Modelling and prediction of the stress rupture behaviour of single crystal superalloys

This paper is concerned with modelling and prediction of the stress rupture properties of several single crystal superalloys. The stress rupture theory is based on a modified form of the damage mechanics equations of Kachanov and Rabotnov. The ultimate tensile strength (UTS) is included and acts as an upper bound in these equations, which corrects for the fact that failure should occur when the effective stress reaches the UTS, rather than infinity, and allows the curvature in stress rupture data to be modelled effectively. The interpretation of damage on a microscopic scale is discussed. The time–temperature behaviour of these alloys is shown to obey a stress modified Sherby–Dorn parameter. A consideration is made for the changing strength of the material with temperature by comparing rupture data at constant fractions of the UTS rather than at constant stress. The activation energy used in the Sherby–Dorn parameter is similar to that for self-diffusion of nickel. By basing the rupture model on physical properties (namely the UTS and activation energy for diffusion), predictions of rupture behaviour of new alloys can be made when these corresponding physical properties are determined. However, such predictions of behaviour can only be made accurately between alloys of relatively similar composition. A major distinction is shown to exist between non-rhenium containing first generation alloys, and alloys containing rhenium, due to the increased rupture strength and different rafting kinetics of the latter.