Materials Research Lecture

This talk addresses ten frequently-asked questions regarding the local atomic packing order and topological arrangements in the structures of metallic glasses and their parent supercooled liquids, as well as the correlations between these structural features and the properties of these amorphous metals. We illustrate why 1) dense random packing of hard spheres or stereochemical models do not suffice, and 2) why the amorphous structure is better dissected on the basis of quasi-equivalent atomic packing motifs, but cannot be perceived as "clusters + empty space". 3) The common approaches used to probe the atomic structure in metallic glasses are briefly summarized. Furthermore, we explain 4) at what compositions (Cu-centered) full icosahedra constitute the dominant and characteristic short-range-ordered coordination, and demonstrate their evolution and extent during prolonged structural relaxation and the resulting reduction in potential energy and slow-down of dynamics, as well as the formation and percolation of networks formed by interpenetrating connection of icosahedra to constitute a stiff backbone over extended range. We also 5) differentiate full-icosahedra based short-range ordering from the generally favorable five-fold bonds, and 6) look at the accompanying preference of a particular type of Zr-centered Kapser coordination polyhedra, to compare the Cu-based perspective with a Zr-centric view. In addition, we will 7) show other non-icosahedral ordering varieties in alloys with different atomic size ratios, and 8) comment on the most "liquid-like" local environments (referred to as geometrically unfavored motifs, GUMs) to identify soft spots that contribute the most to low-frequency vibrational modes and shear transformations. The structural perspective also 9) guided our search to land a high-toughness BMG with record-breaking damage tolerance, and 10) enabled us to identify a structural signature of the temperature-dependent heat capacity of supercooled liquids and their fragility. Taken together, the ten issues addressed set the stage for understanding structure-property relations in metallic glasses and liquids. In particular, the considerable diversity in the short-range ordering, with favored and unfavored local atomic configurations coexisting in an inhomogeneous amorphous structure, suggests that by tailoring the population and contribution of these local motifs via controlled rejuvenation of the amorphous structure in thermomechanical processing, the desired deformability of the glass may be enhanced. We also comment on a quantitative and universal prediction of the shear modulus of the glasses, using a dynamics variable related to vibrationally probed/accessed space as a structural indicator.

More about the speaker:

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 Johns Hopkins University. Prof. Ma has published ~290 papers (w/ ~18,000 citations and h index=70) and presented ~110 invited talks at international conferences. He is an elected Fellow of ASM, APS, and Materials Research Society (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, nanostructured metals, plasticity mechanisms, and in situ transmission electron microscopy of small-volume materials.