The classical concept of Nabarro creep was extended for a general dislocation microstructure. The specific mechanism of the creep involved the generation and annihilation of vacancies at dislocation jogs acting as non-ideal sources and sinks for vacancies. This mechanism caused the climb of dislocations, allowing for local volume and shape change. In the first step, the kinetics of generation/annihilation of vacancies for each individual jog was derived by means of the thermodynamic extremal principle. In the second step, the creep rate tensor was formulated; including the chemical potential of vacancies as a chemical term and the hydrostatic stress and stress deviator as mechanical terms. Closed-form equations for the creep rate were derived for isotropic polycrystals. For a sufficient number of different Burgers vectors in the system, the volumetric and shape change could be obtained. As special cases, pure uniaxial and pure shear loading of a face-centered cubic single crystal were treated. Particular attention was paid to the Kirkendall effect which was observed more or less only qualitatively in many systems. The associated deformation was caused predominantly by a deviation of the local vacancy-site fraction, from its equilibrium value, caused by diffusion in the system. Solution concepts had recently been presented in order to account for local isotropic swelling/shrinkage due to the generation and annihilation of vacancies. In this case, only the hydrostatic part of the local stress tensor had been accounted for. However, application of the present theory also permitted incorporation of the deviatoric components of the local stress tensor; yielding a significant improvement in the quantitative treatment of the Kirkendall effect by using the corresponding creep strain tensors. The present model was applied to the estimation of the creep rate in the ferritic P-91 type creep resistant steel at very low applied stresses.

Chemically and Mechanically Driven Creep Due to Generation and Annihilation of Vacancies with Non-Ideal Sources and Sinks. F.D.Fischer, J.Svoboda: International Journal of Plasticity, 2011, 27[9], 1384-90