[011] CREEP IN A SINGLE-CRYSTAL NICKEL-BASE SUPERALLOY AT 1033-K

TEM investigations are performed on [011] single crystals of SRR99 after creep at 1033 K under a load of 680 MPa. At the creep rate minimum, multiple slip in the matrix occurs on four octahedral slip systems. Octahedral cross slip or cubic slip is not activated. Due to the superposition of coherency and external stresses in small matrix channels, deformation is concentrated in the ''roof'' matrix channels. During secondary creep, screw interfacial dislocations cross slip in the cube gamma/gamma' interfaces. In this stage, gamma' precipitates are sheared by Shockley super partials. Common shearing of matrix and gamma' particles by Shockley super partials is observed. The high creep rate in the [011] orientation is considered to be caused by the stress concentration in roof matrix channels and the few activated slip systems.