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Problems in Theories of Low Stress Creep Mechanisms
Kloc, Luboš ; Sklenička, Václav ; Fiala, J.
Viscous creep behaviour of many materials including creep resistant steels was observed at temperatures close to one half of absolute melting point and at very low stresses. The experimental results were interpreted as Coble diffusional creep and/or Harper-Dorn dislocation creep. Some of the results are in a very good agreement with the Coble theory of diffusional creep and support an idea that the diffusional creep is an important deformation mechanism under certain loading conditions. Nevertheless, another results are not fully compatible with the theoretical predictions. Some authors dispute the role or even the very existence of the diffusional creep and offer the Harper-Dorn creep mechanisms instead. In fact, there are many theories trying to deescribe Harper-Dorn creep mechanism, but none of them is capable to explain all observed properties. Transition (or primary) stage observed at low stress creep regime exhibits also some properties which contradict all existing theories. The observed effects which cannot be explained by the current theories are: i) a large scatter of creep rates observed for coarse grain materials, ii) creep rates much higher than those predicted by the diffusional creep theory while the creep rate dependence on grain size is maintained, iii) transition stage duration is independent of stress, iv) transition strain is dependent on temperature. Despite the problems in theoretical description, the experiments show that the viscous creep regime in structural materials must be considered as an important behaviour of the structural materials.
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