CYCLIC DEFORMATION OF A DIRECTIONALLY SOLIDIFIED COBALT‐BASE SUPERALLOY

An investigation has been carried out on the cyclic deformation and changes in microstructure of a directionally solidified cobalt‐base superalloy. The tests are conducted at 700 °C and 850 °C in air under different total strain amplitudes. The alloy tested at 700 °C exhibits an initial hardening, a short saturation stage and an evident secondary hardening, while the alloy at 850 °C suffers continuous cyclic hardening until fracture. TEM examinations indicate that the initial hardening of the alloy at 700 °C is caused by the pile‐ups of dislocations and stacking faults at the stacking fault intersections, while the stress saturation is due to the weakening of obstacles against the dislocation movement. The secondary hardening has a contribution from the formation of sessile dislocation tangles. The early stage of continuous hardening of the alloy at 850 °C is related to the pile‐ups of dislocations and stacking faults at the intersections, and the later stage is controlled by the interaction between precipitates and dislocations.