Ph.D. Oral Defense - Julia R. Greer
Julia R Greer
jules83 at stanford.edu
Mon May 9 13:53:43 PDT 2005
I will be defending my dissertation on Tuesday, May 17th, at 1pm in the CIS-X Auditorium.
Refreshments will be provided at 12:45. Please find the attached abstract below. I really hope some of you will be able to come.
Size Effect in Plasticity of Metals at the Sub-Micron Scale
Julia R. Greer (Advisor: Professor William D. Nix)
Department of Materials Science and Engineering
The results of both experimental studies and molecular dynamics simulations indicate that crystals exhibit strong size effects at the sub-micron scale. In experimental studies, the size effects are usually explained by the presence of strain gradients. One of the more prominent strain gradient plasticity theories was developed by Nix and Gao. While this model accurately predicts the observed increase in hardness as the deformation volume decreases, it fails to describe the discrete behavior indicative of nucleation of dislocations at the indentation depths shallower than ~100nm. By contrast, more recent experiments and some molecular dynamics (MD) simulations suggest that the yield strength of crystalline materials depends on the specimen size even without strain gradients and scales with the sample size through a power relationship. In the scope of this work, results of uniaxial compression experiments of free-standing gold single-crystalline and polycrystalline "nanopillars" without imposing significant stress/strain gradients are presented. These Au cylinders are created by focused ion beam (FIB) machining and by Microlithography/Electroplating techniques. They are subsequently compressed using a Nanoindenter fitted with a custom-fabricated diamond flat punch. Compressive stresses and strains, as well as pillar stiffnesses are determined from the test data. The experiments show that the flow stresses of these pillars increase significantly with decreasing pillar volume, reaching stresses as high as 0.8 GPa for the smallest pillars. These results indicate a 50x flow stress increase compared to that of bulk gold which is reported to be ~20 MPa at 2% strain. These high strengths appear to be controlled by dislocation starvation, unique to very small crystals. TEM studies and dislocation starvation model results are discussed.
Julia R. Greer
Ph.D. Candidate in Materials Science and Engineering
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