Primary creep in single crystal superalloys: Origins, mechanisms and effects Original Research Article

Creep deformation of the CMSX-4 nickel-base single crystal superalloy is studied in the range 750–850 °C. Emphasis is placed on elucidating the factors causing primary creep when the tensile stress is orientated within 20 °C of the technologically important 〈0 0 1〉 direction. It is demonstrated unambiguously that primary creep occurs only if a threshold stress of approximately 500 MPa is exceeded; thereafter the accumulated primary creep strain is proportional to the magnitude by which the threshold stress is surpassed. A very high dependence of the strain rate on the applied stress is observed. Transmission electron microscopy confirms that the stress threshold is associated with the movement of image dislocation ribbons through the γ′ precipitates by so-called stacking fault shearing. The conditions necessary for the nucleation and propagation of image dislocation ribbons and the cessation of their movement are considered; it is demonstrated that the anisotropy of creep deformation for tensile loading within 20 °C of 〈0 0 1〉 can be rationalised.