Dislocation Structure In A Single Crystal Nickel Base Superalloy During High Cycle Fatigue At 870 Degrees C

Dislocation structure and its impact on the mechanical behavior of a gamma' strengthened nickel base single crystal alloy under high cycle fatigue (HCF) at 870 degrees C was studied. The results indicate that HCF lifetime declines with increase of applied stress amplitude. In the early stages, the fatigue deformation occurs by forced bowing of dislocations through the narrow gamma matrix channels on {111} planes. During mid-term fatigue stages, most of the dislocations formed in the matrix are located in the gamma/gamma' interfaces, whose intersection and reaction produce new dislocation segments and three-dimensional dislocation networks. The Burgers vectors of the dislocations in the network are 1/2<110>. The interaction of cyclic stress with high temperature induces the precipitation of homogeneous globular gamma' particles, which is beneficial to fatigue strength. At the end of fatigue test, the cyclic stress leads to the formation of persistent slip bands moving through the gamma matrix channels and gamma' particles. Dislocation shearing through the gamma'phase is found occasionally. The extrinsic stacking faults is also observed.