Valery Levitas—Schafer 2050 Challenge Professor and faculty member of aerospace engineering and mechanical engineering—and his post-doctoral associate Mahdi Javanbakht, have published the paper “Phase transformations in nanograin materials under high pressure and plastic shear: nanoscale mechanisms,” in the high-impact journal Nanoscale.
There are two main challenges in the discovery of new high-pressure phases and the transformation of this discovery into technologies: finding conditions to synthesize new high-pressure phases and finding ways to reduce the phase transformation pressure to an economically reasonable level.
Based on the results of pressure–shear experiments in the rotational diamond anvil cell, superposition of plastic shear on high pressure is a promising way to resolve these problems.
Levitas and Javanbakht are looking at ways to resolve this problem by providing more insight into nanoscale mechanisms of phase transformation under high-pressure and large plastic shear.
In their paper, the two researchers explain generic mechanisms of coupled nucleation and evolution of dislocation and high-pressure phase structures in the nanograin material under pressure and shear utilizing the previously developed advanced phase field approach.
They found that the strong stress concentration near dislocation pile-ups increases the local thermodynamic driving force for phase transformation, which allows one to significantly reduce the applied pressure. The applied pressure is 3 to 20 times lower than the transformation pressure under hydrostatic conditions and 2 to 12.5 times smaller than the phase equilibrium pressure.
Plasticity plays a dual role: in addition to creating stress concentrators, it may relax stresses at other concentrators, thus competing with phase transformation. Some ways to optimize the loading parameters have been found that lead to methods for controlling phase transformations. 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 transformations without shear.
In addition to the search for new high-pressure phases and reduction in transformation pressure, phase transformations under pressure and shear occur in shear bands in geophysics (especially during the initiation of earthquakes), penetration of projectiles in materials, and shear ignition of energetic materials. Strain-induced transformations under high pressure also take place in various technological applications (e.g., cutting and polishing of germanium and silicon), as well as silicon and boron carbides, high-pressure torsion and extrusion, and transformations during friction. Still, in all cases nucleation starts at the nanoscale and developed approach can be useful for all these problems.