Bulk Metallic Glasses XIV: Structures and Mechanical Properties I
Sponsored by: TMS Structural Materials Division, TMS: Mechanical Behavior of Materials Committee
Program Organizers: Peter Liaw, University of Tennessee; Hahn Choo, University of Tennessee; Yanfei Gao, University of Tennessee; Yunfeng Shi, Rensselaer Polytechnic Institute; Xie Xie, The University of Tennessee; Gongyao Wang, The University of Tennessee; Jianzhong Jiang, Zhejiang University

Tuesday 8:30 AM
February 28, 2017
Room: 33A
Location: San Diego Convention Ctr

Session Chair: Takeshi Egami, The University of Tennessee; Katharine Flores, Washington University

8:30 AM  Keynote
Dynamic Atomic Cooperativity in Liquids and Glasses: Takeshi Egami1; 1University of Tennessee
    It is well-known that supercooled liquid shows behavior different from that of simple liquid. We show this behavior actually has nothing to do with supercooling, but is an intrinsic property of any liquid associated with dynamic cooperativity crossover related to the lifetime of phonon. Above the cooperativity crossover temperature, TA, atoms are dynamically uncorrelated. But below TA atomic dynamics become cooperative and correlated in space and time. We propose to call the liquid below TA “cooperative liquid”. Viscosity increases rapidly with increased cooperativity, culminating in the glass transition. Therefore TA is a more fundamental temperature than the glass transition temperature Tg. Externally applied stress suppresses cooperativity, leading to decrease in viscosity and glass-to-liquid transition, resulting in mechanical failure. We discuss simulations and scattering experiments which support this view. This work is supported by the Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division.

9:00 AM  Invited
Flexibility Volume as a Universal Structural Parameter to Quantitatively Predict Metallic Glass Properties: Evan Ma1; 1Johns Hopkins University
    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 space and interactions, measurable both computationally and experimentally. This indicator deterministically predicts the shear modulus, which is at the heart of key properties of MGs. The flexibility volume correlates strongly with atomic packing topology, and with the activation energy for thermally activated relaxation or the propensity for stress-driven shear transformations, providing a structural underpinning of the mechanical heterogeneities. The concrete structure-property correlations discovered are robust and prognostic for all compositions, processing conditions and length scales. We advocate flexibility volume as a single-parameter indicator, in lieu of the widely cited“free volume”, in understanding the structural state and properties of MGs.

9:20 AM  Invited
Deformation Induced Heterogeneities in Metallic Glasses: Robert Maass1; 1University of Illinois at Urbana-Champaign
    Monolithic bulk metallic glasses (BMGs) are generally considered to be free of internal length scales. However, the atomically disordered structure is known to exhibit fluctuating properties at the nanometer scale, which can be revealed with distributions of pop-in stresses or elastic modulus variations. How such structural fluctuations evolve upon the application of load, and how deformation may alter/introduce internal length scales is not yet well understood. Here we discuss recent experimental insights into structural heterogeneities of as-cast BMGs, how the application of an elastic load affects those, and how strain localization can lead to length scales. In particular, we will address changes in nano-scale elastic properties, critical stress distributions, and fluctuations of internal stresses. The results extend from the nano- to the micrometer scale, and reveal unexpected long-range fluctuations in a material that is atomically disordered. We discussed our findings in the context of damage accumulation in BMGs.

9:40 AM  Invited
Hierarchical Heterogeneities in Bulk Metallic Glasses: Peter Tsai1; Kelly Kranjc1; Katharine Flores1; 1Washington University
    Structural heterogeneities play a critical role in determining the plastic deformation mechanisms in metallic glasses. Here, modulus mapping with a nanoindenter probe is used to detect heterogeneities in a Zr-based glass. As-cast samples displayed a networked arrangement of elastically stiff and compliant regions that was strongly correlated with the temperature gradient during quenching. Upon annealing, the statistical distribution of the measured modulus values narrowed while bifurcating into two distinct peaks. Laser pulsed samples showed a more pronounced heterogeneous elastic microstructure with decreasing cooling rate. While the widths of the modulus distributions varied with processing, the maps yielded similar sizes and spacing of the heterogeneities under all conditions, on the order of 100 nm. Nanoindentation arrays performed with an indent spacing of 3 μm also showed a high degree of spatial variability of the hardness and modulus. These results suggest the existence of a hierarchy of heterogeneities in the metallic glass.

10:00 AM  Invited
A Study on the Formation and Propagation Behavior of Shear Bands in Metallic Glasses: Ke-Fu Yao1; Guan-Nan Yang1; Yang Shao1; 1Tsinghua University
    It is known that metallic glasses deform through formation and propagation of shear bands. But the shear band formation, propagation, and fine structure within shear band and its evolution are still not clear, despite of that considerable attentions have been focused on studying the basic structure of shear bands and their related properties. Recently, we have investigated and characterized the formation and propagation behaviors of shear bands in metallic glasses by using high-resolution transmission electron microscopy, including the temperature rise in the shear bands by a model considering the simultaneous heat generation and dissipation based on heat transfer theory. It shows that there exists fine structure change within the shear bands. Nano-void formation and coalescence within the shear bands have been clearly observed. When the stress drop caused by shear band formation is small, the temperature rise is small. Then, the deformation mechanism of metallic glasses has been discussed.

10:20 AM Break

10:40 AM  Invited
An Assessment of Ternary Bulk Metallic Glasses: Correlations between Structure, Glass Forming Ability & Stability: Kevin Laws1; Daniel Miracle2; Dmitri Louzguine-Luzgin3; Larissa Louzguina-Luzgina3; 1University of New South Wales; 2Air Force Research Laboratory,; 3Tohoku University
    We present an assessment of over 840 distinct ternary bulk metallic glasses from 75 ternary alloy systems. For each of these glasses, the structure is quantified by coordination and packing efficiency utilising the efficient cluster-packing (ECP) model, using reported glass constitutions as input. Published glass transition (Tg), crystallization (TX) and liquidus temperatures (Tl) are used to compute thermal stability parameters. Comparisons between packing efficiency, stability parameters and critical casting size are presented and the strongest correlations are identified. The frequency and relative glass forming ability of specific topological families is also assessed. Results indicate a strong correlation between the average packing efficiency of constituents and glass forming ability. Further, specific topologies are preferred for bulk glass-forming alloys and others are absent, falling in-line with previously postulated topological exclusion considerations.

11:00 AM  Invited
High Pressure, High Temperature Structural Study of Zr-based Glasses: Wojciech Dmowski1; Stanislaw Gierlotka2; Yoshihiko Yokoyama3; Takeshi Egami1; 1University of Tennessee; 2Institute of High Pressure Physics of the Polish Academy of Sciences; 3Tohoku University
    There have been observations that hydrostatic pressure affects glass temperature (Tg), induces structural relaxation, and delays crystallization of bulk metallic glasses (BMG). We investigated structure and microstructure of Zr-based bulk metallic glasses after high pressure quenching form the supercooled melt. We observed formation of variety of structures: single phase glasses, glass-crystalline composites, and nanocrystalline phases. The final structure deepened on both pressure and temperature. However, all samples were crystallized when heated above Tg+150 deg even at 8 GPa. Using high energy X-ray diffraction, pair density function analysis and TEM we identified distinct glass and glass-nanocrystalline structures. Mechanical testes were done on single phase glasses. We also performed molecular dynamics simulation of the high pressure quenching and obtained model glasses. The MD data qualitatively agrees with the experimental PDF. Work supported by the U.S. DOE, DE-AC05-00OR-22725.

11:20 AM  Invited
Effect of Stress on Crystallization Pathways in Metallic Glasses: M. Naeem1; S. Lan1; B. Wang1; Yang Ren2; Xun-Li Wang1; 1City University of Hong Kong; 2Argonne National Laboratory
    Ca-Mg-Cu bulk metallic glasses have low density, and their elastic and bulk moduli are comparable to human bones which make them potential candidate for biomedical applications. They also have low characteristic temperatures, e.g., Tg, just above 100C. It is therefore crucial to investigate their phase stability or resistance against crystallization at high temperatures and under stress, in order to explore their suitability for potential applications. In-situ high-energy synchrotron diffraction measurements show that when thermally annealed without load, the crystallization process started right from the beginning with rapid growth. However, when a compressive stress was applied simultaneously during annealing, there was no evidence of crystallization; only structure relaxation was observed. It is suggested that the applied stress restricted the atomic mobility, which suppressed structure relaxation and crystallization.

11:40 AM  Student
Discovering a Unique Thermal-driven Glass-glass Transition in Metallic Glass: Qing Du1; Xiongjun Liu1; Qiaoshi Zeng2; En Ma3; Hui Wang1; Yuan Wu1; Z.P. Lu1; 1University of Science and Technology Beijing; 2Center for High Pressure Science and Technology Advanced Research; 3Johns Hopkins University
    Analogous to the well-known polymorphism in crystalline materials, the polyamorphic transition in glassy matters is also a ubiquitous and intriguing phenomenon in the natural world, such as the glassy water (amorphous ice) can exit in two distinct forms: low- and high-density amorphous ices, under different pressures. Recently, pressure-induced glass-glass transitions (GGTs) have also been observed in some lanthanide-based and Ca-Al metallic glasses, where the GGT seems impossible due to their densely-packed atomic structures. In this talk, we will report a non-pressure-induced GGT in a metallic glass. In particular, the following information will be delivered: (1) a unique thermal-induced GGT under ambient pressure condition is discovered in this metallic glass;(2) both in-situ synchrotron X-ray experiments and atomistic simulations are performed for observation of structure revolution;(3) discuss the difference between the present and the previously reported GGTs in metallic glasses, and the importance of this finding.