New Rules of Coupled Severe Plastic Deformations, Phase Transformations, and Microstructure Evolution under High Pressure

1. It was found [3,4] for severely predeformed Zr that (a) crystallite size and dislocation density in both phases in a single-phase state, the minimum pressure for the strain-induced α-ω PT in Zr, and the pressure-dependent yield strength of ω-Zr are getting steady and independent of plastic strain tensor and strain path ; (b) crystallite size and dislocation density in ω-Zr and (with some outliners) α-Zr during PT are independent of pressure, plastic strain tensor, and its path, and depend on the volume fraction of the high-pressure phase only. Obtained results can be used to find economic ways of grain refinement during PT and obtain nanocomposite with optimal properties.
2. Coupled experimental-analytical-computational approaches, utilizing synchrotron X-ray diffraction, are developed [5,6] to solve an inverse problem and find fields of all components of stress and plastic strain tensors and friction rules before, during, and after α-ω PT in strongly plastically predeformed Zr. In addition, the kinetics of strain-induced PT is described in the whole sample.
3. We revealed in situ various unexpected plastic strain-induced PT phenomena in Si [7]. Thus, for 100 nm Si, strain-induced PT Si-I→Si-II initiates at 0.3 GPa versus 16.2 GPa under hydrostatic conditions; for 30 nm Si, it is 6.1 GPa versus ∞ since it does not occur. For 100 nm Si-I→Si-III, it is 0.6 GPa vs. ∞. The predicted theoretical correlation between the direct and inverse Hall-Petch effect of the grain size on the yield strength and the minimum pressure for strain-induced PT is confirmed. Retaining Si-II at ambient pressure and obtaining reverse Si-II→Si-I PT are achieved for the first time. Retaining single-phase Si III is achieved at much more economic conditions than the currently known.
4. A new theory is developed for the plastic strain-induced olivine-spinel PT in a shear band, which resolves multiple existing puzzles in the mechanism of deep-focus earthquakes [8]. Conceptual experimental confirmation is obtained [9]. The obtained results offer a new fundamental understanding of strain-induced PTs under high pressure in DAC/RDAC. They create new opportunities in material design, synthesis, and processing of nanostructured materials by severe plastic deformations at low pressure, methods of controlling PTs and nanostructures, and searching for new high-pressure phases and phenomena.