Athermal strength of pure aluminum is significantly decreased by severe plastic deformation and it is markedly augmented by subsequent annealing

Over the last two decades, it has been considered that fine crystal grains produced by severe plastic deformation (SPD) lead to an extraordinarily high metal strength. The present study reveals that this understanding is basically incorrect. In our uniaxial tensile tests on industrial pure aluminum at an ultralow strain rate of similar to 10-7/s, we observed that SPD accompanied by grain refining significantly softened the material. The fundamental strength effective for real structures and structural materials should mean an eternal capability to bear stresses caused by external forces, which is independent of time, that is, athermal. We tried to extract quantitatively the athermal (time-independent) strength from the total strength measured in uniaxial tensile tests under the assumption that the total stress can be additively divided into athermal and thermal (time-dependent) components. As a result of systematic experimental investigation, we found that the athermal strength is significantly reduced by SPD and then markedly increased by subsequent low-temperature annealing. In addition, we confirmed that SPD promotes an increase in the time dependence (viscosity) of the material and that subsequent annealing removes most of the viscosity caused by SPD. The material processed by SPD acquires its prominent time-independent strength after low-temperature annealing.