Deformation of Metals under High Pressure: Multiscale Stress Fields, Plasticity, and Phase Transformations

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Non-technical abstract:Processes that involve high pressures and plastic deformations are quite common in material synthesis and technologies, e.g. in high-pressure torsion (or twisting). The main objective of these technologies is to produce high-pressure phases and nanostructures that possess unique physical properties important for engineering applications. However, an understanding of the physical mechanisms and ways to characterize and control simultaneous evolution of plastic deformations, phase transformations, and microstructure under high pressure is lacking. The goal of the project is to conduct the first coupled experimental, theoretical, and computational multiscale study of non-uniform stresses and strains, plastic flow, and phase transformations in several technically-important metals under high pressure and shear deformation. The experimental study will be performed in a rotational diamond anvil cell, a unique device in which material is compressed by two diamond anvils to high pressure and then twisted. The transparency of the diamonds allows for measurements of strains and study of various phase transformations directly in the loaded sample. The experimental study will be combined with advanced multiscale modeling, enabling extraction of all deformational and transformational material properties at high pressure. New science produced in the project is expected to impact existing and future technologies for synthesis of the nanograined high-pressure phases. Due to the interdisciplinary nature of the proposed work, two graduate and two undergraduate students will be trained to learn, develop, and apply cutting-edge experimental and computational techniques to a variety of complex systems.Technical abstract:Processes involving high pressures and plastic deformations are quite common in material synthesis and technologies, in nature (e.g. in geophysics), and in physical experiments. High pressure usually causes phase transformations in solids and plastic straining significantly changes the microstructure, thermodynamics, and kinetics of phase transformations. However, an understanding of the physical mechanisms and ways to characterize simultaneous evolution of dislocations and phase transformations under high pressure is lacking. The goal of the project is to conduct the first coupled experimental, theoretical, and computational multiscale study of stress and strain fields, dislocational plasticity, and strain-induced phase transformations in Zr, Fe, Ce, and CeP under high pressure and shear deformation. The experimental study will be performed in a rotational diamond anvil cell, a unique device in which material is compressed to high pressure and then twisted, while providing an opportunity for in situ measurements and study of various phase transformations and strains. Molecular dynamics and a nanoscale phase field approach will be used to model stress-field and nucleation at defects, coupled to the synchrotron X-ray microdiffraction measurements. The kinetics of strain-controlled phase transformations will be determined in terms of volume fraction of phases. At the macroscale, the evolution of the fields of the stress tensor, displacements, plastic strain, and the volume fraction of high-pressure phases within the entire sample in a rotational diamond anvil cell will be measured and simulated. As a result, all deformational and transformational material properties will be determined at high pressure. The hypothesis that the pure volumetric phase transformations in Ce and CeP are affected by plastic shear will be checked.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.