Phase transformations in nanograin materials under high pressure and plastic shear: nanoscale mechanisms† Valery I. Levitas * a and Mahdi Javanbakht b There are two main challenges in the discovery of new high pressure phases (HPPs) and transforming this discovery into technologies: finding conditions to synthesize new HPPs and finding ways to reduce the phase transformation (PT) pressure to an economically reasonable level. Based on the results of pressure–shear experiments in the rota- tional diamond anvil cell (RDAC), superposition of plastic shear on high pressure is a promising way to resolve these problems. However, physical mechanisms behind these phenomena are not yet understood. Here, we elucidate generic mechanisms of coupled nucleation and evolution of dislocation and HPP structures in the nanograin material under pressure and shear utilizing the developed advanced phase field approach (PFA). Dislocations are generated at the grain boundaries and are densely piled up near them, creating a strong concentrator of the stress tensor. Averaged shear stress is essentially larger in the nanograin material due to grain boundary strengthening. This leads to the increase in the local thermodynamic driving force for PT, which allows one to significantly reduce the applied pressure. For all cases, the applied pressure is 3–20 times lower than the PT pressure and 2–12.5 times smaller than the phase equilibrium pressure. Interaction between nuclei leads sometimes to their coalescence and growth of the HPP away from stress concentrators. Plasticity plays a dual role: in addition to creating stress concentrators, it may relax stresses at other concentrators, thus competing with PT. Some ways to optimize the loading parameters have been found that lead to methods for controlling PT. Since such a local stress tensor with high shear stress component cannot be created without plastic deformations, this may lead to new transformation paths and phases, which are hidden during pressure induced PTs. There are two main challenges in the discovery of new HPPs and transforming this discovery into technologies: nding conditions to synthesize new HPPs and nding ways to reduce the PT pressure to an economically reasonable level. Various new HPPs with unique properties have recently been discovered experimentally: new superhard phases of carbon, 1,2 BC 5 , 3 B-BN, 4 and BC 2 N, 5,6 supposedly highly energetic phases of polymeric nitrogen, 7 CO 2 , 8 and ionic boron. 9 Many others have been pre- dicted theoretically 10–14 but have not yet been synthesized, because of kinetic barriers or because the proper transformation path could not be realized under quasi-hydrostatic pressure and known phases appeared instead. PT pressure for most of these phases is too high for technological realization. Based on the results of pressure–shear experiments in rotational Bridgman anvils, 15 rotational diamond anvil cell (RDAC), 16–24 high pressure torsion, 25–30 and ball milling 31–33 superposition of plastic shear on high pressure can in principle resolve these problems. Indeed, we recently obtained a new high-density amorphous phase of SiC under a pressure of 30 GPa and large shear, 16 which was not obtained under hydrostatic pressure up to 130 GPa. Phase IV of fullerene C 60 (which is believed to be harder than diamond) was rst revealed under pressure and shear in the RDAC 17,18 and then reproduced under high pressure and temperature. Highly ener- getic polymeric phases of nitrogen and sodium azide 19,20 and superhard phase of single wall carbon nanotube 21 were obtained under pressure and shear in the RDAC. Also, plastic shear reduces the PT pressure by a factor of 2 to 10 for some PTs 18,22–25,29 – e.g., in Si and Ge, 23 rhombohedral BN to superhard cubic BN, 18 Zr and Zr–Ni alloys, 25,29 and disordered nanocrystalline hexagonal BN to wurtzitic BN. 24 Despite the fundamental and applied importance and various intriguing phenomena, our understanding of the mechanisms and theoretical description is in its infancy. Macroscopic continuum thermodynamics fails to describe the signicant reduction in PT pressure. Indeed, let the PT occur when the mechanical part of the thermodynamic driving force W (transformation work) reaches a critical value k – i.e., W ¼ p3 0t + sg t ¼ k, where p and s are the pressure and shear stress, 3 0t < 0 and g t are the volumetric and shear transformation strains. Let g t ¼23 0t ¼ 0.2, PT pressure under hydrostatic a Departments of Aerospace, Mechanical, and Material Science Engineering, Iowa State University, Ames, Iowa, USA. E-mail: vlevitas@iastate.edu; Fax: +1 484 208 9691; Tel: +1 515 294 9691 b Department of Aerospace Engineering, Iowa State University, Ames, Iowa, USA † Electronic supplementary information (ESI) available: A PFA to interaction of PTs and dislocation evolution is developed and material properties are described. See DOI: 10.1039/c3nr05044k Cite this: Nanoscale, 2014, 6, 162 Received 11th September 2013 Accepted 15th October 2013 DOI: 10.1039/c3nr05044k www.rsc.org/nanoscale 162 | Nanoscale, 2014, 6, 162–166 This journal is © The Royal Society of Chemistry 2014 Nanoscale COMMUNICATION