Combined Severe Plastic Deformations and Phase Transformations in Germanium and Silicon under High Pressure

Germanium and silicon have broad applications in electronic and optoelectronic devices, solar panels, and micro-/nano-electromechanical systems. Under high pressures, they possess multiple transformations into phases with unique electronic properties. However, required pressures are prohibitively high for engineering applications, and special technologies are needed for retaining useful high-pressure phases under normal conditions. This project support's research that will explore how large plastic deformations can drastically reduce the phase transformation pressures, manipulate transformation paths to desired phases, and retain them under normal conditions. It will include in situ experimental studies utilizing a new unique device, dynamic rotational diamond anvil cell, coupled to two-scale modeling and simulation efforts. Fundamental relationships and new rules for coupled large plastic straining, microstructure, and phase evolution look to be established. In addition to economic material synthesis, the results intend to be applicable to optimizing surface processing (polishing, turning, scratching, etc.) of strong brittle semiconductors and developing regimes of ductile machining, analyzing their friction and wear. These efforts have the potential to boost domestic research and manufacturing of semiconductors in the US. The project will provide opportunities to educate and train postdoc, graduate, and undergraduate students in the cross-disciplinary fields of multiscale mechanics, high-pressure and severe plastic deformation science, and nanostructured materials, through research in the PI's laboratory and at synchrotron radiation facilities at Argonne National Laboratory. The goal of the research is to find new fundamental rules for combined severe plastic deformations, multiple strain-induced phase transformations in germanium and silicon, and the evolution of the grain size and dislocation density under high pressure for broad ranges of strain rates and particle/grain sizes using dynamic rotational diamond anvil cell with rough diamonds. New methods of measurements and post-processing of in-situ, real-time synchrotron X-ray diffraction patterns look to be established. Phase-field approach to the interaction between discrete shear bands and multiphase transformations in a polycrystalline sample seek to be developed. Physically-based macroscale theory looks to be developed and utilized for finite-element simulation of sample behavior. Iterative coupling between experiments and simulations will allow refinement, calibration, and verification of all models and determination of the evolution of fields of stress, plastic strain, and strain rate tensors, the volume fraction of phases, grain size, and dislocation density. Various formulated hypotheses will be verified. New fundamental science intends to represent the foundation for producing the nanograined phases and multiphase structures by severe plastic deformation and for processes occurring during surface processing and friction. 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.