Thermal-mechanical and isothermal fatigue behaviour of the nickel-base superalloy IN 792 CC

Many components used in high temperature applications are exposed to complex thermal-mechanical loadings during operation. For this reason the effect of start-stop-cycles with thermal-mechanical fatigue (TMF) as consequence was investigated by means of In-Phase (IP) and Out-of-Phase (OP) TMF tests. The fatigue life of the gamma'-hardend nickel-base cast superalloy IN 792 CC decreases with increasing maximum temperatures T-max of the TMF cycles, due to the increasing plastic deformations and the increasing mean stress (OP-TMF) or increasing intergranular damage (IP-TMF), respectively. These relations can be satisfactorily described using the Manson-Coffin-relationship or the damage parameters of Smith-Watson-Topper and Ostergren. By contrast, the influence of different phase shifts between temperature and mechanical loading also cannot be approximately described with one consistant relation between damage parameters and fatigue life. The evaluation of TMF loadings based on results from isothermal LCF-tests with the same frequency and respective mechanical strain leads always to an overrating of the fatigue life, even if the temperature of the isothermal test is the maximum temperature of the TMF cycle. This applies when comparing mechanical loading values as well as when comparing damage parameters.