Creep and Tensile Behavior of a Nickel-Based Single Crystal Superalloy With a Bimodal γ′ Precipitation

High temperature aircraft engines components made from single crystal nickel-based superalloys typically have homogenous γ/γ′ microstructure after conventional solution and aging heat treatments. However, manufacturing processes as well as service conditions may alter the material’s microstructure which can lead to a multimodal γ′ precipitation with regular secondary precipitates (400 to 500 nm) and smaller (10 to 100 nm) tertiary precipitates. In this study, such a bimodal microstructure was first obtained after a heat treatment study and the impact of such a microstructure on mechanical behavior was then investigated. Tensile and creep properties are sensitive to such a bimodal microstructure below 900 °C with a 60 to 120 MPa reduction in yield stress and a creep lifetime reduced by factors of 2 to 15 compared to a unimodal reference microstructure. It is suggested that tertiary γ′ precipitates facilitates secondary γ′ shearing reducing tensile and creep properties. Above 900 °C, no difference in tensile and creep behavior has been observed between both kinds of microstructure as tertiary precipitates are rapidly dissolving.