Effect of hydrostatic pressure on plastic-flow properties of iron single crystals

Abstract

Hydrostatic pressure increases the yield strength and work-hardening rate of steel. The effect has been found to be greater than that predicted by current theories of plastic flow in metals. To aid in developing more representative plasticity theory that will improve our ability to predict the behavior of steel during complex working and forming operations, and to improve our understanding of the phenomena, critical experiments have been conducted on polycrystalline steels and on iron single crystals. In the present work, the effect of superimposed hydrostatic pressures up to 1104 MPa (160 ksi) on the deformation behavior of iron single crystals of three different orientations has been studied at room temperature. The three orientations investigated were chosen so that slip occurred on (110) planes or on (112) planes in both the twinning and anti-twinning directions. The results showed that both the critical resolved shear stress (crss) and the initial work-hardening rate were increased by hydrostatic pressure. However, the predominant effect of hydrostatic pressure was to increase the initial work-hardening rate, which suggests that the normal stress on the slip plane had a larger effect on dislocation interactions than on the initial movement of dislocations. Analysis of the slip systems and the dislocation structures present after deformation under hydrostatic pressure indicated that the same slip systems operated and the same kinds of dislocation distributions were formed as observed after deformation at 1 atmosphere. These results suggest that the predominant effect of hydrostatic pressure was to retard the generation of mobile dislocations resulting from cross-slip or other dislocation interactions because of the increase in volume associated with an increase in dislocation density, thereby resulting in a greater work-hardening rate under hydrostatic pressure than at atmospheric pressure. The increase in crss with increasing pressure was independent of orientation and was about twice the amount and can be accounted for by the change in shear modulus or the added effect of pressure on the dislocation volume. If the assumption is made that the increase in crss over that which can be accounted for was due to an activated process that controls deformation, then the activation volume for that process is about 0.1 atomic volume.