Materials Research Lecture

In this talk, I will address the fundamental atomic mechanism of deformation by focusing on metallic glass. Metallic glass is not only a new class of structural and functional material, but also a simple model system for studying disordered or amorphous solids. Under external applied stress, the atoms in ordered crystalline metals displace away from their equilibrium positions in neatly the same way and when the stress reaches the so-called "yield point," this highly correlated atomic motion is replaced by local deformation caused by the known crystal defect, dislocation. But although the same macroscopic mechanical behaviors are observed, what happens in metallic glasses? As in crystals, should there be an equivalent defect that causes localized deformation? Or is it just the spontaneous formation of the atomic displacement "pattern" when the system reaches some sort of instability? From a practical point of view, how can one control and manipulate the mechanical properties by varying the size of the sample (as has been done in crystalline metals)?

To answer these questions, I shall go through two topics relevant to the above questions. The first regards searching for theoretical strength, and the other, the size effects on mechanical properties of metallic glasses. Theoretical strength represents the highest value of strength that a defect-free material can achieve. In the course of pursuing this strength value, we can perceive the underlying deformation mechanisms. Using a real space random potential model, we show that the maximum strength in metallic glasses is comparable to that of crystals. In addition, the deformation process is highly de-correlated with atoms moving in their own random potential wells. Therefore, the "defects" are present intrinsically in the amorphous solids that eventually lead to localization and a series of other mechanical properties unique to this class of materials, i.e. serrated flow, modulus softening, and large elastic limit.

I will also show that without extended structural defects (such as dislocations and grain boundaries), the strength is related to the compatibility of the characteristic length scales between the sample size and some intrinsic material process during deformation.  We have identified several relevant characteristic length scales. Our results indicate short-range correlation length in deformation fluctuations in metallic glass and an ever increasing "capillary" effect generated from surface that ultimately control the mechanical response of metallic glasses at small scales.

More about the speaker: Professor Mo Li received his Ph.D. in Applied Physics in 1994 and M.S. in Materials Science in 1990 from California Institute of Technology under the supervision of Prof. William L. Johnson and William A. Goddard. After a brief staying as a postdoctoral fellow at Caltech and the Argonne National Laboratory, he joined Morgan Stanley & Co. in New York. He came back to academia in 1998. From 1998 to 2001 he was an assistant professor at the Johns Hopkins University. Currently, he is a professor at the Georgia Institute of Technology. He is the recipient of the National Thousands Talent Program Award of China and the Alexander von Humboldt Young researcher award. Professor Li's research focuses on understanding fundamental properties and processes of materials, and predicting material behaviors. The approaches used in his research are a blend of those from statistical physics, solid state physics, materials science, metallurgy, mechanics and large scale, high performance computing. His research focuses on algorithm development, simulation, and theoretical analysis.