Effect of Threshold Stress on Anisotropic Creep Properties of Single Crystal Nickel-Base Superalloy SRR99

The influence of orientation on the creep properties of the nickel-base single crystal superalloy SRR99 was investigated at 760 °C and various levels of applied stress. It was found that the effect of anisotropy was strikingly prominent at this temperature. Deformation mechanisms of single crystals with three principal orientations at 760 °C/790 MPa were explained according to detailed observations of dislocation arrangements by means of transmission electron microscopy (TEM). Liability to shearing γ′ precipitates for [011] orientation and co-planar slip for [11] orientation resulted in poor creep strength and short stress rupture life. It was also found that the apparent primary creep strain could be measured when the applied stress was increased to 565 MPa. Modified power law equation was adopted and the concept of a threshold stress σ0 which determines dislocation looping or shearing to be activated was then involved. Through detailed calculations, the threshold stress was obtained to further analyse the distribution of the applied stress and better rationalize the anisotropic creep behaviour in the stereographic triangle in combination with TEM observations.