Constant-load tertiary creep in nickel-base single crystal superalloys

Accelerating or tertiary creep in nickel-base single crystal (SX) superalloys, for constant-load tensile tests represents a “dynamic steady state” between the engineering strain rate and the corresponding actual or true stress. Experiments on nominally [0 0 1]-oriented CMSX-10 at 900 °C revealed this for initial stresses of 500–700 MPa for conditions of non-rafting γ′ precipitates. Analysis of results obtained elsewhere on [0 0 1]-oriented CMSX-10, for conditions of γ′ rafting at temperatures up to 1100 °C also indicate this dynamic steady state. This state is described by a power-law between stress and strain rate and one test provides the stress exponent over a stress range of 100 MPa. This suggests an economical method for evaluating initial microstructure-property relationship. A time-based elasto-viscous (non-linear) creep model with only three material parameters predicts tertiary creep as long as the deformation is homogeneous. The onset of inhomogeneous deformation can be predicted by comparing the model output with the experimental observations using a normalized strain-rate technique. For [0 0 1]-oriented CMSX-10 at 900 °C and 500–700 MPa, inhomogeneous deformation is predicted for strains greater than 12%. This was verified by conducting interrupted tests and measuring the profile of the cross-sectional shape along the entire originally uniform gauge length. However, fractured specimens revealed very significant non-uniform deformation in the form of necking at two regions. The elongation of pores and cracking at pores walls are recognized as creep enhancement factors that lead to double necking and final fracture at one of them. A survey of constant-load creep results obtained in different laboratories on different nickel-base SX alloys (CMSX-4, CMSX-10, TMS75) at different temperatures and stresses confirm the hypothesis of dynamic steady state (including true strain rate versus true stress), during tertiary stage.