Shear driven formation of nano-diamonds at sub-gigapascals and 300 K

Abstract

The transformation pathways of carbon at high pressures are of broad interest for synthesis of novel materials and for revealing the Earth's geological history. We have applied large plastic shear on graphite in rotational anvils to form hexagonal and nanocrystalline cubic diamond at extremely low pressures of 0.4 and 0.7 GPa, which are 50 and 100 times lower than the transformation pressures under hydrostatic compression and well below the phase equilibrium. Large shearing accompanied with pressure elevation to 3 GPa also leads to formation of a new orthorhombic diamond phase. Our results demonstrate new mechanisms and new means for plastic shear-controlled material synthesis at drastically reduced pressures, enabling new technologies for material synthesis. The results indicate that the micro-diamonds found in the low pressuretemperature crust could have formed during a large shear producing event, such as tectonic rifting and continued plate collision, without the need to postulate subduction to the mantle.

Tensorial stress−strain fields and large elastoplasticity as well as friction in diamond anvil cell up to 400 GPa

“…This value is significantly smaller than the theoretical shear strength of 96.6 GPa at zero pressure, which grows with pressure. 36 It is important that the regions in which maximum normal and shear stresses occur do not overlap.…”

“…Due to significant increase in the sticking zone, an increase in p max above 240-300 GPa leads to decrease in the maximum range of pressure and contact stress σ c at which plastic flow, Coulomb or plastic friction occur. The only way to increase these ranges for characterization of plastic flow and contact friction is to use torsion under a fixed force in rotational DAC, 12,15,22,[34][35][36] for which FEM simulations 29,37 show that the sticking zone is localized near the center.…”

“…obtained results have also applied importance for study of various sample materials within W gasket. Knowledge of the distributions of all (generally 12) components of stress and plastic strain tensors in a sample will allow study of their (instead of pressure alone) effect on phase transformations, chemical reactions, 12,[14][15][16][20][21][22][34][35][36][37] and various physical properties. In comparison with research under hydrostatic pressure, this will add up to 11 new dimensions to the parametric space for studying these processes, searching for new phases and materials, drastically reducing the required pressure for synthesis of new and known materials with unique properties, and understanding processes in the deep interiors of the Earth and other planets.…”

Finite-strain scale-free phase-field approach to multivariant martensitic phase transformations with stress-dependent effective thresholds

“…From the other side, there are well-documented experimental results showing that under plastic straining PT may occur at a pressure much below than the phase equilibrium pressure. Thus, the phase equilibrium pressure is estimated as 10.5 GPa for Si I -Si II (Voronin et al (2003)) and is in the same range for Si I -Si III (Blank and Estrin (2013) (Gao et al (2019)). For α − ω PT in Zr, phase equilibrium pressure is 3.4 GPa under hydrostatic conditions, while under shear, this PT was obtained at 1.2 GPa (Pandey and Levitas (2020)).…”

“…This issue is discussed in detail. While it sounds catastrophic, there are many experimental results for different material systems demonstrating that under plastic straining PT may occur at a pressure well below than the phase equilibrium pressure (Blank and Estrin (2013); Gao et al (2019); Pandey and Levitas (2020)). Such a counterintuitive behavior was explained in Levitas (2004); Levitas (2018, 2015); Esfahani et al (2020) by strong stress tensor concentration produced by defects induced by plastic deformation, e.g., dislocation pileups.…”