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

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 a〈112¯〉 dislocation ribbons through the γ′ precipitates by so-called stacking fault shearing. The conditions necessary for the nucleation and propagation of a〈112¯〉 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.