Plasticity, Phase Transformations, and their Interaction under High Pressure in Silicon

Iowa State University, Ames IA

Investigators

Abstract

The interaction between crystalline phase transformations and permanent plastic deformation under high pressure in materials is broadly found in several technological applications; however, knowledge of this phenomenon is scarce because of the specialized equipment needed to observe it. This award fills the gap by supporting an integrated experimental, theoretical, and computational study of this interaction in silicon. Silicon is chosen as a representative material because it exhibits numerous types of phase transformations and is widely used in electronics and micro-electromechanical systems, as well as solar cells. The knowledge gained on the fundamental aspects of phase transformation-plasticity interaction under high pressure will also have broader implications on the quantitative modeling and optimization of surface processes, such as polishing, turning, and scratching, for brittle semiconductors and ceramics. As part of the project, there will be opportunities to educate and train graduate and undergraduate students through special courses and research, parts of which will be conducted at the Argonne National Laboratory, with an emphasis on underrepresented students. The objective of this research is to investigate the effect of plastic flow on phase transformation under high pressure in silicon. Coupled in situ high-throughput experiments and modeling will yield quantitative characterization and understanding of how plastic straining drastically reduces the transformation pressure, promotes the formation of novel phases, retains high-pressure phases at ambient pressure, and changes transformation paths between different phases. Silicon samples will be compressed and sheared in a rotational diamond anvil cell. Synchrotron X-ray diffraction and absorption, Raman spectroscopy, and displacement measurements will be used for in-situ diagnostics of the deformation and transformation processes. A microscale phase field approach for coupled phase transformations and localized shear bands (e.g., due to dislocation pileups) will be developed and used to study the interplay between multiple phase transformations and mechanisms of plasticity in single grains and polycrystalline aggregates. Phase transformation criteria, strain-controlled kinetic equations, and the pressure- and plastic strain-dependent yield strengths of all phases and phase mixtures will be obtained from the experiments and incorporated in a macroscale phenomenological model. This model will be used in the finite element simulations of deformation-transformation processes in diamond anvils. A search for new phases will also be performed as an application of the computational framework. 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.