A New Hyperbolic Tangent Modelling Approach for Creep of the Single Crystal Nickel-Based Superalloy CMSX4

Rolls-Royce has developed a hyperbolic tangent representation of creep, referred to as CT07 [1], which explicitly relates the accumulated creep strain to the current time, stress and temperature. Although, such an approach has been successfully applied to a number of nickel-based superalloys, it will be shown that it does not adequately capture the shapes of the creep curves and rupture times of the single crystal CMSX4 at temperatures below 800 degrees C and above 1000 degrees C. This result arises from the implicit assumption made in CT07 that the generic shape of the creep curve is tertiary dominated, which is not observed in CMSX4. This paper presents a new hyperbolic tangent formulation that accounts for sigmoidal creep, the large primary creep strains observed at low temperatures, the tertiary dominated creep behaviour at high temperatures (850-1000 degrees C) and the secondary creep dominated response above 1000 degrees C. A microstructure-explicit creep model has been used to derive parameters, such as the incubation time at low temperatures, which feed into the new hyperbolic tangent formulation. It will be shown that the new hyperbolic tangent model predictions of the CMSX4 creep response are in good agreement with the experimental data over a wide range of stresses and temperatures