Thermo-mechanical fatigue behavior and life modelling in nickel based single crystal superalloy

Mechanical strain-controlled thermal mechanical fatigue (TMF) tests were performed on thin-walled tubular specimens made of nickel based single crystal superalloy DD6, and the effects of thermal cycles, phase shift and load-controlled mode on stress-strain behavior and fatigue lifetime were studied. The experimental results indicated that the lifetime of TMF was shorter than that of isothermal fatigue (IF) at the peak temperature due to the asymmetry of stress and additional damage induced by thermal cycles. In addition, the lifetime under out-of-phase (OP) cycles was less than that under in-phase (IP) cycles in mechanical strain-controlled mode, which was in contrast with that in stress-controlled mode. According to the results of viscoplastic analysis based on slip systems with Walker constitutive model, the correlation between the damage generated in single crystal superalloy under different TMF loadings and the microscopic parameters on slip systems was identified. Moreover, the max Schmid stress, max slip shear strain rate, cyclic Schmid stress ratio and slip shear strain range were selected as the damage parameters, and then a new life model based on microscopic parameters was proposed for the TMF life prediction. The predicted TMF life of DD6 under different phase shift and different load-controlled mode was within a factor 2 of the experimental life.