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4.26】 Dr. Hao Zhang
Topic:String-like Collective Motion in Grain Boundaries...
 
2011-04-19 | 文章来源:外事办        【 】【打印】【关闭

Topic:String-like Collective Motion in Grain Boundaries and Nanoparticles

Speaker: Dr. Hao Zhang, Chemical and Materials Engineering, University of Alberta, Canada

Time: 9:00 AM, 26, April 2011

Venue: Room 403, R&D Center, IMR CAS

Welcome to Attend!

 

String-like Collective Motion in Grain Boundaries and Nanoparticles 

Hao Zhang

Chemical and Materials Engineering, University of Alberta

Polycrystalline materials can be viewed as composites of crystalline “grains” separated from one another by thin “amorphous” grain boundary (GB) regions. While GBs have been exhaustively investigated at low temperatures (T), where these regions are relatively ordered, much less is known about them at higher T where they exhibit structural disorder, and where characterization methods are limited. The time and spatial scales accessible to molecular dynamics (MD) simulation are appropriate for investigating the dynamical and structural properties of GB at elevated T and we exploit MD to explore basic aspects of GB dynamics as a function of T. It has long been hypothesized, based on the processing characteristics of polycrystalline materials, that GBs have features in common with glass-forming liquids. We find remarkable support for this uggestion, as evidenced by string-like collective motion, transient caging of atom motion, and non-Arrhenius T dependence of GB mobility. Evidently, the frustration caused by the inability of atoms in the GB region to simultaneously order with respect to competing grains is responsible for this similarity. The paradigm that grains are encapsulated by a “frustrated fluid” provides a powerful conceptual model of polycrystalline materials. The same numerical metrologies have been employed to study the interfacial dynamics in nanoparticles (NP). Instead of a simple fluid layer on the NP surface, we find a prevalence of string-like collective atomic motions where the geometrical nature of these collective excitations is found to be quantitatively like the collective atomic motions found in glass-forming liquids. We consider a phenomenon that might be of experimental interest in relation to understanding the catalytic properties of NP- the effect of alloying on the NP interfacial dynamics.

Biography:

Before joining the University of Alberta in September 2007, Dr. Zhang was a postdoctoral research associate at Princeton University. He received B.E. and M.S. degrees in Materials Science and Engineering from Tsinghua University and PhD degree in Mechanical and Aerospace Engineering from Princeton University.

The general area of research of Dr. Zhang is computational and theoretical materials science. Specifically, he is interested in the structure and the properties of defects, atomic-level mechanisms of defect migration and microstructural evolution in polycrystalline materials. In particular, Dr. Zhang focuses on examining anisotropic properties in grain boundaries, identifying the atomistic mechanism by which grain boundaries migrate, exploring the fundamental mechanisms behind the plastic deformation of nanocrystalline materials, investigating mechanical response in nanostructured materials, understanding interfacial dynamics in nanoparticle, and predicting microstructural evolution in polycrystalline metals.

For more information, please visit http://www.ualberta.ca/~hao7/

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