Effect of Phase Angle on Thermo-Mechanical Fatigue Lives of Alloy 617

Alloy 617 is an important candidate material for the boilers and turbines in the advanced ultra-super critical (A-USC) energy system. Here, the understanding of thermo-mechanical fatigue failures is a critical issue to be concerned. In this program the TMF lives were investigated, in comparison with the isothermal fatigue lives. A special attention was paid to the effect of strain/temperature phase angle. It was found that the TMF lives under the out-of-phase condition were significantly shorter than those under both the in-phase TMF and the isothermal low-cycle fatigue lives. This behavior was discussed considering the understanding of crack propagation process, where a new simple energy density parameter was proposed based on non-linear fracture mechanics analysis. The analysis strongly suggested that the phase angle effect would be attributed to the level of mean stress that depended on the TMF test conditions. The similar approach was applied to the TMF lives of a single crystal Ni-based superalloy, showing the similar conclusion.