Low cycle fatigue behavior of a directionally solidified cobalt base superalloy

An investigation has been made of the low cycle fatigue behavior and microstructure evolution of a directionally solidified cobalt-base superalloy at room temperature, 700 and 850°C under the control of different total strain amplitudes. The results show that at room temperature the cyclic hardening of the alloy appears during the first few cycles, and then a long saturation stage begins. At 700°C, the alloy exhibits a pronounced initial hardening, and a secondary hardening after a short saturation. At 850°C, the alloy shows a continuous cyclic hardening until fracture. Examination by TEM indicates that the initial hardening of the alloy at room temperature is caused by the pile-ups of the stacking faults at the stacking fault intersections, while the saturation is related to the formation of the hexagonal close-packed (HCP) zones and twins. The mechanism of initial hardening at 700°C is similar to that at room temperature, while the stress saturation is due to interaction obstacle to stacking-fault becoming weaker, because of thermal activation. The secondary hardening is attributed to the formation of sessile dislocation tangles. The continuous cyclic hardening at 850°C is related to the interaction between the precipitates (M23C6)and dislocations.