Investigation on microstructure evolution and fracture morphology of single crystal nickel-base superalloys under creep-fatigue interaction loading

Creep-fatigue interaction behavior of a single crystal nickel-base superalloy was studied at 980 °C and the microstructure was quantitatively analyzed. A three-stepped plate specimen had been designed to simultaneously observe microstructure changed under three different stresses. It was found that the distribution of γ′ size was basically consistent with lognormal probability scale under all three loading conditions. The average size of γ′ precipitates increased with the increasing of stress at the same interrupted time. Four different layers including oxide layer, γ′-free layer, γ′-reduced layer and γ/γ′ layer can be observed on the cross-section of the specimens. The thickness of γ′-free layer also increased with the increasing of stress. Numerous dimples and square-shaped facets were observed at the fracture surface and furthermore propagation of cracks from micropores to the boundary of the facets could be found, which showed the fracture behaviours of both creep and fatigue damage.