On the microstructural origin of primary creep in nickel-base superalloys

The nature of primary creep in nickel-base superalloys is strongly correlated to the different hardening species present in the material. In fine-grained single-phase material the classical assumption of a homogeneous dislocation distribution enables the prediction of the transition from normal via sigmoidal to inverse primary creep with decreasing applied stress σ. In coarse-grained material the back stress σb of hard subgrain boundaries evolving during plastic deformation must be additionally taken into account. Second-phase particles influence creep in a 2-fold manner via reducing the effective stress σeff, namely directly by the stress σb* for particle overcoming, and indirectly by increasing the dislocation density p. The proposed approach accounts for the observed pronounced normal primary creep in particle-strengthened superalloys.