The role of hydrostatic pressure in severe plastic deformation

The contribution presents several experimental examples which show that the presence of an enhanced hydrostatic pressure - as compared to conventional large deformation modes - is one of the main features of severe plastic deformation (SPD). At the example of systematic high pressure torsion experiments with Cu at room temperature, strength measurements after deformation showed that the onset strains of deformation stages III, IV, and V are not affected by the pressure applied; however, the related onset flow stresses increase by at least 10% of the values of low pressure torsion, per GPa of pressure increase. During deformation, increases of flow stresses by at least 40% of the values of low pressure torsion, per GPa of pressure increase, have been found. From comparisons with tests on Ni, the increases appear to grow with the materials melting temperature. For a theoretical explanation of flow stress increases the pressure induced changes of i) the elastic moduli, and ii) the formation energy of lattice defects. While contribution i) is almost negligible, contribution ii) accounts for an increase of flow stress during deformation by about 15% per GPa of pressure increase. The difference left to experiment has to be attributed to a third contribution, i.e., the pressure specific evolution of the structure. For this contribution, a modification of the model of Zehetbauer and Les is introduced which is based on the pressure caused decrease of lattice diffusion. The latter is thought to restrict the diffusion controlled annihilation of dislocations, thus leading to a higher density of vacancies, dislocations and/or grain boundaries causing the higher stress level observed.