Formation of a dislocation back stress during creep of copper at low temperatures

Copper gives creep strain versus time curves at 75 °C that look very similar to those recorded at much higher temperatures. Thus, for example, an extended secondary stage where the strain rate is constant is observed. Considering the high creep exponent that can be up to 75, one would expect a creep curve with rapidly increasing strain rate but that is not found. The difference to creep of pure metals at high temperatures is so large that we can talk about an entirely new material class with respect to creep. To explain the observations a recently developed dislocation model (Sandstrom, 2017) for cell structures is used. A new creep model is presented where a back stress based on the dislocations in the cell walls is introduced. Unbalanced sets of dislocations without matching dislocations of opposite signs are formed in the cell walls. Since the unbalanced content is not exposed to static recovery, it forms a stable back stress. It is shown that the computed back stress can fully explain the observations and reproduce both creep curves and results for slow strain rate tensile tests.