Plastic deformation of nickel under high hydrostatic pressure

Samples of nickel were deformed in torsion under very high hydrostatic pressures (up to 8 GPa) up to strains being much larger than those reached under normal pressure conditions. The obtained stress-strain curves were fitted by the model of Zehetbauer and Kohout which has been developed from the work hardening model of Zehetbauer designed for conventional large strain deformation. The configuration parameters of dislocations were determined from strain dependent dislocation densities which were measured by Multiple Whole X-ray Bragg Peak Profile Analysis (MXPA). Using actual materials and physical constants of Ni, the obtained values of fitting parameters could be related to realistic numbers of physical quantities hidden in those. The main part of the deformation stress increase observed with increasing hydrostatic pressure can be ascribed to the increase of dislocation density which arises from the restriction of diffusion controlled annihilation mechanisms. This restriction is related to the pressure-induced decrease of diffusion, i.e. to the increase of vacancy migration enthalpy which is due to the shrinking of interatomic spacing when the hydrostatic pressure is enhanced. However, the calculated concentrations of deformation induced vacancies for pressures 5 GPa and beyond are unacceptably high. Data from measurements of microhardness and MXPA after unloading show that beyond a pressure of 4 GPa both the strength and the dislocation density is not being increased anymore which suggests the onset of spontaneous vacancy and dislocation annihilation due to the high overall concentration of vacancies reached. This failure of the model is not surprising since it has not considered so far any limits in the vacancy concentration. (C) 2004 Elsevier B.V. All rights reserved.