A NEW HYPERBOLIC TANGENT MODELLING APPROACH FOR THE CREEP BEHAVIOUR OF THE SINGLE CRYSTAL NICKEL-BASED SUPERALLOY CMSX4

Rolls-Royce has developed a hyperbolic tangent representation of creep, referred to as CT07 [1], whic h explicitly relates the accumulated creep strain to the curr ent 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 t he single crystal CMSX4 at temperatures below 800°C and above 1000°C. This result arises from the implicit assumpti on 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 str ains observed at low temperatures, the tertiary dominated creep behaviour at high temperatures (850-1000±C) and the secondary creep dominated response above 1000±C. A microstructure-explicit creep model has been used to derive parameters, such as the incubation time at low temperature s, 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.