报告1：Strength Statistics of Single Crystals and Metallic Glasses under Small Stressed Volumes
主 持 人：高志文 副教授
The plastic deformation of crystalline and amorphous metals/alloys shows a general trend of "smaller is stronger". However, most studies have been focused on finding and reasoning the scaling slope or exponent in the logarithmic plot of strength versus size. Here we show that the universal picture should rather be the thermally activated nucleation mechanisms in small stressed volume, the stochastic behavior as to find the weakest links in intermediate sizes of the stressed volume, and the convolution of these two mechanisms with respect to variables such as indenter radius in nanoindentation pop-in, crystallographic orientation, pre-strain level, sample length as in uniaxial tests, and others. Experiments that cover the entire spectrum of length scales and a unified model that treats both thermal activation and spatial stochasticity have discovered new perspectives in understanding and correlating the strength statistics in a vast of observations in nanoindentation, micro-pillar compression, and fiber/whisker tension tests of single crystals and metallic glasses.
报告2：Nanomechanics of Atomic Friction
主 持 人：高志文 副教授
For well-defined contacts with sizes less than hundreds of nanometers, the friction force-displacement curves are characterized by the stick-slip behavior, which are very sensitive to surface atomic structure, lattice mismatch or incommensurability, sliding velocity, surrounding temperature, to name a few. While the stick-slip behavior can be modeled by the one-degree-of-freedom Tomlinson model, it cannot explain the role of lattice structure and interface defects. Molecular simulations, on the other hand, suffer the temporal limitations and thus have difficulties in modeling the dependence on velocity and temperature. In this work, a Peierls-type model is developed which views the sliding process as the initiation and gliding passage of interface dislocations with diffused cores. The dependence of the friction behavior on the contact size is naturally due to the introduction of the dislocation core size. The spatially inhomogeneous nature of rate-limiting processes is successfully determined, from which a quantitative comparison to the thermally activated friction behavior can be made. A number of experiments in literature will be compared.
主 持 人：高志文 副教授
Yanfei Gao is a Professor at Department of Materials Science and Engineering, University of Tennessee, USA. He received his BS degree in Engineering Mechanics and a dual BS degree in Computer Science from Tsinghua University in 1999, and PhD from Princeton (advised by Prof. Zhigang Suo) in 2003. After a two-year post-doctoral training at Brown University, he joined the University of Tennessee and Oak Ridge National Laboratory in 2005. His research activities have been focusing on deformation and failure of advanced structural materials such as metallic glasses, superalloys, high entropy alloys, and structural ceramics. He has published more than 120 journal papers, many of which are in Journal of the Mechanics and Physics of Solids and Acta Materialia. His research has been mostly sponsored by the US National Science Foundation and Department of Energy.