Analysis of the Effect of Nonhydrostatic Compression Conditions on Direct Phase Transformations in Carbon

We have analyzed the effect of deviations from hydrostatic compression conditions on the driving force for transformations of graphite to dense phases. We consider transformations of the hexagonal (2H) and rhombohedral (3R) modifications of graphite to lonsdaleite and diamond by martensitic mechanisms, and also a direct diffusional transformation. Comparative estimates of the driving forces were made for hydrostatic and uniaxial compression schemes, and also for shear loading. We show that a loading scheme combining hydrostatic compression with uniaxial compression is more favorable for the martensitic transformation of 2H-graphite to lonsdaleite, while shear loading and uniaxial loading are more favorable for the transformations of 3R-graphite; and lonsdaleite is formed in the first case, while diamond is formed in the second case.