The influence of high hydrostatic pressure on the flow stress of polycrystals of high-purity copper (OFHC) and of tough pitch copper (TPC) has been studied at room temperature and compared with that obtained with pure aluminium polycrystals which has already been reported by the authors. Pressurizing under pressures up to 15000kg/cm2, tensile tests at atmospheric pressure and under 12000kg/cm2 using the differential pressure method, and tensile tests under a constant hydrostatic pressure of 12000kg/cm2 have been carried out. Results obtained are as follows: (1) Pressurizing up to 15000kg/cm2 gives no effect on the flow stress of OFHC copper at atmospheric pressure. As for TPC, a remarkable increase of flow stress at atmospheric pressure is observed at the beginning of deformation after pressurizing under 12000kg/cm2. The increase of the flow stress falls rapidly with increasing strain and becomes almost zero at about 40% strain. The repeated pressurizing during tensile testing has the same effect as that due to the pressurizing applied only once before deformation. (2) The change of flow stress observed by the differential pressure method with the pressure difference between atmospheric pressure and 12000kg/cm2 is about 2.2% for OFHC copper and about 2.6% for TPC. The change in OFHC copper agrees with that in the shear modulus of copper measured by Lazarus. (3) Tensile tests under a constant pressure of 12000kg/cm2 show that the flow stress of OFHC copper increases by about 2% at the beginning of the deformation and by about 5.5% under about 15% strain or more as compared with that at atmospheric pressure. As for TPC, the change in the flow stress is approximately expressed as a sum of the amount measured for OFHC copper and that due to the effect of pressurizing for TPC.
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 to a pressure of 12000kg/cm2, tensile tests using the differential pressure method between atmospheric pressure and 12000kg/cm2 and tensile tests under a constant pressure of 12000kg/cm2 have been carried out. The results obtained are as follows: (1) For zone-refined iron and molybdenum, no effect of pressurizing is found on the stress-strain behaviour at atmospheric pressure while a decrease of about 2.4% in flow stress is observed for carbon-deoxydized iron in the region of Liiders deformation after pressurizing. This effect seems to be similar to the previous result which was 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 change of low 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 amounts of change coincide well with those of shear moduli of steel and molybdenum under hydrostatic pressure. (3) In the stage of Liiders deformation for iron, the change in the 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, an activation volume of about 0.2 atomic volume is obtained. (4) The increase in work-hardening rate under hydrostatic pressure is 1 to 2% for iron in the region of uniform deformation. For molybdenum it increases with strain and becomes constant (about 3%) at about 20% strain.
An austenitic 18-8 stainless steel was tested in tension under hydrostatic pressure and the iiiiiie iand the id the ie influence of hydrostatic pressure on its stress-strain behaviour was discussed. Tensile tests under constant pressures of 12000kg/cm2 and atmospheric pressure and tests during which the ambient pressure was changed from 12000kg/cm2 to atmospheric pressure and vice versa were carried out at room temperature. Also, the change in incuctance of a coil wound around the specimen was measured to evaluate the amount of a-martensite induced by plastic deformation. The results obtained are as follows:(1) Pressurizing at 12000 kg/cm2 gives no effect on the stress-strain relation of the annealed specimen at atmospheric pressure. (2) In the magnetic measurement, a-martensite cannot be observed over the range of less than about 10% strain, whereas at a larger strain it begins to be induced and its amount increases similarly with increasing strain under both 12000kg/cm2 and atmospheric pressure. But e-martensite is observed by X-ray diffraction when the specimen is deformed under 12000 kg/cm2. (3) When the specimens are deformed under a constant pressure of 12000 kg/cm2, the increase in flow stress against that at atmospheric pressure is very large at the beginning of deformation, decreases with increasing strain, and then becomes almost constant (about 12%) above about 15% strain or more. (4) When the ambient pressure is changed between atmospheric pressure and 12000kg/cm2 on the way of deformation, the flow stress changes 0 to 4% at small strains and, about 12% (pressure raising) and 19% (pressure releasing) at 30 to 40% strain. (5) The phenomena mentioned above in (3) and (4) can be explained by the fact that e-martensite is induced rapidly at the beginning of deformation resulting in work-hardening under high hydrostatic pressure, while it easily transforms to a-martensite under a low tensile stress at atmospheric pressure at large strains.
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