3. Results
3.1. Cyclic response of specimens subjected to TMF loading
During cyclic loading, all hysteresis loops were recorded and cyclic hardening/ softening curves are reported. Hysteresis loops in four types of thermomechanical cycling (IP-TMF, IPD-TMF and OP-TMF and OPD-TMF) between 250 °C and 700 °C with total strain amplitude 0.6% shows Fig. 1. The first loop and the loop at half-life are shown. Since the dwell was always applied at the maximum temperature for IPD-TMF cycle it coincided with maximum tensile strain and for OPD-TFM cycle with maximum compression strain. In the first cycle, the hysteresis loops with the dwell do not significantly differ from those without dwell since the amount of relaxation is small. With increasing the number of cycles material cyclically hardens and the stress relaxation also becomes larger. This leads to the increase of the plastic strain range and also to the modification of the stress cycle asymmetry.
The evolution of the stress amplitude σa plotted vs. the number of cycles N of the material subjected to the OPD-TMF constant strain rate cycling shows Fig. 2. The cyclic response is characterized by rapid cyclic hardening from the onset of the cyclic straining. The pronounced cyclic hardening leads to the saturation of the stress amplitude for all total strain amplitudes (Fig. 2a). The domain of saturation takes more than 90% of the fatigue life. The final very rapid decrease of the stress amplitude is connected with the appearance of the dominant crack and the distortion of the shape of the hysteresis loop. The plastic strain amplitude vs. the number of cycles (Fig. 2b) decreases rapidly at the start of cycling. Later, at the number of cycles corresponding to maximum stress amplitude, the slope of the plot in semilogarithmic coordinates decreases. This decrease is for the majority of the fatigue life very mild and for high strain amplitudes plastic strain amplitude is nearly constant which corresponds to the saturation of the stress amplitude (see Fig. 2a). The evolution of the mean stress during OPD-TMF shows Fig. 2c. The initial slightly positive mean stress increases during the fatigue life up to 90 MPa. The rapid drop of the mean stress in the final stage of cycling is connected with the appearance of macroscopic crack.