Postdoctoral Researcher, University of California, Berkeley
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“Nonequilibrium Deformation and Engineering of Materials”
Materials are rarely under thermodynamic equilibrium despite that most existing theories assume so. Shock wave, with an ultrashort time scale, is an extreme example of nonequilibrium phenomenon which either hinters or promotes phase transformations in materials that are often omitted by equilibrium phase diagrams. On the other hand, nonequilibrium techniques that subject materials to extreme temperatures, stress state, and/or electrical fields, producing metastable microstructures not accessible by conventional processes. These “abnormalities” beg outstanding scientific questions such as how material deforms in conditions that are far from equilibrium, and can we harness these nonequilibria to manufacture new materials? In the first part of the webinar, I will discuss how the nanosecond pulsed lasers with power on the order of terawatts, can create extreme, nonequilibrium conditions which combine unprecedented high pressures, strain rates and temperatures. I will summarize our efforts on laser shock compression of four covalently bonded materials, namely, silicon, germanium, boron carbide and silicon carbide. These materials are known to exhibit high Peierls-Nabarro stress and negative Clapeyron slope. The shock deformation microstructure has been revealed by high-resolution scanning/transmission electron microscopy.
Our work indicates stress-induced, solid state amorphization is a generalized deformation mechanism. In the second part of the webinar, I will highlight our recent work towards nonequilibrium defect engineering using electrical pulsing and cryogenic pressing. We demonstrate that dislocation configuration in a model material (Ti-7at%Al) can be altered from localized 2-D planar slip to homogeneous 3-D wavy slip by the application of high intensity electrical pulse, which leads to an enhanced ductility. I will also talk about our recent success in using cryogenic pressing as an effective way to engineer desirable multi-scale, coherent interfaces in Titanium alloys which achieves unprecedented strength-ductility synergy. I will conclude my talk with discussions on the new opportunities of nonequilibrium material science enabled by novel experimental mechanics and multi-scale characterization techniques.
Shiteng Zhao, Ph.D., Postdoctoral Researcher, University of California, Berkeley
Shiteng Zhao is currently a postdoc fellow in the Department of Materials Science and Engineering, University of California, Berkeley. He is also a research affiliate with the National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory. Prior to his postdoc appointment, Shiteng received his Ph.D. from University of California, San Diego in the Department of Mechanical and Aerospace Engineering, his M.S. of metallurgical engineering from RWTH Aachen University in Germany, and his B.S. of materials science and engineering from the University of Science and Technology Beijing, China. Shiteng’s research interests lie in the characterization, microstructural design, and mechanical behavior of materials, especially under extreme loading conditions of high temperature, high pressures, and high strain rates. He also uses both in-situ and ex-situ electron microscopy to reveal the deformation mechanisms of the materials.
Recently, Shiteng received the Acta Materialia Student Award and Gareth Thomas Materials Excellence Award for his work in the shock-induced amorphization in covalently bonded materials.