Deformation behavior and crack mechanism of a first generation single-crystal Ni-based superalloys under thermomechanical fatigue loading

Abstract

A research investigation was conducted to analyze thermomechanical fatigue (TMF) in a first-generation nickel-based single-crystal superalloy. The analysis was conducted within a temperature range of 450–950 °C while employing a strain ratio of −1 during two phases, in-phase (IP) and out-of-phase (OP). TMF life data were collected at varying mechanical strain amplitudes. The IP and OP TMF lifetimes were compared under identical strain conditions. A detailed analysis of the fractures and microstructures of the tested specimens was performed to investigate the crack mechanism. In the IP specimens, the formation of cracks closely followed the direction perpendicular to the principal stress within the material, which was primarily associated with the presence of micropores. Internal cracks tended to develop at locations with strain concentrations and subsequently propagate outward from the surface. The main mechanism behind these cracks was the interaction between creep and fatigue damage. Conversely, in the OP TMF specimens subjected to an oxide scale formed on the fracture surface, leading to fatigue cracking at multiple locations. The cracks gradually extended within the material before transitioning into the (111) crystallographic planes, where localized deformation bands occurred, affecting an area depleted of the γ' phase. This crack mechanism predominantly involved interactions between oxidation and fatigue. Among the various fatigue life prediction models, the Ostergren model exhibited strong agreement with the predicted data points for the fatigue life.