A unified constitutive model for the thermomechanical fatigue response of a nickel-base superalloy Rene 80

This work describes the development of a nonisothermal constitutive model to predict the thermomechanical fatigue (TMF) response of a Nickel base superalloy, Rene 80. Nonisothermal deformation mechanisms and temperature history effects are modeled using state variables. The isothermal constitutive model developed by Ramaswamy and Stouffer was chosen as a starting point since it could predict the tensile, creep and fatigue response of Rene 80 successfully at several temperatures. However, the TMF response could be predicted only approximately.; The flow equation of the Ramaswamy-Stouffer model was re-written in the form of an Arrhenius equation with explicit temperature dependence. The model parameters were re-calculated, and the isothermal predictions were correlated with the data at five test temperatures between 538C and 1093C. Thus both the deformation mechanisms in Rene 80, ie. planar slip at low temperatures and dislocation climb at high temperatures were modeled successfully by the constitutive equations. Material parameters were verified using isothermal and nonisothermal tensile calculations at intermediate temperatures. The revised isothermal model was also able to predict the initial TMF response successfully using parameter interpolation.; The subsequent TMF response suggests that temperature history is important during nonisothermal deformation. The high and low temperature deformation mechanisms in Rene 80 produce characteristic microstructures which interact under nonisothermal conditions to produce extra hardening that is not present during isothermal deformation. A state variable approach has been developed to account for interactions between low and high temperature dislocation structures during TMF tests. This approach to model nonisothermal deformation mechanisms is consistent with the state variable approach used to model isothermal response. The nonisothermal model could successfully predict the initial and saturated TMF response, and block isothermal response from several tests.