Influence of High Hydrostatic Pressure on the Flow Stress of Pure Iron and Molybdenum Polycrystals

The influence of high hydrostatic pressure on the flow stress of zone-refined pure iron, carbon-deoxydized iron and molybdenum polycrystals has been studied at room temperature. Pressurizing tests at a pressure of 12000 kg/cm2, tensile tests using the differential pressure method between atmospheric pressure and 12000 kg/cm2 and tensile tests under a constant pressure of 12000 kg/cm2 have been carried out. The results obtained are as follows: (1) For zone-refined iron and molybdenum, no pressurizing effects are found on the stress-strain behaviour at atmospheric pressure, but a decrease of about 2.4% in flow stress is observed for carbon-deoxydized iron in the region of Lüders deformation after pressurizing. This effect seems to be similar to the previous results which are explained by the generation of free dislocations around inclusions of FeO under hydrostatic pressure. (2) According to the results of tensile tests using the differential pressure method, the rate of change of flow stress is about 2.1% for zone-refined iron, 2.8% for carbon-deoxydized iron and 0.7% for molybdenum in the region of uniform deformation. These rates of change coincide well with those of shear modulus of steel and molybdenum under hydrostatic pressure. (3) In the stage of Lüders deformation for iron, the rate of change in flow stress is much greater (5.6 to 5.9%) than that of the shear modulus of steel under the same pressure. Assuming that the difference is due to the activation volume for the thermally activated process which controls the deformation, the activation volume of about 0.2 atomic volume is obtained. (4) The rate of increase of work-hardening under hydrostatic pressure is 1 to 2% for iron in the region of uniform deformation. For molybdenum, it increases from zero strain and becomes constant (about 3%) at about 20% strain.