 at 1050 °C was also carried out. Quantitative metallographic assessment is used to investigate the influence of the multiaxial stress state on the microstructural evolution of nickel-based single crystal superalloys. The results show that the evolution of the γ/γ′-microstructure in specimens with square and triangular penetration pattern film cooling holes is related to the kinetics of the stress redistribution during tensile creep. A minimum stress level is necessary for rafting to occur at a specific experimental temperature. The contrast between the mechanical results and microstructural findings shows the effect of the maximum principal stress and stress triaxiality on the microstructural evolution of nickel-based single crystal superalloy specimens with close-packed film cooling holes. The rafting orientation of the γ/γ′-microstructure is shown to be sensitive to the direction of the maximum principal stress. Finally, the γ/γ′ topological inversion is faster under higher ligament efficiency.