报告人：Prof. E. Ma, Department of Materials Science and Engineering, Johns Hopkins University
E. Ma did his graduate work at Tsinghua University (China) and Caltech, followed by postdoc sojourns at MIT and Univ. of Michigan. He was an assistant professor at LSU and is currently a professor in the Department of Materials Science and Engineering at the Johns Hopkins University. Prof. Ma has published ~300 peer-reviewed papers (w/ ~19,000 SCI citations and h index=71; and ~24,700 citations and h index=83, according to Google Scholar), and presented ~115 invited talks at international conferences. He is an elected Fellow of ASM, APS, and MRS. Dr. Ma has also been an adjunct professor at Xi’an Jiaotong Univ. (China) since 2009. His current research interests include amorphous metals (metallic glasses), chalcogenide phase-change alloys for memory applications, strength-ductility synergy in nanostructured metals, plasticity mechanisms, and in situ transmission electron microscopy of small-volume materials.
Quantitatively correlating the amorphous structure in metallic glasses with their physical properties has been a long-sought goal. Here we introduce “flexibility volume” as a universal indicator of the structural state, to correlate with properties on both atomic and macroscopic levels. The flexibility volume is assessed via atomic vibrations that probe local configurational space and interaction between neighboring atoms, and is defined in a simple form to be measurable both computationally and experimentally. We show that this indicator deterministically predicts the shear modulus, which is at the heart of key properties of metallic glasses. The flexibility volume correlates strongly on the one hand with atomic packing topology, and on the other hand with the activation energy for thermally activated relaxation and the propensity for stress-driven shear transformations. As such, the flexibility volume provides a structural underpinning of the mechanical heterogeneities. The concrete structure-property correlations discovered are robust and prognostic for all metallic glass compositions, processing conditions and length scales. All these advantages advocate flexibility volume as a useful single-parameter indicator, in lieu of the widely cited but ambiguous “free volume”, to bridge the configurational state a metallic glass is in with its properties.