The back stress concept in power law creep of metals: A review

Abstract

The idea that the plastic deformation of crystalline solids takes place under only a part of the applied stress and not under the entire stress (represented by the difference between the applied stress σ and a back stress σ B) is used to discuss the well-known discrepancy between the natural third-power dependence of the steady state creep rate on the applied stress and the experimentally observed significantly stronger dependence in metals and solid solution alloys exhibiting metal class behaviour. The back stress is identified with the internal stress measured by a dip test technique and introduced into the third-power law creep equation. This approach has led to a satisfactory, although essentially phenomenological, interpretation of the third-power law. Some results of complete unloading experiments in steady state creep as well as the results of internal stress measurements during backward straining after the complete unloading are presented and discussed on the basis of the model of soft and hard regions of dislocation structure. It is suggested that the internal stress measured in steady state creep of metals and solid solution alloys exhibiting metal class behaviour represents a level of internal backward stresses acting in the soft regions (cells or subgrains), whereas in the hard regions (cell walls or subboundaries) large, forward internal stresses act. The dislocation processes in the hard regions most probably govern the dependences of the creep rate on both the temperature and the applied stress. However, the forward internal stresses acting in hard regions are related to the backward internal stresses acting in soft regions (and thus to the measured internal stress) through the dislocation substructure. Argon and Takeuchi's theory of internal stress and creep is briefly described and compared with some experimental results including the measured internal stress. It has been found that this theory does not account satisfactorily for the experimental results. Some other attempts to reconcile the natural third-power law with the significantly stronger applied stress dependence of the steady state creep rate observed experimentally are briefly discussed. The need for a sound theory of internal stress and more detailed knowledge of dislocation processes in cell walls and/or subboundaries in creep for greater understanding of creep mechanisms is emphasized.