Bulk Metallic Glasses XIV: Structures and Mechanical Properties II
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 2:00 PM
February 28, 2017
Room: 33A
Location: San Diego Convention Ctr

Session Chair: Kostas Georgarakis, Cranfield University; Shigenobu Ogata, Osaka University

2:00 PM  Keynote
Rejuvenation of Metallic Glasses: A. Greer1; Kostas Georgarakis; 1University of Cambridge
    In the context of extending the range of glassy states [1], there has recently been much interest in the possible 'rejuvenation' of metallic glasses by a variety of techniques. Rejuvenation is the opposite of relaxation, and involves taking the glass to a state of higher energy (that would be characteristic of formation by quenching from the liquid at a greater rate). Rejuvenated metallic glasses are of interest for several reasons, but principally because they exhibit better plasticity. Both experimental studies and atomistic simulations have advanced our understanding of rejuvenation, its mechanisms, its potential, and its limits. This presentation reviews the recent work and identifies priorities for future work. [1] YH Sun, A Concustell, AL Greer, Thermomechanical processing of metallic glasses: extending the range of the glassy state, Nature Reviews Materials, 1 (2016) 16039.

2:30 PM  Invited
Plasticity--toughness Connections in Ductile Metallic Glasses: Upadrasta Ramamurty1; 1Indian Institute of Science
    "What controls the toughness of a bulk metallic glass (BMG)?" is a question that the BMG community has long been seeking answer for. While parameters like the Poisson's ratio can give a broad indicator as to whether a BMG will be brittle or ductile, they simply cannot be utilized for a detailed understanding. Recently, it was suggested that elastic properties such as the shear modulus can be correlated to the fracture toughness of ductile BMGs. Since the measured toughness depends intricately on the plastic dissipation of the stress concentrations at the crack-tip, the physical basis and the causative connection for a correlation can be argued. Through some recent data in this presentation, I will argue that it is the shear band mediated plasticity that controls toughness.

2:50 PM  Invited
Atomistic Study on Pressure-promoted Thermal Rejuvenation of Metallic Glass: Shigenobu Ogata1; Narumasa Miyazaki1; Masato Wakeda1; 1Osaka University
    Rejuvenation is the structural excitation of glassy materials, and is a promising approach for improving the macroscopic deformability of metallic glasses. This atomistic study proposes the application of compressive hydrostatic pressure during the glass-forming quenching process and demonstrates highly rejuvenated glass states that have not been attainable without the application of pressure. The pressure-promoted rejuvenation process increases the characteristic short- and medium-range order, even though it leads to a higher-energy glassy state. This "local order" – "energy" relation is completely opposite to conventional thinking regarding the relation, suggesting the presence of a well-ordered high-pressure glass/high-energy glass phase. We also demonstrate that the rejuvenated glass made by the pressure-promoted rejuvenation exhibits greater plastic performance than as-quenched glass, and greater strength and stiffness than glass made without the application of pressure. It is thus possible to tune the mechanical properties of glass using the pressure-promoted rejuvenation technique.

3:10 PM  
Exploring the Spectrum of Mechanical Properties and Structural States in Metallic Glasses via Physical Vapor Deposition: Daniel Magagnosc1; Gang Feng2; Le Ye3; Xuemei Cheng3; Daniel Gianola4; 1University of Pennsylvania; 2Villanova University; 3Bryn Mawr College; 4UC Santa Barbara
    Whereas the structural state of metallic glasses is controlled to a limited extent by varying the cooling rate, physical vapor deposition of organic and metallic glasses has been shown produce a wider range of structures. However, the isochemical bounds of mechanical behavior have yet to be elucidated. Here, sputter deposition from a Pd77.5Cu6Si16.5 alloy target while varying the substrate temperature is employed to isochemically control the structural state and mechanical response. Increasing the deposition temperature from 333 K to 461 K results in a 33.5% increase in hardness to 9.69 GPa in amorphous films. Further increasing the temperature leads to a decrease in hardness. Changes in hardness are mirrored by systematic changes in deformation behavior with the most distinct shear bands observed alongside the greatest hardness. The variation in mechanical response, encoded at the time of deposition, indicates wide variations in glass structural state and improved control over structural state.

3:30 PM Break

3:50 PM  Invited
Inverse Notch Effect in Bulk Metallic Glasses: Jie Pan1; Haofei Zhou2; Yi Li3; Huajian Gao2; 1 Institute of Metal Research, Chinese Academy of Sciences; 2Brown University; 3Institute of Metal Research, Chinese Academy of Sciences
    Understanding notch-related failure is crucial for the design of reliable engineering structures. However, substantial controversies exist in the literature on the notch effect in bulk metallic glasses (BMGs), and the underlying physical mechanism responsible for the apparent confusion is still poorly understood. Here we investigate the physical origin of an inverse notch effect in a Zr-based metallic glass, where the tensile strength of the material is dramatically enhanced, rather than decreased (as expected from the stress concentration point of view), by introduction of a notch. Our experiments and molecular dynamics simulations show that the seemingly anomalous inverse notch effect is in fact caused by a transition in failure mechanism from shear banding at the notch tip to cavitation and void coalescence. Based on our theoretical analysis, the transition occurs as the stress triaxiality in the notched sample exceeds a material-dependent threshold value.

4:10 PM  
Dynamics of Inherent Structure Energy Evolution in Metallic Glasses: Yue Fan1; Takuya Iwashita2; Takeshi Egami2; 1University of Michigan, Ann Arbor; 2University of Tennessee, Knoxville
    The studies on dynamics in glassy materials are particularly challenging because of their strongly disordered atomic nature. In the picture of potential energy landscape (PEL), the evolution of inherent structure (IS) energy can be attributed to the competitions between activations and relaxations. In particular, whether glasses will undergo aging or rejuvenation depends on whether the activation barrier is smaller or larger than the relaxation energy. We prepared 6 metallic glass samples with different thermal histories (from 10^13K/s to 10^9 K/s), and further explored their activation barrier and relaxation energy spectra through advanced atomistic sampling technique. The spectra are found sensitive to the systems’ IS energy. Stemmed from such dependence, a self-consistent equation in describing IS energy evolution has been derived. Without any empirical parameter, this equation can well explain the equilibrium line of supercooled liquid and the cooling curves at different rates directly obtained by MD studies.

4:30 PM  Invited
New Soft Magnetic FeCoNi(P, C, B) High-entropy Bulk Metallic Glasses with Large Supercooled Liquid Region: Yanhui Li1; Wei Zhang1; Tianlong Qi1; 1Dalian University of Technology
    The developed high-entropy bulk metallic glasses (HE-BMGs) provide a new strategy to design and synthesis BMGs. More importantly, the HE-BMGs possess excellent mechanical and physical properties inherited from the advantages of both HEAs and BMGs, and show great potential for practical applications. In this work, we reported new FeCoNi(P, C, B) HE-BMGs with large supercooled liquid region (ΔTx), and excellent soft magnetic and mechanical properties. Fully HE-BMG rods were successfully prepared by copper mold casting method. The developed HE-BMGs possess low glass transition temperature (Tg) and large ΔTx. The alloys also exhibit good soft magnetic and mechanical properties. The mechanism of good thermal stability of the supercooled liquid for the present HE-BMGs was discussed in terms of the precipitated phases during crystallization. The developed HE-BMGs with good thermal stability, and soft magnetic and mechanical properties give excellent promise for making MEMS/NEMS devices by thermoplastic processing.

4:50 PM  Invited
Quasi-Elastic Neutron Scattering and Machine Learning Studies of the Arrhenius Crossover Phenomenon and Its Correlation with the Kinetic Fragility in Glass-Forming Metallic Liquids: Abshishek Jaiswal1; Yang Zhang1; 1University of Illinois at Urbana-Champaign
    Most metallic glasses are produced by quenching high-temperature metallic liquids sufficiently fast that the structural relaxation becomes essentially “frozen”. Therefore, an in-depth understanding of the relaxational dynamics of the metallic liquids and its connection to the kinetic fragility is important to unveil the atomic origin of the glass-forming abilities. We performed Quasi-Elastic Neutron Scattering (QENS) measurements of the mean effective diffusion coefficient of glass-forming metallic liquids in the generalized hydrodynamic regime and used machine learning algorithms to analyze the simulated atomic trajectory. We observed a universal Arrhenius crossover from high-temperature Arrhenius to low-temperature super-Arrhenius behavior at reduced Arrhenius crossover temperature θA = TA/Tg. By comparing with many other molecular and network liquids, we found a distinct correlation between the reduced Arrhenius crossover temperature θA and the kinetic fragility index m. These observations provide a way to estimate the low-temperature glassy characteristics (Tg and m) from high-temperature liquid quantities (Eand θA).

5:10 PM  Student
A High-Throughput Approach to Identifying Metallic Glasses and Characterizing Their Mechanical Properties: Juan Wang1; Peter Tsai2; Katharine Flores2; 1Department of Mechanical Engineering and Materials Science, Washington University in Saint Louis; 2Institute of Materials Science and Engineering, Washington University in Saint Louis
    The conventional trial and error approach for exploring glass formation in multicomponent alloy systems is a time-consuming endeavor and provides little insight into critical cooling rates and thermal stability. In this work, multiple alloy systems were explored using laser-deposition fabrication that enables the evaluation of a large continuous composition range in a single fabrication step. Topographically-smooth surface regions were rapidly identified using optical microscopy, and then confirmed to be amorphous via TEM. By varying laser processing parameters, the optimum glass-forming compositions within a fabricated library could be experimentally deduced. Furthermore, nanoindentation was used as a high-throughput characterization tool to establish compositional trends of indentation hardness and modulus within a large region of composition space. These trends in turn may provide valuable insight for designing good glass formers with improved plasticity.