Discrete Models of Plastic Deformation of Solids Under the Action of High Hydrostatic Pressure
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DISCRETE MODELS OF PLASTIC DEFORMATION OF SOLIDS UNDER THE ACTION OF HIGH HYDROSTATIC PRESSURE L. Yu. Kozak
UDC 669.3: 539.89: 537.322.11: 536.424.1
By using a two-dimensional discrete model, we show that the crystal lattice is transformed from the stable equilibrium state into a state of unstable equilibrium under high hydrostatic pressures. As a result of this transformation, the intensity of the processes of plastic deformation increases. Keywords: crystal lattice, interatomic distance, hydrostatic pressure, instability, plastic deformation.
It is known that the solids, even very brittle, become plastic under the action of high hydrostatic pressures. Moreover, the plastic solids become more plastic [1–3]. Thus [3], 20 steel becomes highly plastic at pressures of 15,000–25,000 dan/mm 2 (∼ 1500–2500 MPa) so that its relative narrowing becomes as high as 99%. Under the same conditions, this parameter is equal to 58% for a superduty steel (HB 234), to ∼ 30% for the gray cast iron, to 20% for the rock salt, and to ∼ 25% for marble. The effect of high hydrostatic pressures is explained by weakening of the tensile stresses, which inhibits the formation of ruptures and increases the level of plastic strains [2, 4]. The results of metallographic and electron-microscopic studies and the X-ray diffraction analysis demonstrate that high hydrostatic pressures improve the mechanical properties of materials. The level of plasticity of brittle metals under pressures is increased due to the removal of defects. Any actual metals and alloys always contain defects, microcracks, pores, etc. The metals are additionally densified as a result of “curing” of the micro- and macrodefects and macropores. By decreasing the level of tensile stresses in the body, high hydrostatic pressures inhibit the processes of crack initiation and propagation. In the process of plastic deformation of commercial metals and alloys, high hydrostatic pressures impede the intercrystalline deformation, which promotes brittleness. As a result, we can observe a phenomenon paradoxical under the ordinary conditions, i.e., a simultaneous increase in the levels of plasticity and strength of the material (both the ultimate and yield strengths and the degree of plastic elongation increase) [4–7]. Under the analyzed conditions, special attention is given to the behavior of dislocations. It is believed that the enhancement of plasticity of solid bodies is connected with the acceleration of the multiplication of dislocations and the processes of their climbing and annihilation. High hydrostatic pressures strengthen the interaction of dislocations and strongly affect their dynamics [3, 8, 9]. At the same time, the influence of high hydrostatic pressures is not restricted to structural changes. In most solids, we detect changes in the phase diagrams and in the type of chemical bonds; moreover, we observe the polymorphic transformations [10, 11]. Thus, the classical semiconductors (germanium and silicon) are transformed into metals at room temperature under pressures of 900
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