Bulk Metallic Glasses XIV: Structures and Characterization
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

Wednesday 2:00 PM
March 1, 2017
Room: 33A
Location: San Diego Convention Ctr

Session Chair: Paul Voyles, University of Wisconsin, Madison; Jörg Löffler, ETH Zurich

2:00 PM  Invited
Nucleation and Metastable Phase Formation Studied via Calorimetry at Ultrafast Heating and Cooling Rates: Jörg Löffler1; 1ETH Zurich
    Using novel chip-based fast differential calorimetry (FDSC) we have been able to determine thermophysical properties at several 105 K/s heating and cooling rates. In this way we can measure complete time-temperature-transformation diagrams in the undercooled liquid range upon heating and cooling, even for moderate glass formers (see, e.g., Appl. Phys. Lett. <B>104</B> (2014) 251908), and determine the corresponding critical rates of glass formation. At such high rates we were also able to detect new phase transition paths, e.g. a solid-state phase transformation via melting for a Au-based glass, and describe the underlying thermodynamics (Nature Comm. <B>7:11113</B> (2016), doi:10.1038/ncomms11113). These phase transition paths are also seen in binary metallic glass-forming liquids where various metastable phases form as a function of the ultrafast heating and cooling rates. Thus as they appear to be a common feature of many metallic systems, they may need to be considered in future phase transition theories.

2:20 PM  Invited
In-situ Synchrotron High-energy X-ray Diffraction Study of an Amorphous/Nanocrystalline NiTi Alloy during Recrystallization Process: Cun Yu1; Bachir Aoun2; Lishan Cui1; Yinong Liu3; Yang Ren2; 1China University of Petroleum; 2Argonne National Laboratory; 3The University of Western Australia
    NiTi shape memory alloys form amorphous/nanocrystalline mixtures after severe deformation or during film fabrication. To utilize the shape-memory-effect and superelasticity, the mixtures need to be recrystallized via thermal treatment. We used in-situ synchrotron high-energy diffraction coupled with pair-distribution-function (PDF) analysis to study atomistic structural evolution of a severely cold drawn NiTi wire during thermal annealing. It was found that the wire consisted of mainly amorphous phase (~92%) with some nanocrystalline domains. PDF analysis revealed that the amorphous regions underwent structural relaxation via atomic rearrangement above 100 C, which is correlated with an exothermal anomaly in the differential scanning calorimetry curve, before recrystallization. The nanocrystalline domains exhibit a strong cold work induced lattice strain anisotropy, which is gradually released upon heating above 200 C. With further heating nano-grains started to grow accompanied by reducing microstrain. The results will also help us understand the microstructures of nanocomposites with bulk metallic glass matrix.

2:40 PM  Invited
Medium-range Structure and Glass-forming Ability of Metallic Glasses: Jason Maldonis1; Pei Zhang1; Paul Voyles1; 1University of Wisconsin, Madison
    We have used fluctuation microscopy and hybrid reverse Monte Carlo optimization to investigate the relationship between medium-range order (MRO) and glass forming ability (GFA) in metallic glasses. In Zr-Cu-Al alloys, we find a mixture of two types of MRO: crystal-like structure with approximate 6-fold rotational symmetry and icosahedral-like structure with approximate 5-fold symmetry. On annealing, Zr50Cu45Al5, an alloy with high GFA, becomes more icosahedral-like and less crystal-like. Zr50Cu35Al15, an alloy with lower GFA, becomes more crystal-like and less icosahedral-like. The difference in the stability of the two structures suggests that high GFA requires destabilizing crystal-like structures, in addition to stabilizing non-crystal-like structures. Pd82Si18 has a similar mix of crystal-like and non-crystal-like MRO, but unlike Zr-Cu-Al, the two MRO types do not have different short-range order, as characterized by their Voronoi polyhedron indices. Attempts to characterize the short-range structure using rotational alignment of nearest-neighbor clusters and machine learning will be discussed.

3:00 PM  Invited
The Early Stages of Shear Band Development: Gerhard Wilde1; 1University of Muenster
    Plastic deformation in metallic glasses is mostly localized in so-called shear bands. So far, their actual physical properties remain fairly unknown. Here, experimental data on the atomic diffusion within shear bands have been obtained. The experimental results indicate unambiguously that the diffusivity is largely enhanced. Relaxation studies reveal the presence of two processes with different time scales. A new approach based on analytical transmission electron microscopy methods allows quantitative determination of the local mass density, composition and structural states with nanometer resolution. Thus, shear bands and the surrounding matrix can be analyzed, including also the local structures that are analyzed by fluctuation electron microscopy. The experimental results indicate a complex coupling between applied stress, shear and the local structural heterogeneity inherent to the glass state and are discussed with respect to the early stages of shear band activation and on the properties characterizing these “defects” in deformed metallic glasses.

3:20 PM  
Combinatorial Assessment of Metallic Glasses Using High-throughput Characterization: Ryan Ott1; Fanqiang Meng2; Jie Geng2; Matthew Besser2; Matthew Kramer2; 1Ames Laboratory (USDOE); 2Ames Laboratory (USDOE)
    Predicting glass formation and subsequent crystallization pathways in a multi-component phase space is highly complex due to the far from equilibrium nature of both vitrification and devitrification. Furthermore, experimental characterization of the solidification and phase selection behavior of highly-driven systems is complicated by their strong dependence on small changes in alloy composition and different processing conditions. Combinatorial thin film synthesis and in situ synchrotron X-ray diffraction provides a unique platform to rapidly study a range of glass forming systems. In marginal glass-forming systems, nucleation of nearby simple crystalline phases competes with glassy phases over narrow composition ranges. Since the range of glass-forming ability is also dependent on cooling rates, selected compositions were examined via melt processes (e.g. melt spinning and laser surface tracing) to obtain a more comprehensive understanding of the crystallization pathways during different non-equilibrium processing conditions.

3:40 PM Break

4:00 PM  Invited
Nanoscale Crystallization in Bulk Metallic Glasses and Its Implications on Glass-forming Ability: Dong Ma1; Alexandru Stoica1; 1ORNL
    A variety of families of bulk metallic glasses, such as Vitreloy 1 and Pd-P-base BMGs, exhibit a two-stage, sequential nano-crystallization upon heating above their glass transition temperatures. This commonality suggests a potential connection between glass-formation ability and nanoscale crystallization. To better understand this relationship, we have investigated the kinetics of nanoscale crystallization in Zr-based bulk metallic glasses at multiple length scales using neutron and synchrotron X-ray scattering, complemented by microscopic studies. The role of diffusion and chemical interactions in determining the crystallization pathway will be discussed, and its implications for understanding the glass-forming ability will be presented. Our findings may provide useful insights for developing bulkier metallic glasses.

4:20 PM  Invited
Entropy Contributions in Strong and Fragile Metallic Glasses: Hillary Smith1; Andrew Hoff1; Chen Li2; Tabitha Swan-Wood3; Fred Yang1; Dennis Kim1; Marios Demetriou1; Brent Fultz1; 1California Institute of Technology; 2University of California, Riverside; 3California State University, Channel Islands
    The contribution of vibrational entropy to the excess entropy of the liquid over the crystal at the glass transition has been measured for Cu50Zr50 and Cu46Zr46Al8. Cu50Zr50 is among the most fragile metallic glass formers, while the addition of 8% Al results in the strong glass former Cu46Zr46Al8. We determined the excess entropy of the glass transition in both metallic glasses to be at least 95% configurational in origin, consistent with the classic Adam-Gibbs description of the glass transition. Calorimetry and inelastic neutron scattering measurements performed during in situ heating were used to obtain the total and vibrational entropies, respectively. These results will be discussed in the context of Potential Energy Landscape theory and molecular and network glasses, which show significant contributions from vibrational entropy.

4:40 PM  
Atomic Dynamics in La-based Metallic Glasses by X-ray Photo Correlation Spectroscopy: Xiaodong Wang1; Jin Zhang1; Qing Yu1; Qingping Cao1; Jianzhong Jiang1; 1Zhejiang University
    The atomic dynamics and its structure dependence in a glass are fundamental issues but still little understood for decades. Through the state-of-the-art x-ray photo correlation spectroscopy (XPCS), we reveal that the atomic dynamics in a -relaxation pronounced La-Al-Ni metallic glass slows down by one order of magnitude upon annealing but is still comparable in a La-Al-Cu glass when heating from low to high temperatures. In contrast, both ascast samples exhibit an anomalous slow dynamics compared to the annealed ones when initially cooling from high temperatures. This inconsistency in relaxation times during heating and cooling can be ascribed to the lower stability of ascast samples against crystallization, in which intermittent ageing dynamics is also observed. It is suggested that the reported intermittent ageing mode may be not the intrinsic feature of the glass but the combination of glass and its crystalline phases.

5:00 PM  Invited
Real-time Studies of the Evolution of Atomic Structures of Bulk Metallic Glasses: Wei Zhang1; Jiawei Mi1; 1University of Hull
    The atomic structures of metallic glasses are very sensitive to the heating/cooling rates during processing. Understanding precisely how atomic structures evolve at different heating and cooling conditions, and identifying the critical points where transitions occur between amorphous and crystalline structures have been always the centre of metallic glass research. In this work, we used synchrotron X-ray total scattering and pair distribution function method to investigate, in real-time, the local atomic structural evolution of a Vitreloy-1 alloy subject to different heating and cooling rates. Total X-ray scattering patterns were acquired every second during the experiments of different heating and cooling rates. The evolution of scattering spectra in Fourier space and pair distribution functions in real space identified the critical temperatures for phase transition. Detailed analyses revealed that the reconfigurations of Zr-Zr, Zr-Cu, Cu-Cu partial atomic pairs play the dominant roles in controlling the evolution and changes of atomic structures.

5:20 PM  Invited
Tracing the Pathway of Metallic Liquids to Vitrification: Kostas Georgarakis1; 1Cranfield University
    Metallic glasses are formed when a melted alloy is cooled rapidly enough so that crystallization is avoided upon solidification. However, the way the atomic structure of the liquid transforms to the glassy structure is generally unknown, mainly because the acquisition of structural information in the supercooled liquid regime is hindered by crystallization. Recent advances in containerless solidification methods offer new possibilities to study the undercooled metallic liquids using synchrotron and neutron radiation and to probe the structural evolution during vitrification. Here we report on the in-situ vitrification of glass forming liquids in a high-energy synchrotron beam. The structural evolution indicates fast atomic rearrangements in the super-cooled liquid, enhancing strongly the short (SRO) and medium (MRO) range atomic order. The evolution of the atomic structure points to three non-crystalline regimes in the pathway of metallic liquids to vitrification [K. Georgarakis, A.R. Yavari, et al, Acta Mater. (2015)].