ON THE MICROSTRUCTURAL ORIGIN OF PRIMARY CREEP IN NICKEL-BASE SUPERALLOYS

The nature of primary creep in nickel-base superalloys is strongly cor related to the different hardening species present in the material. In fine-grained single-phase material the classical assumption of a homo geneous dislocation distribution enables the prediction of the transit ion from normal via sigmoidal to inverse primary creep with decreasing applied stress sigma. In coarse-grained material the back stress sigm a(b) of hard subgrain boundaries evolving during plastic deformation m ust be additionally taken into account. Second-phase particles influen ce creep in a 2-fold manner via reducing the effective stress sigma(ef f), namely directly by the stress sigma(p) for particle overcoming, a nd indirectly by increasing the dislocation density rho. The proposed approach accounts for the observed pronounced normal primary creep in particle-strengthened superalloys. (C) 1997 Elsevier Science S.A.