High Temperature Low Cycle Fatigue of GH133 Superalloy

The cyclic stress-strain responce and microstructures(TEM) were studied in LCF samples of 15Cr-27Ni-1.8Ti-Fe alloy. The strain control LCF tests were performed at 600* under two different wave mode:R=-1 and fixed eeeemax. Comparing the results of these two kind tests, the detrimantal effect of average tensile stress was proved. This was comcerned with the influenceof tensile stress on the movement of dislocations and the fromation ocracks.
The observations of dislocations structure indicated that during the cycling both mechanisms causing cyclic hardening and cyclic softening were taking place. Cyclic strain led to generate the stress-field at the interface of γ and γ' phases first. As a result of increasing stress field the dislocations were created.The density of dislocation was further increased by the move, ment of dislocations themself. The interactions between dislocations and γ particles degenerated from pair dislocation cutting γ' particles to single dislocation cutting γ'particles. It led to form more faults and APB'S, and the γ' particles became propressively harder to shear and ultimately led to dislocations looping around particles.
A typical dislocation structure of persistant slip hand, laddie-like stracture was seen asasatureted dislocation structure in the slip band,and the more laddie-like structure formed, the more cyclic softening happened.
The slip band extended to grain boundaries and twin boundaries, The grain diclocations interacted with boundaries, and led to complex boundary dislocations structure the density of dislocations near grain boundaries andtwin boundaries was much higher than others since the boundaries can actedas the barrier and dislocation generator. This would cause to form grainboundary cracks and twin boundary cracks.