The role of stacking fault shear in the primary creep of [001]-oriented single crystal superalloys at 750°C and 750 MPa

The structure of the a〈112〉 dislocation ribbons in CMSX-4 single crystal superalloy crept at 750°C and 750 MPa has been characterised using transmission electron microscopy. Calculations are performed which confirm that the macroscopic creep deformation occurring under these conditions, i.e. the accumulation of primary creep, is principally a result of the movement of the ribbons through the γ/γ′ structure. Emphasis is placed on clarifying the factors which control the movement of the fault: it appears that the rate controlling step is the entry of the final a/3〈112〉 dislocation into the γ′ phase, which has the effect of removing the superlattice extrinsic fault. Further observations suggest that the a/2〈110〉 dislocations play a vital role not only in nucleating the ribbons, but also in terminating their long-range movement through the microstructure. It is proposed that this latter effect results in the breakdown of primary creep and thus the transition to secondary ‘steady-state’ creep.