Non Technical Abstract: The ease with which materials like metals bend, stretch and deform depends on the defects inside the crystals. Learning how such defects interact with one another is difficult since there aren't any experimental tools for imaging the three dimensional interactions of these defects with one another at the atomic scale. This project entails looking at tiny particles called colloids that are suspended in a fluid and ordered into crystals. Crystals made from these particles exhibit many of the same defects and defect interactions found in their atomic counterparts, but the particles are big enough and slow enough to examine using microscopes. This project will use a new computational technique developed by the principle investigator and the co-principle investigator to determine the forces that crystal defects exert on one another through these particles. In addition, the forces these defect interactions produce on a large scale will be measured using equipment developed by the principle investigator. The defects being investigated include vacancies, where single particles are missing from the crystal, dislocations, where an entire plane of atoms is missing, grain boundaries, where crystals with different orientations meet, and cracks, where particles lose contact with one another. Comparison of the forces driving these defects to move and interact with one another on different length scales is enabling the design and manipulation of crystals to determine their large scale mechanical properties. The combination of basic research involving fundamental issues in condensed matter physics with practical issues in experiment development provides an excellent training ground; students working on this project will recieve training in state-of-the-art experimental and theoertical techniques which will provide them with an excellent preparation to enter positions in academia, government laboratories, and high-tech industry, given their background in both basic and applied work.

Technical Abstract

The dominant mechanisms for creating large irreversible strain in crystals is the nucleation and dynamic evolution of defects including vacancies, dislocations, and grain boundaries. Such defects are central to our understanding of yield, work hardening, fracture, fatigue, and time-dependent elasticity in atomic crystals. A major hurdle faced by the materials science community is to understand how the spatially heterogeneous and history dependent local stress environment created by such defects further affects their evolution and determines the bulk mechanical properties of the crystal. Using a newly developed technique called Stress Assessment from Local Structural Anisotropy (SALSA) this project is mapping out stress inhomogeneities that arise from such defects and their interactions in colloidal crystals. Crucially, SALSA allows for mapping out these stresses at the single particle scale. A triaxial confocal rheometer is being used to measure bulk properties and determine how the local stress inhomogeneities lead to the macroscopic response of the crystal. The project focuses on several major defect mediated phenomena found in crystals. A nonlinear dynamics theory of vacancy interactions with other defects is being developed. In addition these techniques are being used to study the interaction of dislocations with vacancies, other dislocations, stacking faults and grain boundaries. A combination of bulk compression and shear will determine how the mechanical response of polycrystalline domains under shear varies as the film thickness approaches the length scale of a single grain. Finally, depletion interactions is being used to form attractive crystals in order to measure the stresses arising from crack propagation in such crystals. This research is enabling a fundamental understanding of the connection between the stress fields driving defect mobility and the resulting bulk mechanical response of the crystal. The projects chosen reflect the crucial roles played by vacancies, dislocations, grain boundaries, and cracks in mediating these properties.

National Science Foundation (NSF)
Division of Materials Research (DMR)
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Paul Sokol
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Cornell University
United States
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