Our recent in-situ experimental results on plastic strain-induced phase transformations (PTs) and chemical reactions under compression and shear in a rotational diamond anvil cell (RDAC) are presented. They include a reduction in PT pressure up to one to two orders of magnitude due to plastic straining, novel strain-controlled kinetics, and the appearance of new phases. For example, plastic shear reduced graphite-diamond PT pressure from 70 GPa under hydrostatic conditions to 0.7 GPa. The four-scale theory was developed, and corresponding simulations were performed. First-principles and molecular dynamics simulations were used to determine analytical criteria for lattice instability under all six components of the stress tensor, demonstrating a strong reduction in PT pressure under nonhydrostatic loading. Nano- and scale-free phase-field approaches to coupled PT and dislocations are developed and applied to nucleation at dislocation pileups in a bicrystal and polycrystal at the nano- and microscales, respectively. The possibility of reducing PT pressure by more than an order of magnitude due to stress concentration at the tips of dislocation pileups is proven. A strain-controlled kinetic equation is derived and used in the large-strain macroscopic theory for coupled PTs and plasticity. At the macroscale, the sample's behavior in DAC and RDAC is studied using FEM. A coupled computational-experimental method is developed to calibrate and verify the macroscopic model and reproduce all tensorial and transformational fields in a sample. Various simulation predictions are confirmed experimentally using in situ synchrotron X-ray diffraction. Materials under study include Zr, Si, carbon, BN, Fe and its alloys, and olivine. Some applications to mechanochemical material synthesis and to geophysical problems, such as new PT-based mechanisms for deep-focus earthquakes, are considered. REFERENCES [1] Levitas V.I. Strain-Induced Phase Transformations, Chemical Reactions, Microstructure Evolution, and Severe Plastic Deformations under High Pressures. Prog. Mater. Sci., 158, 101625, 2026. [2] Dhar A., Levitas V.I., Pandey K. K. et al. Quantitative kinetic rules for plastic strain-induced α-ω phase transformation in Zr under high pressure. Npj Computational Materials, 10, 290, 2024. [3] Levitas V.I., Dhar A., Pandey K.K. Tensorial stress-plastic strain fields in α-ω Zr mixture, transformation kinetics, and friction in diamond anvil cell. Nat Comm, 14, 5955, 2023.