Topic：Probing Mechanical Properties of Nanostructured Materials via Ultra-Large Scale Molecular Dynamics Simulations 

Speaker：Prof. Dr. Huajian Gao (Walter H. Annenberg Professor of Engineering )

　　　　　　　　 Brown University, USA

Location：Rm.468, Lee Hsun Building

Time：9：30 - 11：30,  Dec. 23

Welcome to attend！

 

Probing Mechanical Properties of Nanostructured Materials via Ultra-Large Scale Molecular Dynamics Simulations 

Huajian Gao
Walter H. Annenberg Professor of Engineering
Brown University 

The rapid development of synthesis and characterization of nanostructured materials as well as unprecedented computational power have brought forth a new era of materials research in which experiments, simulation and modeling are performed side by side to probe the unique mechanical properties of nanoscale materials. As a prominent example of recent advances in this area, we have collaborated with Drs. Ke Lu and Lei Lu on dislocation nucleation governed softening and maximum strength in nano-twinned metals. In conventional metals, there is plenty of space for dislocations to multiply so that the strength of material is controlled by dislocations interaction with grain boundaries (Hall-Petch strengthening) and other obstacles. For nanostructured materials, in contrast, multiplication and motion of dislocations are severely confined by the nano-scale geometries so that continued plasticity can be expected to be source-controlled. Nano-grained polycrystalline materials were found to be very strong but brittle, owing to the fact that both nucleation and motion of dislocations are effectively suppressed by the nano-scale crystallites. We report a dislocation source controlled mechanism in the newly-developed nano-twinned metals in which there are plenty of dislocation nucleation sites while dislocation motion is not confined. We show that dislocation nucleation plays the governing role in the strength of such materials, resulting in their softening below a critical twin thickness. Ultra-large-scale molecular dynamics simulations and a kinetic theory of dislocation nucleation in nano-twinned metals show that there exists a transition in deformation mechanism, which occurs at a critical twin boundary spacing where the strength is maximized, from the classical Hall-Petch type of strengthening due to dislocation pile-up and cutting through twin planes to a dislocation nucleation governed softening mechanism with nucleation and motion of partial dislocations parallel to the twin planes (twin boundary migration). The simulations indicate that the critical twin boundary spacing for the onset of softening in nano-twinned Cu and the maximum strength depend on the grain size: the smaller the grain size, the smaller the critical twin spacing, and the higher the maximal strength of the material.

Should time permit, I will also discuss a number of other projects in my research group, including plastic strain recoveries [1] and early Baushinger effect in free standing nanocrystalline films with grain sizes on the order of 50 nm.

[1] X. Li, Y. Wei, W. Yang and H. Gao, “Competing grain-boundary- and dislocation-mediated mechanisms in plastic strain recovery in nanocrystalline aluminum,” 2009, Proceedings of the National Academy of Sciences of USA, Vol. 106 (38),16108-16113.

 

About Prof. Dr. Huajian Gao

Huajian Gao received his B.S. degree from Xian Jiaotong University in 1982, and his M.S. and Ph.D. degrees in Engineering Science from Harvard University in 1984 and 1988, respectively. He served on the faculty of Stanford University between 1988 and 2002, where he was promoted to Associate Professor with tenure in 1994 and to Full Professor in 2000. He served as Director at the Max Planck Institute for Metals Research between 2001 and 2006 before joining the Faculty of Brown University in 2006. At present, he is the Walter H. Annenberg Professor of Engineering at Brown.

Professor Gao has a background in engineering science and applied mechanics. His research is focused on the understanding of basic principles that control mechanical properties and behaviors of materials in both engineering and biology. His research group studies how metallic and semiconductor materials behave in thin film and nanocrystalline forms, and how biological materials such as bone, gecko and cell achieve their mechanical robustness through structural hierarchy. He has broad collaborations with scientists in the United States, Europe and China.

He is an author/co-author of more than 250 scientific papers with more than 8000 citations and an h-index of 47. He is co-editor-in-chief of the Journal of the Mechanics and Physics of Solids (2006), the flagship journal of his field. He is also Editor-in-Chief of Acta Mechanica Sinica and International Journal of Applied Mechanics, and serves on the editorial boards of 8 other international journals.

He is the recipient of numerous academic honors ranging from the NSF Young Investigator Award (1993), John Simon Guggenheim Memorial Fellowship Award (1995) to Distinguished Scholars and Artists Advisor for the Guggenheim Foundation (2008-09). Among his recent honors are the 2009 Robert Henry Thurston Lecture Award from the American Society of Mechanical Engineers; the Visiting Investigator Program Award (2007) from the Agency for Science, Technology and Research, Singapore; the 2005 Wissenschaftspreis des Stifterverbands (research innovation award) from the Max Planck Society; the Society of Engineering Sciences Young Investigator Award (2005); the Chang Jiang Scholar Visiting Professorship Award at Tsinghua University and Outstanding Overseas Young Investigator Award from the Ministry of Education of China (2000-05); and various lectureships.