Inclusions in diamonds with solubility changes and phase transformations

Abstract

Abstract Liquid-metal inclusions are sealed into diamond crystals during crystal growth at high temperatures and pressures. The pressure and temperature of these inclusions change as the diamond is lowered to room temperature and atmospheric pressure and during subsequent heating and cooling cycles at atmospheric pressure. Liquid-metal inclusions that dissolve carbon follow different P – T curves than liquid metal inclusions that do not. A discontinuous change of the pressure in the inclusion occurs when the liquid metal solidifies during cooling. This discontinuous change is caused by the molar volume and carbon solubility difference between the liquid and solid metal. Contrary to common belief, the residual pressure in these inclusions can be large and positive at room temperature when graphite precipitates from supersaturated solid metal inclusions. The formation of carbides in the inclusion following solidification can also have a significant effect on the pressure in the inclusion. Internal stresses are generated in the diamond that are proportional to the difference between the pressure in the inclusion and the external pressure around the diamond. These stresses increase as the cube of the inclusion diameter and fall off as the cube of the distance from the inclusion. Small positive or negative temperature fluctuations (±30 °C) while the diamond is growing can generate large stresses that produce plastic flow around inclusions. Plastic flow produces an increased atomic defect density around the inclusion as well as permanent internal elastic stresses in the diamond far from the inclusion. These internal stresses may affect the mechanical properties of the diamond. When the pressure in the inclusion falls below the diamond-graphite equilibrium line, graphitization can occur on the walls of the inclusion. Because graphite has a molar volume 50% greater than diamond, such graphitization reduces the volume of the inclusion cavity and increases the pressure in the inclusion. Multiple inclusions generate stresses that are proportional to the inclusion concentration.