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 gamma/gamma ' 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 gamma ' 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 degrees 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 gamma ' precipitates facilitates secondary gamma ' shearing reducing tensile and creep properties. Above 900 degrees C, no difference in tensile and creep behavior has been observed between both kinds of microstructure as tertiary precipitates are rapidly dissolving.