Brown University
Joint Materials/Solid Mechanics Seminar Series
Departments of Mechanical Engineering and Materials Science and Engineering. Johns Hopkins University, Baltimore, MD, USA 21218, hemker@jhu.edu | |
Abstract: Micro-tensile testing and TEM have been employed to characterize the small-scale and scale-specific mechanical behavior of materials for MEMS and nanocrystalline thin films. MEMS structures are often deposited far from microstructural equilibrium and the mechanical properties of these materials will be shown to be strongly processing, microstructure and temperature dependent. The extremely high hardness and strength of nanocrystalline materials have been well-documented, but operative deformation mechanisms in these materials have proven to be much harder to quantify. In the second study, recent experimental observations of twinning and grain growth in nc-Al will be used to underscore the novel deformation mechanisms operative in nanocrystalline metals. Of particular interest is the extended plasticity that occurs as a result of stress-induced discontinuous grain growth. Efforts to model this growth with traditional driving forces have proven less than satisfactory, and the importance of grain boundary pinning and the role of stress assisted grain boundary migration appear to be more important. The significance of this finding with respect to the reliability of thin film nanocrystalline devices cannot be ignored, as the mechanical behavior of these structures appears to not only be different than that of microcrystalline metals but dynamic as well.
Biography: Kevin Hemker is currently a Professor in the Department of Mechanical Engineering at Johns Hopkins University, with joint appointments in the Departments of Materials Science and Engineering and Earth and Planetary Sciences. He was a NSF National Young Investigator, an invited Professor at the EPFL (1995) and the University of Paris XIII (2001), and received the ASM Materials Science Research Silver Medal in 2001. His research group is engaged in research on: the identification of processing-microstructure-property relations in materials for microelectromechanical systems (MEMS), characterization and modeling of thermal barrier coatings, deformation of nanocrystalline metals and HREM characterization of dislocation core geometries in metals and alloys. He has authored or co-authored over 100 journal articles and conference proceedings, co-edited three books and is an editor of Scripta Materialia.
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