Internal stress and activation energy of creep

The stress σ acting on a moving dislocation can be divided into two components: the effective stress σ* that opposes the velocity dependent lattice friction and the internal stress σi that overcomes long-range stress fields generated by neighbouring dislocations or other obstacles. The activation energy of plastic deformation has to be determined for the same state of structure, i.e. at an identical level of the internal stress, and at the same applied stress Q i * = − k ∂   ln ⁡ ε ˙ ∂ ( 1 / T ) σ i , σ , where ε ˙ is the creep rate, T the absolute temperature and k is the Boltzmannʹs constant. A new method for determining this quantity is proposed. It is shown that at zero applied stress this energy is identical to the apparent activation energy of creep determined at constant applied stress.