The effect of strain reversal on hardening due to high-pressure torsion was investigated using commercially pure aluminium. Hardening was lower for cyclic high-pressure torsion as compared to monotonic high-pressure torsion. When using a cycle consisting of a rotation of 90° per half cycle, there was only a small increase in hardness if the total amount of turns was increased from 1 to 16. Single reversal high-pressure torsion processing involves torsion in one direction followed by a (smaller) torsion in the opposite direction. It was shown that a small reversal of 0.25 turn (90°) reduces hardness drastically, and that decrease was most marked for the centre region. These behaviours and other effects were interpreted in terms of the average density of geometrically necessary dislocations and statistically stored dislocations. A model was presented that describes the experimental results well. A key element of the model was the assumption that at the very high strains developed in severe plastic deformation processes such as high-pressure torsion, the dislocation density reaches a saturation value. The model indicates that the strength/hardness was predominantly due to geometrically necessary dislocations and statistically stored dislocations.

Microstructure Development and Hardening during High Pressure Torsion of Commercially Pure Aluminium: Strain Reversal Experiments and a Dislocation Based Model. J.Zhang, N.Gao, M.J.Starink: Materials Science and Engineering A, 2011, 528[6], 2581-91