The effects of the hydrostatic pressure in a deep ocean on the mechanical behaviour of metals

Abstract

The nucleation of a microcrack in a metal by dislocation coalescence is governed principally by shear stresses and is therefore, to a first approximation, not influenced by the application of a hydrostatic pressure. Its propagation is, however, made more difficult by a hydrostatic pressure because it is governed principally by the tensile components of stress normal to the plane of the crack. As a result, a metal which is brittle at ambient pressure may become ductile when a hydrostatic pressure is applied, i.e. the ductile-brittle transition temperature may be lowered. A simple calculation shows that this decrease in transition temperature should be approximately proportional to the pressure and should be several tens of degrees for the magnitude of pressure found in a deep ocean. A limited number of experiments carried out in laboratory pressure chambers, and reported in the literature, show that this calculation is at least approximately correct. Some recent work reported in the literature has shown that in some cases the application of a pressure may cause mechanical property changes which persist when the pressure is removed. These have been tentatively attributed to effects such as the irreversible generation of dislocations at inclusion-matrix interfaces. The pressures used in this work were much greater than those encountered even in the deepest ocean. Nevertheless, it is worth examining the possibility that similar, if smaller, effects occur when a metal is exposed to a deep ocean environment. Experiments aimed at detecting such effects have been conducted on eight materials—five steels and three titanium alloys. The steels included AISI 1018, AISI 4130, tempered martensitic steels having nominal yield stresses of 130,000 and 170,000 psi, and a maraging steel with a nominal yield stress of 180,000 psi. The three titanium alloys had α, α/ β, and β structures respectively. Tensile and Charpy V-Notch specimens of all eight materials were exposed to a pressure of 25,000 psi, which corresponds to a depth about 50 per cent greater than the deepest known parts of the oceans, for 30 min. Their properties, and the properties of control specimens, were then studied over a range of temperature. The results show that pressurization caused small property changes in four of the eight materials, the AISI 1018 and AISI 4130 steels, the 170,000 psi martensitic steel, and the α β titanium alloy. Preliminary experiments in which tensile specimens of the AISI 1018 steel and the α/β titanium alloy were subjected to 100 pressure cycles suggest that these property changes are not cumulative on cyclic pressurization.