Yunfeng Shi, Rensselaer Polytechnic Institute
Michael J. Demkowicz, Massachusetts Inst. of Technology
A. Lindsay Greer, University of Cambridge
Despina Louca, University of Virginia
Symposium Support Corning Inc
NN2: Structural Characterization of Amorphous Solids
Monday PM, November 26, 2012
Hynes, Level 1, Room 101
2:30 AM - *NN2.01
Coherent Diffraction of Nanocrystalline and Amorphous Solids
Jian Min Zuo 1 Ran Ke 1 Jiong Zhang 1
1Univ of Illinois, Urbana-Champaign Urbana USAShow Abstract
Coherent diffraction using X-ray and electrons holds great promises for determining the atomic position and chemical identity of individual atoms in a small nanostructure. However, achieving such goal requires significant improvement in our capabilities of collecting 3D, high quality, diffraction data and robustness of inversion algorithms. At University of Illinois, we have developed coherent electron diffraction based approaches for structure determination of nanocrystalline and noncrystalline nanostructures and thin-films. Coherent electron beams of a few to tens of nanometers are formed using a field-emission electron source and the probe forming lens of electron microscopes. Diffraction patterns are recorded from individual nanostructures or local areas of thin-films with sub-Angstrom information transfer. We use two approaches to extract structural information; one is based on pair-distribution function analysis to obtain quantitative, averaged, structural information and the other to image the nanostructure using iterative phase retrieval with help of electron imaging. Using these techniques, we have investigated the structure of ultrafine, noncrystalline, nanoparticles and 2D amorphous carbon film. In case of 2D amorphous carbon film, we have succeeded in using electron irradiation to transform single-layer and few-layers graphene into amorphous and characterized the structure using electron diffraction. Results from these studies together with electron diffraction approach will be presented. Future challenges and opportunities using state of art facilities for studying amorphous solids will also be discussed.
3:00 AM - *NN2.02
Structural Studies of Plastic Process Zones around Crack Tips in Metallic Glasses
Todd Hufnagel 1
1Johns Hopkins University Baltimore USAShow Abstract
The fracture toughness of metallic glasses can be remarkably high, given their lack of ability to work harden or of the presence of microstructural features associated with toughening mechanisms in metals. Presumably fracture toughness results from the development of a plastic process zone around the crack tip, but the characteristics and development of this plastic zone are not well understood. In this talk, we describe recent experiments using high-energy x-ray scattering to study the plastic process zone around fatigue pre-cracks in Zr-based metallic glasses under load. Far away from the crack tip, we can use the shift in the x-ray scattering peak to map the elastic strain as a function of position in two dimensions. Within the process zone, the strain measurement becomes unreliable, because the x-ray scattering pattern is affected by plastic deformation processes. A comparison of the strain calculated from the peak shift with an analytical calculation of the strain around a mode I crack allows us to determine the extent of the plastic zone as a function of the imposed stress intensity and testing temperature. As expected, the size of the plastic zone decreases with temperature, which correlates with a decrease in the critical stress intensity at fracture. We also report on preliminary studies of direct real-space (pair correlation) structural analysis of the material inside the plastic zone, compared with undeformed material away from the crack tip. We discuss the implications of these results for our understanding of fracture of metallic glasses, and the potential for future studies of both single-phase glasses as well as metallic-glass-matrix composites.
3:30 AM - *NN2.03
In-situ TEM Observation of Super Elastic Strain Limit in Metallic Ni60Nb40 Glassy Films
Q. K. Jiang 1 P. Liu 2 Y. Ma 1 Q. P. Cao 1 X. D. Wang 1 D. X. Zhang 3 X. D. Han 2 Z. Zhang 1 Jianzhong Jiang 1
1Zhejiang University Hangzhou China2Beijing University of Technology Beijing China3Zhejiang University Hangzhou ChinaShow Abstract
On monolithic Ni-Nb metallic glass films, we experimentally revealed 6.6% elastic strain limit by in-situ transmission electron microscopy observations. The origin of high elastic strain limit may link with high free volume in the film, causing the rearrangement of loosely bonded atomic clusters (or atoms) upon elastic deformation. This high elastic limit of metallic glass films will shed light on new application fields for metallic glasses, and also trigger more studies for deformation mechanism of amorphous materials in general.  Q.K. Jiang, P. Liu, Y. Ma, Q. P. Cao, X. D. Wang, D.X.Zhang, X. D. Han, Z. Zhang and J.Z. Jiang, Scientific Reports (2012), in press.
4:30 AM - *NN2.04
Nanoscale Structure and Structural Relaxation in a Bulk Metallic Glass from Fluctuation Electron Microscopy
Jinwoo Hwang 1 Zenon Melgarejo 1 Yunus Kalay 2 3 I. Kalay 2 3 Don Stone 1 Matt Kramer 2 3 Paul Voyles 1
1University of Wisconsin, Madison Madison USA2Ames Laboratory Ames USA3Iowa State University Ames USAShow Abstract
We have combined nanometer-scale medium-range order structural information from fluctuation electron microscopy (FEM) experiments with short-range order information in from an empirical interatomic potential in a hybrid reverse Monte Carlo refinement to determine the structure of Zr50Cu45Al5 bulk metallic glass . The refined models show icosahedral nearest-neighbor clusters of atoms like other current models, but they also show clusters with more four- and six-sided faces on the Voronoi polyhedra. Both types of clusters group together to form nanometer-scale “superclusers”. The icosahedra form chains, and the other clusters form compact shapes with four- and six-fold rotation symmetry. A result, we call them “crystal-like”. As a function of structural relaxation by annealing at 0.85Tg, the fraction of crystal-like clusters decreases, and the fraction of icosahedral clusters increases. These new features are a result of including FEM data in the refinement. A model created by Monte Carlo minimization of the energy alone has icosahedral short range clusters, but not in chains, and does not contain the crystal-like clusters at all. The structure factors of all the models are indistinguishable and in good agreement with experiment.  J. Hwang, Z. H. Melgarejo, Y. E. Kalay, I. Kalay, M. J. Kramer, D. S. Stone, P. M. Voyles, Phys. Rev. Lett. 108, 195505 (2012).
5:00 AM - NN2.05
Structural Irreversibility and Enhanced Fragility under Fatigue in Zr-based Amorphous Solids
Despina Louca 1 G. Wang 2 Peter Liaw 2 Yunfeng Shi 3
1University of Virginia Charlottesville USA2University of Tennessee Knoxville USA3Rensselaer Polytechnic Institute Troy USAShow Abstract
The effect of fatigue on ZrCuAl amorphous metals induced by mechanical cyclic loading is investigated using inelastic neutron scattering and the pair density function analysis of neutron diffraction data. With cooling, the local atomic structure undergoes a reorganization that is directly related to the number of fatigue cycles, accompanied by a suppression of the atomic fluctuations. A structural restructuring occurs within a 4 Å radius and intensifies with increasing the compression cycles, whereas the vibrational density of states is attenuated as the intensity shifts towards the elastic, zero-energy transfer, peak. The combined static and dynamic structural effects are a signature of the microscopic changes brought upon by fatigue, and together may be the onset for subsequent behaviors following extended cyclic loading such as fracture. Even after the load is removed, the structural changes described here remain and increase with repeated cyclic loading which is an indication that the lattice deforms even before shear bands are formed.
5:15 AM - *NN2.06
Microscopic Understanding of Mechanical Deformation in Metallic Glasses and Supercooled Liquids
Yue Wu 1 2 Magdalena T Sandor 1 Meng Zhang 2 Lin Liu 2
1University of North Carolina Chapel Hill USA2Huazhong University of Science and Technology Wuhan ChinaShow Abstract
Deformation mechanisms of metallic glasses and supercooled liquids are fundamental issues for understanding the glassy state and the glass transition. With regard to the glassy state of metallic systems, anelasticity has been recognized for a long time but its microscopic understanding, analogous to the Zener and Snoek relaxation effects of crystalline systems, is still missing. A critical step toward atomic level understanding is the identification of measurable atomic level structural parameters that respond to anelastic deformations. We demonstrate that the electric-field-gradient (EFG) tensor measured by 27Al NMR in glassy La50Ni15Al35 is such a parameter. EFG tensor at Al sites is a sensitive measure of the site symmetry at Al sites. The measured EFG change upon anelastic deformation demonstrates that a significant fraction of Al sites undergo anelasticity-induced enhancement of atomic site symmetry. Systematic analysis indicates that high symmetry Al-centered clusters, such as icosahedral clusters, play an important role in the process of anelastic deformation of La50Ni15Al35 metallic glass. This provides an important step toward a microscopic understanding of anelasticity in metallic glasses. With regard to the supercooled liquid state, systematic measurements of viscoelastic properties indicate that the flow mechanism exhibits characteristics of facilitated dynamics. Strain rate softening and flow instability are observed in some Zr-based alloys in the supercooled liquid state. It is shown that such behaviors cannot be explained by commonly employed explanations based on the mean field theory. The relevance of such behavior to the nature of glass transition is discussed.
5:45 AM - NN2.07
Recoilless Fraction in Amorphous and Nanocrystalline FeCuNbSiB System
Monica Sorescu 1 Tianhong Xu 1 Steven Herchko 1
1Duquesne University Pittsburgh USAShow Abstract
Differential scanning calorimetry, X-ray diffraction, and room temperature Mossbauer spectrum measurements of Fe73.5Cu1Nb3Si13.5B9 (Finemet) alloy have been carried out in order to study its structural and magnetic properties as a function of annealing temperature. The DSC profile of as-quenched Finemet showed two exothermic peaks at 530 and 702 degrees C, respectively, corresponding to two crystallization processes. The Finemet alloy remains amorphous at 450 degrees C with one broad peak in the XRD pattern and one broad sextet in the Mossbauer spectrum. When the Finemet alloy was annealed at 550 degrees C, only well indexed body-center-cubic phase was detected. After being annealed at 650 and 750 degrees C, the XRD patterns showed the coexistence of alpha-Fe(Si) and Fe-B intermetallic phases with the increase in XRD peak intensities. The Mossbauer spectra of annealed Finemet alloy could be fitted with 4 or 5 sextets and one doublet at higher annealing temperatures, revealing the appearance of different crystalline phases corresponding to the different Fe sites above the crystallization temperature. The appearance of the nanocrystalline phases at different annealing temperatures was further confirmed by the recoilless fraction measurements using our recently developed dual absorber method. Its unique features made it possible to determine the recoilless fraction of the nanocrystalline state and the amorphous grain boundary phase in this alloy.
NN1: Distinctions and Commonalities in Amorphous Solids
Monday AM, November 26, 2012
Hynes, Level 1, Room 101
9:00 AM - *NN1.01
The Deformation of Hard-sphere Colloidal Glasses
Frans Spaepen 1 Katharine E. Jensen 1 David A. Weitz 1
1Harvard University Cambridge USAShow Abstract
Micron-size, hard-sphere particles in colloidal suspension can be prepared as dense assemblies with a structure that closely resembles that of simple glasses. The structure and dynamics of these glasses are studied by tracking the particles in space and time by confocal microscopy. The conditions for glass formation vs. crystallization for monodisperse colloids, as well as the dependence of the glass structure on those conditions, have been explored. The anelastic and plastic deformation of these glasses occurs by, respectively, reversible or irreversible local shear transformations. The local strains associated with these transformations can be analyzed by the Eshelby inclusion model. This Eshelby-type deformation appears to be an intrinsic excitation mode of these glasses, as it is also observed in stationary systems subject only to thermal agitation. Deformation, then, is the result of directional biasing of these excitations by the stress.
9:30 AM - *NN1.02
Phosphate Glasses and the Effects of Short-, Intermediate- and Long-range Order on Properties
Richard Brow 1
1Missouri Samp;T Rolla USAShow Abstract
New spectroscopic techniques, particularly NMR techniques, and refined diffraction and modeling approaches provide quantitative details about the network structures of oxide glasses that then can be used to tailor glass properties. Phosphate glasses are of interest for many optical, bio-medical, and electronic applications, and detailed structural information is available to consider the role of chemical order, from the nature of the individual polyhedra that constitute the glass-forming and modifying networks to the alignment of chains and the creation of structural anisotropies, on the macroscopic properties of these glasses. The effects of composition on the connectivity of the phosphate network can be understood from the overall O/P ratio, but the short-range coordination environment of the modifying cations, and the creation of a modifier sub-network, must also be considered to account for the compositional dependence of the glass properties. NMR studies of the intermediate range site connectivities in chemically-ordered polynary networks of aluminophosphate and borophosphate glasses use similar crystal chemical models and help in the design of useful glass compositions. Finally, anisotropic phosphate glass fibers can be produced with ordered chains that alter glass properties and the implications of such long-range order will be discussed.
10:00 AM - *NN1.03
Effect of Molecular Orientation and Entanglements on Mechanical Properties of Amorphous Polymers
Mark O. Robbins 1 Ting Ge 1
1Johns Hopkins Univ. Baltimore USAShow Abstract
The competition between strong covalent bonds along polymer backbones and weak van der Waals interactions between molecules helps to stabilize amorphous states. It also leads to a range of orientational and conformational degrees of freedom that can affect mechanical response. The talk will first discuss the variation of yield stress with temperature, rate and prestress. The effect of prestress on yield will be related to strain hardening after yield. The shear stress for a wide variety of strain histories can be collapsed when plotted against measures of molecular orientation on either the scale of backbone bonds or the end-to-end vector of molecules. The orientation should be reflected in optical birefringence or other experimental measurements. The talk will next discuss the effect of chain length and entanglements. Entanglements are topological constraints related to the fact that polymers can not pass through each other. They are difficult to identify with measures of short range order, but can be tracked using methods based on the concept of a primitive path - the minimum length the polymer can be reduced to without crossing other polymers. The role of entanglements in strain hardening, craze formation and the mechanical strength of polymer welds will be described. Entanglements enforce an affine deformation at large scales during uniaxial shear and craze formation. The growth in strength with welding time is directly related to evolution of interfacial entanglements.
11:00 AM - *NN1.04
Stress State and Temperature Effects on Flow and Fracture of Bulk Metallic Glasses
John Lewandowski 1
1Case Western Reserve University Cleveland USAShow Abstract
The effects of changes in stress state and test temperature on the flow and fracture behavior of bulk metallic glasses continues to be investigated. Previous work has investigated the effects of changes in stress state on the flow/fracture criterion over a range of superimposed hydrostatic pressures and normal stresses. These will be summarized along with more recent work where the effects of changes in both test temperature and superimposed pressure will be presented.
11:30 AM - *NN1.05
Brittle-to-ductile Transition in Densified Silica Glass
Liping Huang 1
1Rensselaer Polytechnic Institute Troy USAShow Abstract
Molecular dynamics (MD) simulations were carried out to study densified silica glass prepared by pressure-quenching or potential tuning. The density and Poisson&’s ratio of silica glass increases with the increase of the quenching pressure and with the decrease of the oxygen-oxygen repulsion within SiO4 tetrahedra. Uniaxial tension, nano-indentation and fracture tests all show a brittle-to-ductile transition with increasing density and Poisson&’s ratio of silica glass. In densified silica glass, more and more silicon atoms become five coordinated with oxygen atoms with the increase of density. These five-coordinated silicon atoms create additional energy dissipation pathway and facilitate shear flow during mechanical tests and play a critical role in the mechanical behaviors of densified silica glass. In addition, the correlation between densification, shear flow and Poisson&’s ratio is examined and a qualitative explanation is proposed.
12:00 PM - *NN1.06
Structure/Property Relations in Ultra Stable Glasses
Juan de Pablo 1
1University of Chicago Chicago USAShow Abstract
There is considerable interest in identifying structure-property relations in glasses. Structural studies of glassy materials have benefitted from insights provided by molecular simulations of model glass forming liquids. In particular, simulations have provided support for the existence of dynamic and mechanical heterogeneity at the level of small groups of molecules or particles. In general, however, the cooling rates employed in simulations have been many orders of magnitude faster than in experiments, thereby adding some level of ambiguity to direct comparisons between theory and experiment. Recently, experiments have shown that glasses having unusually large thermal and kinetic stability can be prepared by a vapor deposition process. Inspired by such experiments, we have devised a strategy that allows one to prepare highly stable glassy materials, in silico, having thermal and kinetic characteristics comparable to those observed in experimental vapor-deposited glasses. In this work, we will describe that strategy, along with results for several glass forming materials. Results to date indicate that model stable glasses exhibit relaxation times that are typically 19 to 24 orders of magnitude longer than those of ordinary glasses prepared by gradual cooling of the liquid. Consistent with experiment, such glasses also exhibit higher density, lower energy, and higher mechanical constants than ordinary glasses. More importantly, the structural origin for the extraordinary stability of vapor deposited glasses has been clearly identified, thereby offering new insights that could be useful for design of stable amorphous materials deep in the potential energy landscape.
12:30 PM - *NN1.07
Crystallization during Bending of a Metallic Glass Detected by X-Ray Microscopy
Alain Yavari 1 2 Konstantinos Georgarakis 2 1 Jerzy Antonowicz 3 Mihai Stoica 4 Gavin Vaughan 5 Nobuyuki Nishiyama 2 Mingwei Chen 2 Michel Pons 1
1INP Grenoble Saint-Martin-d'Hamp;#232;res France2Tohoku University Sendai Japan3Warsaw University of Technology Warsaw Poland4IFW Dresden Germany5European Synchrotron Radiation Facility Grenoble FranceShow Abstract
Without the availability of slip systems and dislocation glide as in crystalline materials, metallic glasses resist irreversible deformation to elastic strains of 2% or more before undergoing heterogeneous plastic flow via the formation of shear bands. Observation of crystallite formation under compressive load was previously obtained by transmission electron microscopy TEM. Here we present results of non-destructive X-ray diffraction microprofiling of the section of a bent glassy Pd40Cu30Ni10P20 ribbon in transmission using a synchrotron microbeam. Crystallization was clearly detected but only on the compression side of the neutral fibre. The experimental results and crystal nucleation frequency analysis confirm previous suggestions that shear band crystallization contributes to extended strain to fracture of metallic glasses under compressive load, a mechanism that does not intervene under tension. The phenomenon is sensitively dependent on the volume change that accompanies crystallization.
Yunfeng Shi, Rensselaer Polytechnic Institute
Michael J. Demkowicz, Massachusetts Inst. of Technology
A. Lindsay Greer, University of Cambridge
Despina Louca, University of Virginia
Symposium Support Corning Inc
NN4: Mesoscale Modeling of Amorphous Solids and Beyond
Tuesday PM, November 27, 2012
Hynes, Level 1, Room 101
2:30 AM - *NN4.01
Interplay between Metallic Glass Deformation and Free Volume Evolution: A Study Based on Shear Transformation Zone Dynamics Simulations
Lin Li 1 Eric Homer 2 Christopher Schuh 1
1MIT Cambridge USA2Brigham Young University Provo USAShow Abstract
The interplay between deformation behaviors of metallic glasses and the evolution of free volume is investigated via a meso-scale model, shear transformation zone (STZ) dynamics. By incorporating a structure-related state variable, the STZ dynamics model evolves via two competing processes: STZ activation that creates free volume vs. diffusive rearrangement that annihilates it. The two physical-related rate-controlling processes give rise to equilibrium excess free volume that can be connected to flow viscosity via the phenomenological Vogel-Fulcher-Tammann relation in relaxed structures above the glass transformation temperatures. On the other hand, the excess free volume allows glasses to deform at low temperatures via shear localization into shear bands, even in presence of internal stress distributions that arise upon cooling after processing. The development of the model as well as its application to metallic glass processing and deformation are discussed.
3:00 AM - *NN4.02
Heterogeneously Randomized STZ Model of Metallic Glasses: Softening and Extreme Value Statistics during Deformation
Ju Li 1 Pengyang Zhao 2 Yunzhi Wang 2
1Massachusetts Institute of Technology Cambridge USA2The Ohio State University Columbus USAShow Abstract
A nanoscale kinetic Monte Carlo (kMC) model is developed to study the deformation behavior of metallic glasses (MGs). The shear transformation zone (STZ) is adopted as our fundamental deformation unit and each nanoscale volume element (1nm voxel) in the MG is considered as a potential STZ that may undergo inelastic rearrangements sampled from a randomized catalog that varies from element to element, with stress-dependent activation energies. The inelastic transformation sampled out of spatially randomized catalogs (a key characteristic of glass) is then treated as an Eshelby's inclusion and the induced elastic field is solved in the Fourier space using the spectral method. The distinct features of our model, compared to previous work, are the introduction of randomized event catalogs for different nanoscale volume elements, repeated operations within the same element, and a "generation-dependent" softening term to reflect the internal structural change after each deformation. Simulations of uniaxial tension show the important effect of softening on the formation of shear bands, with a size-independent thickness of 18nm. Statistical analysis of the accumulated strain at the 1nm voxel level is carried out and sample size effect on the extreme value statistics is discussed.
3:30 AM - *NN4.03
Compositional Dependence of Mechanical Properties from Effective Medium Approach
Joseph Poon 1 Alexander Petersen 1 Andrew Cheung 2 Gary Shiflet 2
1University of Virginia Charlottesville USA2University of Virginia Charlottesville USAShow Abstract
An effective medium approach is being developed for a high-throughput, compositionally dependent, study of mechanical properties of metallic glasses and metallic glass-crystal composites. Multiple-particle effects are being incorporated to expand beyond the single-particle effective medium approximation. The adaptation of effective medium approach to simulate properties on the atomic scale is discussed. Future theoretical studies will incorporate certain aspects of the atomistic structure of metallic glasses to compare with experimental results. Prospective metallic glasses, identified via the prognosticative model, with potentially favorable hardness values, elastic moduli and high damage tolerance will be studied.
4:30 AM - *NN4.04
Topological Constraint Theory of Glass
John C. Mauro 1
1Corning Incorporated Corning USAShow Abstract
A microscopic physical description of the glassy state has long eluded even the top scientists in condensed matter physics owing to the complicated non-crystalline nature of glass structure. Currently many theorists turn to molecular dynamics or other atomistic simulations to determine the structure of various glass compositions. However, while the available computing power has increased exponentially over the past several decades, it will be at least another 20-30 years before enough computing power is available for direct molecular dynamics simulations of glass on a realistic laboratory time scale. Fortunately, topological constraint theory provides another path forward, focusing on the key microscopic physics governing the thermal, mechanical, and rheological properties of glass, while filtering out unnecessary details that ultimately do not affect the macroscopic properties of a glass. Topological constraint theory has met with much success in predicting the composition dependence of glass properties and can be used as a tool to enable the quantitative design of new glass compositions. This presentation will provide a brief review of constraint theory, including several recent breakthroughs in the field.
5:00 AM - *NN4.05
Simple Models for Plastic Deformation and Slip Avalanches: From Crystals to Bulk Metallic Glasses to Granular Materials
Karin Dahmen 1 James Antonaglia 1 Tyler Earnest 1 Nir Friedman 1 Georgios Tsekenis 2 1 Matthew Wraith 1 Xie Xie 3 Junwei Qiao 4 Yong Zhang 5 Julia Greer 6 Peter Liaw 3 Jonathan Uhl 1
1University of Illinois at Urbana Champaign Urbana USA2Northeastern University Boston USA3University of Tennessee Knoxville USA4Taiyuan University of Technology Taiyuan China5University of Science and Technology Beijing Beijing China6Caltech Pasadena USAShow Abstract
Many slowly sheared solid materials are known to deform in an intermittent way, with the discrete events in the data detected as, for example, acoustic emission and serrations in the stress-strain curves. In many materials, power laws govern the statistics of the distributions in the discrete event sizes. A basic micromechanical model with a single tuning parameter (weakening ε), which describes the distributions of the stochastic deformation signature, is introduced. The model is capable of reproducing the observed stress-strain curves, acoustic emissions, related power spectra, and power-law statistics of slip avalanches, including the dependence of the cutoff on the tuning parameter and geometrical properties of slip, with a continuous phase transition from brittle to ductile behavior. Exact universal predictions for the power-law exponents and scaling functions are extracted using the mean-field theory and renormalization group tools. The results agree with recent experimental observations and simulations of dislocation dynamics in nano- and micro-crystals, sheared bulk metallic glasses, and granular materials. Acknowledgements: KD gratefully acknowledges NSF grants DMR-1005209 and DMS-1069224. XX, JQ, and PKL very much appreciate the financial support of the US National Science Foundation, under DMR-0231320, CMMI-0900271, CMMI-1100080, and DMR-0909037, with Drs. C. Huber, C. V. Cooper, D. Finotello, A. Ardell, and E. Taleff as contract monitors. JRG gratefully acknowledges NSF CAREER grant DMR-0748267 and ONR grant N000140910883.
5:30 AM - NN4.06
Electron Trapping Dynamics in a-Si3N4 and Its Application to Multi-scale Dielectric Charging Model
Ravi Vedula 1 Sambit Palit 1 Nathan Anderson 2 Muhammad Alam 1 Alejandro Strachan 2
1Purdue University West Lafayette USA2Purdue University West Lafayette USAShow Abstract
In this paper, we present a multi-scale modeling approach to rigorously calculate the electron trap energy levels and accurately model the dielectric charging phenomenon in a-Si3N4 dielectric, widely used for RF-MEMS devices. First principles charge state calculations are performed on an ensemble of a-Si3N4 structures to quantify the location of trap levels in the energy band gap. We find that the electron traps levels span a range of energy levels of about 1.8 eV below the CB edge with a maximum concentration around 1.6eV. These results are in excellent agreement with recent Trap Spectroscopy by Charge Injection and Sensing experiments on a-Si3N4. The calculations also provide valuable information about the microscopic nature of defects and their corresponding relaxation mechanisms caused by charge trapping. In addition to the commonly postulated III-Si defects (K centers), new defect mechanisms responsible for electron trapping have been identified. Finally, from current transient measurements on MIM capacitors and continuum based dielectric charging model, we show that using a distribution of trap levels from abinitio calculations captures the current transients across a range of voltages (10-20V) and temperatures (300-360K) more accurately while reducing the empiricism in the existing models. We observe an improvement of about 66% in relative error using a mesocale model informed by abinitio calculations. From these calculations, we demonstrate that realistic modeling of the devices requires a reduction in the empiricism of fitting parameters and incorporation of new multi-scale, multi-resolution approach spanning across various spatial and temporal scales.
5:45 AM - NN4.07
Multiscale Approach to Plastic Deformation of Silicate Glasses at the Micron Scale
Etienne Barthel 1 Boris Mantisi 2 Anne Tanguy 2 Remi Lacroix 3 Guillaume Kermouche 3
1CNRS / Saint-Gobain Aubervilliers France2Universite Lyon 1 CNRS UMR 5586 Villeurbanne France3CNRS/ECL/ENISE UMR 5313 Saint-Etienne FranceShow Abstract
Silicate glasses experience plastic deformation at the micron scale. However the micromechanics tools for a better experimental characterization of plastic deformation at the micron scale have become available only recently. Simultaneously, understanding the elementary plastic deformation mechanisms has been facilitated by the progress of Molecular Dynamics. Here we show how we combine continuum scale measurements  with molecular Dynamics (MD) simulations  for a better understanding of the plastic deformation of silicate glasses. On the experimental side, the specific feature of silicate glasses is brittleness at larger scales so that experiments probing the plastic response must be carried out at the 10 micrometer scale or below. For that purpose we use Raman or luminescence spectroscopy under various types of loadings. In situ Raman microspectroscopy experiments were performed under hydrostatic compression in a diamond anvil cell. Post-indentation deformation maps were obtained on microindents. Recently, we have developed micro-pillar experiments to probe a reasonably well defined stress state with significant shear. Etching of the pillars was carried out by Reactive Ion Etching and the design was analyzed by Finite Element Modeling (FEM), taking into account contact conditions, misalignment and substrate compliance . With optimum design of the experiment irreversible deformations of up to 30% were reached . We also show that silica exceeds crystalline oxydes in terms of yield stress over shear modulus ratio, with no sign of size effect, at least above 500 nm. With strong plastic deformation, definite rupture patterns are observed which differ from brittle fracture. To model the plastic response at the continuum lengthscale, FEM analysis of the data has been carried out. A quantitative constitutive equation for silica has been inferred, taking into account both densification and strain hardening. At the atomistic scale, Molecular Dynamics simulations were carried out, using a BKS (Wolf truncated) potential, which is thought to be a good approximant for silica. In these MD simulations, we have performed shear experiments at fixed hydrostatic pressure to identify the yield surface and hardening of the materials. We show that a very reasonable match with the continuum scale constitutive equation is obtained. At the same time, the MD results suggest possible improvements in the form of the constitutive relation used for FEM. We believe this direct dialog between MD and continuum scale approach is successful because there is no intermediate lengthscale in these amorphous materials.  G. Kermouche et al., Acta Materialia 56 (2008) 3222-3228.  A. Tanguy at al. EPL 90 (2010) 16004.  R. Lacroix et al. Int. J. Appl. Glass Sci. 3 36-43 (2012).  R. Lacroix et al., to appear in Acta Mater.
NN3: Molecular Simulation of Amorphous Solids
Tuesday AM, November 27, 2012
Hynes, Level 1, Room 101
9:00 AM - *NN3.01
Cavitation in Amorphous Solids
Pengfei Guan 1 Lu Shuo 4 Michael J. B. Spector 1 Pavan K Valavala 1 Michael L Falk 1 2 3
1Johns Hopkins University Baltimore USA2Johns Hopkins University Baltimore USA3Johns Hopkins University Baltimore USA4Beijing University of Aeronautics and Astronautics Beijing ChinaShow Abstract
Molecular dynamics simulations of cavitation in a Zr50Cu50 metallic glass exhibit a waiting time dependent cavitation rate. On short time scales nucleation rates and critical cavity sizes are commensurate with a classical theory of nucleation that accounts for both the plastic dissipation during cavitation and the cavity size dependence of the surface energy. All but one parameter, the Tolman length, can be extracted directly from independent calculations or estimated from physical principles. On longer time scales aging in the form of shear relaxations results in a systematic decrease of cavitation rate. The high cavitation rates that arise due to the suppression of the surface energy in small cavities provide a possible explanation for the quasi-brittle fracture observed in metallic glasses. Analogous simulations of Fe80P20 reveal that segregation of P on the nanoscale leads to qualitatively different behavior that may be attributable to the idiosyncrasies of the interatomic potential.
9:30 AM - *NN3.02
Atomic Scale Fluctuations Govern Brittle Fracture and Cavitation Behavior in Metallic Glasses
Palla Murali 2 Tianfu Guo 2 R. Narasimhan 3 Yong Wei Zhang 2 Huajian Gao 1
1Brown University Providence USA2Institute of High Performance Computing Singapore Singapore3Indian Institute of Science Bangalore IndiaShow Abstract
We perform atomistic simulations on the fracture behavior of two typical metallic glasses, one brittle (FeP) and the other ductile (CuZr), and show that brittle fracture in the FeP glass is governed by an intrinsic cavitation mechanism near crack tips in contrast to extensive shear banding in the ductile CuZr glass. We show that a high degree of atomic scale spatial fluctuations in the local properties is the main reason for the observed cavitation behavior in the brittle metallic glass. Our study corroborates with recent experimental observations of nanoscale cavity nucleation found on the brittle fracture surfaces of metallic glasses and provides important insights into the root cause of the ductile versus brittle behavior in such materials.
10:00 AM - NN3.03
Intrinsic Ductility of Glassy Solids
Yunfeng Shi 1 Jian Luo 1 Fenglin Yuan 1 Liping Huang 1
1RPI Troy USAShow Abstract
Glasses are usually brittle, seriously limiting their practical usage in wide-range of applications. Thus, one grand challenge in the glass community is to search for glasses with excellent intrinsic ductility (as opposed to extrinsic ductility bestowed from compositing). Recently, it was found among oxide glasses and metallic glasses that the fracture energy of an amorphous solid increases with its Poisson&’s ratio (v), with a sharp brittle-to-ductile transition at v=0.31-0.32. Here we use molecular dynamics (MD) simulations to create three families of glasses (model metallic glasses, amorphous silicon and silica) each with different v by tuning the force fields, varying the processing conditions and testing conditions. Uniaxial tensile tests of bulk glasses and glassy nanowires were carried out, together with fracture simulations with pre-notched samples. For all glassy samples, the failure mode correlates strongly with v. Furthermore, it was found that the critical v of the brittle-to-ductile transition decreases with the average coordination number (CN). Thus, the v-CN diagram can help design toughening schemes via tuning compositions and processing. The observed v-CN diagram can be comprehended by considering the Poisson&’s ratio as a measure of covalency, and the brittle-to-ductile transition as a competition between shear and cleavage.
10:15 AM - NN3.04
Atomistic Simulations of Radiation Damage in Amorphous Metals
Richard Baumer 1 Michael Demkowicz 1 Tomas Oppelstrup 2 Vasily Bulatov 2
1Massachusetts Institute of Technology Cambridge USA2Lawrence Livermore National Laboratory Livermore USAShow Abstract
We performed half-billion atom molecular dynamics simulations of 475keV Nb ion irradiation of an amorphous metal alloy, Cu50Nb50, and used atom-level and coarse-grained analysis to identify: formation of localized liquid regions that rapidly quench back to amorphous structures; emission of stress pulses from liquid regions; and accumulation of plastic strain. These insights point to the mechanisms of radiation damage in metallic glasses.
11:00 AM - *NN3.05
Elastic Responses of Metallic Glasses
Mo Li 1 2 Hao Wang 1
1Georgia Institute of Technology Atlanta USA2Tsinghua University Beijing ChinaShow Abstract
Although considerable efforts have been made, mechanical response and the underlying mechanisms of defect formation and deformation of metallic glasses still remains one of the challenging and open issues in materials science, which include not only the well-known shear localization or shear banding in the plastic regime, but also in the seemingly simple elastic response. In this work, we shall show our recent work in quantifying elastic response of several metallic glasses, including elastic modulus, volume dilatation, loss modulus, and several related issues to plastic deformation and other mechanical properties of the metallic glasses.
11:30 AM - *NN3.06
Comparing Network Structures in Glass and Glassy Polymers Generated by Molecular Dynamics Simulations
Katherine M Sebeck 1 John Kieffer 1
1University of Michigan Ann Arbor USAShow Abstract
Highly crosslinked polymers such as epoxy exhibit brittle glassy behavior, while modified silica networks show increasing degrees of ductility. In both inorganic glasses and network polymers, overall connectivity strongly influences thermo-mechanical properties. This can be controlled by monomer functionalization or degree of cure in polymer systems, while network modifying oxides like Na2O and CaO disrupt the formation of the silicate network. A series of soda lime silicate glasses have been simulated using a charge-transfer potential previously shown to successfully simulate various amorphous silica structures . Similarly, epoxy structures have been generated using a dynamic polymerization technique allowing for natural evolution of the network structure over the course of the simulation, avoiding over-constraint of the system. Examination of local structural defects at the atomistic scale, i.e., sterically hindered polymerization vs. network scission via modifier species, provides insight into the nature of the structure-property relationship in networked amorphous materials. The mechanical properties of these networks are explained in terms of quantitative measure of network topology.  L. P. Huang, M. Durandurdu and J. Kieffer, Nature Materials, 5, 977, (2006
12:00 PM - NN3.07
Studies of the Nanomechanical Behavior of Silica Nanowires: Large-scale Molecular Dynamics Simulations and Finite Element Method
Miguel Diaz Moreno 1 Lilian P. Davila 1
1University of California Merced Merced USAShow Abstract
Oxide nanowires and nanosprings are among the most important 1D nanoscale morphologies of scientific interest due to their potential applications in nanoscale devices, with implications for nanotechnology, bionanotechnology, and many other fields. We have focused our efforts on understanding the nature of glass silica nanowires and their mechanical properties for nanodevice design considerations. These nanowires are readily created in the laboratory nowadays, with resulting unique mechanical and optical properties which could make them useful in small-scale sensing and micro-system applications. We have expanded previous large-scale molecular dynamics (MD) simulations of the mechanical response of glass silica nanowires to include nanowire models with longer length and time scales via finite-element method (FEM) simulations. We focused on the size dependence of the mechanical properties of amorphous silica nanowires under uniaxial compressions via FEM-based simulations. The use of this computational technique allows the investigation of stress distributions and displacement values, which are useful for predicting failure modes. The analysis of the nanowire sizes investigated (4.3 nm < D < 20 nm, 14 nm < L < 1430 nm, where D and L represent the nanowire diameter and length respectively) led to the reported results. The von Mises stress behavior as a function of nanowire diameter is similar to that of the Young&’s modulus reported previously. The shorter nanowire sizes (L=14.3 nm) showed uniform stress distributions with maximum stresses concentrated at the bottom of the nanowires, with no observed buckling. The long nanowire sizes (L=715 nm) revealed striped stress distributions with maximum stresses concentrated at the longitudinal surfaces of the nanowires, with modest buckling. The longest nanowire sizes (L=1430 nm) also showed striped stress distributions concentrated at the longitudinal surface of the nanowires. Finally, analysis of the critical buckling of nanowires of different lengths demonstrated that the shorter nanowires (L=14.3 nm) did not present buckling effects when increasing compressive loads were applied. This MD-FEM approach allows for exploration and comparison against reported experimental and simulated nanowire sizes. The ultimate goal of this project is to contribute to design and fabrication efforts of similar nanowires, currently being considered for opto-electronics and photonic devices.
12:15 PM - NN3.08
Solvent and Confinement Effects on the Thermomechanical Behavior of Amorphous Polymers
Sinan Keten 1 Shawn Mishra 1 Wenjie Xia 1 Luis Ruiz 1
1Sinan Keten Evanston USAShow Abstract
Diffusion and the effect of low molecular weight solvents on polymer microstructure and mechanical behavior is a central problem with broad implications to nanofabrication, biomechanics and multifunctional materials. Ever present water molecules in the form of humidity, as well as retained solvents after materials processing alter the anticipated behavior of materials significantly and contribute to experimental characterization error, as well as deviations from materials performance in service. In particular, presence of low-molecular solvents influences the mechanical behavior of polymers by shifting the characteristic relaxation times and glass transition temperature. This effect can be beneficial in the case of stimuli responsive materials such as shape memory polymers, but it can also be detrimental to device function when solvent is not accounted for especially in large surface area systems such as nanofabricated devices and polymer nano fibers. In this study, we present atomistic and coarse-grained molecular simulations that explain how solvent and confinement effects govern the relaxation and glass-transition behavior of amorphous polymers. We propose a methodology for the spatial variation of glass-transition temperature near surfaces and heterogeneities. Our results can be directly combined with theoretical models to directly quantify variation in thermomechanical properties without empirical inputs, and corroborate recent experimental findings.
12:30 PM - NN3.09
Microscopic Mechanisms of Deformation-induced Crystallization of Metallic Glasses at Low Temperatures
YunWei Mao 1 ChengCai Wang 1 ZhiWei Shan 1 Ju Li 1 2 3 Evan Ma 1 4
1Xiamp;#8217;an Jiaotong University Xi'an China2Massachusetts Institute of Technology Cambridge USA3Massachusetts Institute of Technology Cambridge USA4Johns Hopkins University Baltimore USAShow Abstract
At elevated temperatures, metallic glasses (MGs) crystallize via diffusive actions of individual atoms, in a nucleation and growth transformation. In recent years, however, it has been discovered that MGs can also crystallize (sometimes rapidly) at very low temperatures (such as 77 K), during deformation under high stresses, with little or no temperature rise. Here using an Al-Fe MG as an example, we report recent molecular dynamics (MD) simulation results that reveal the microscopic processes underlying such stress-driven glass-to-crystal transition. We demonstrate cooperative atomic rearrangements that mediate short-to-medium range ordering, leading to metabasin-metabasin transitions. We also illustrate the collective attachment of the ordered patches to nuclei, advancing the interphase interface in an intermittent manner, which is very different from the temperature-driven amorphous-to-crystal transition at high temperatures.
12:45 PM - NN3.10
Effect of Structure on Glass-forming Ability Using Atomistic Simulation
Logan Ward 1 David Riegner 1 Anupriya Agrawal 1 Kevin Laws 2 Katharine Flores 3 Wolfgang Windl 1
1The Ohio State University Columbus USA2University of New South Wales Sydney Australia3Washington University St. Louis USAShow Abstract
Molecular dynamics (MD) provides a powerful tool for directly and unambiguously assessing the structure of amorphous systems under conditions not accessible experimentally, such as the atomic arrangement of amorphous metals with limited glass-forming ability, or structural evolution on time scales less than a few nanoseconds. In this work, we use MD to study the structural characteristics in several binary and ternary metallic glasses systems in order to determine how structure correlates with glass-forming ability. In particular, our work has focused on icosahedral clusters and efficiently packed clusters identified using the effective radius ratio method, which have been hypothesized as key elements for forming metallic glasses in the literature. We have found, however, that there is no obvious correlation between the existence of these structures and glass formation, which suggests that factors other than the presence of favorable local motifs play a dominant role for vitrification. A large portion of our work focuses on the Cu-Zr binary, which offers a particularly rich system for study due to the presence of several experimentally verified glass-forming compositions interspersed among non-glass-forming compositions. In this system, the fraction of icosahedral clusters reaches a maximum in a region outside of the composition range experimentally known for bulk metallic glass formation. Further evidencing the lack of correlation between these clusters and glass formation, the compositions of maximum glass-forming ability in the Cu-Zr binary do not correspond to the maxima in icosahedral or efficient cluster concentrations. In contrast, we have found that the fragility of the supercooled liquid (also calculated using molecular dynamics) correlates better with regions of high glass-forming ability. An important question to be addressed in this context is whether there is a stronger correlation between structure and glass-formation in other systems and whether favorable glass formation can be associated with other structural traits, such as medium range order.
Yunfeng Shi, Rensselaer Polytechnic Institute
Michael J. Demkowicz, Massachusetts Inst. of Technology
A. Lindsay Greer, University of Cambridge
Despina Louca, University of Virginia
Symposium Support Corning Inc
NN6: Structural Models for Amorphous Solids
Wednesday PM, November 28, 2012
Hynes, Level 1, Room 101
2:30 AM - *NN6.01
Local Topology versus Atomic-level Stresses as a Measure of Disorder: Correlating Structural Indicators for Metallic Glasses
Evan Ma 1 Jun Ding 1 Yongqiang Cheng 1 2
1Johns Hopkins University Baltimore USA2Oak Ridge National Laboratory Oak Ridge USAShow Abstract
We present a critical assessment of the choice of structural parameters that may be used to describe the degree of local order/disorder in metallic glasses (MGs) and to establish the relationship between the amorphous structure and the properties of an MG. By summarizing our own quantitative analysis of representative MG models, we compare, and relate to one another, several measures of structural disorder in amorphous alloys, including the topology of local quasi-equivalent clusters, the concept of free volume, and the atomic-level pressure/stresses (or atomic-level strain energy). The model systems used for demonstrating the correlations are characterized by two different and prototypical types of short-range order (SRO). The first is a Cu-rich Cu64Zr36 MG exhibiting pronounced icosahedral SRO, whereas the second is a Pd82Si18 MG representative of solute-lean metal-metalloid systems, in which the center solute atom is surrounded in the nearest-neighbor shell by solvent atoms only, with trigonal prisms as the dominant local motif. Correlations of the structural features with non-affine strains under shear deformation are presented as well.
3:00 AM - *NN6.02
Atomic Defects and Their Impact on the Properties of Covalent Glasses
Himanshu Jain 1 Roman Golovchak 1 Andriy Kovalskiy 2
1Lehigh University Bethlehem USA2Austin Peay State University Clarksville USAShow Abstract
Mott&’s 8-N rule, where N is the number of valence electrons of a given atom, provides a basis for defining the ordered local structure of covalently bonded network glasses such as the chalcogenides. The thermodynamic stability of specific bonds further dictates the preferred local order of these glasses. A deviation from such ‘perfect&’ local structure would mean an atomic defect in the structure. These point defects of covalent glass can be produced, even controlled, by processing conditions similar to those employed with crystals, such as quenching from high temperature or exposure to sufficiently energetic radiation. However, because glass has no long range order and is metastable, the supersaturated concentration of point defects in glass can be orders of magnitude higher than in crystals. Furthermore, the compositional flexibility of amorphous solids allows tuning of the concentration of these defects by choosing non-stoichiometric compositions. However, their stability is threatened by phase separation, eventually the crystallization. We will discuss the formation and stability of the atomic defects in model sulfide and selenide systems, and how they introduce novel properties and new functionalities unique to the glassy state.
3:30 AM - NN6.03
Monitoring Thermally-induced Structural Relaxation of a-Si by Raman Spectroscopy, Electrical Conductivity and Changes in Mechanical Behaviour
L. B. Bayu Aji 1 B. Haberl 1 J. E. Bradby 1 Jim S Williams 1
1Australian National University Canberra AustraliaShow Abstract
Amorphous silicon (a-Si) can be considered a model system for a covalently-bonded amorphous material. However, quite surprisingly the covalent bonding to a large extent complicates understanding of its structural properties compared to those of metallic glasses. For example, different forms of a-Si prepared by different methods, namely by deposition, by ion implantation and by pressure-induced methods, all appear to have different structural properties on a scale of short and medium range order [1,2]. In addition, when annealed below the crystallisation temperature a-Si can undergo relaxation to a state that is close to that of an ideal continuous random network . Such structural differences can also give rise to dramatically different mechanical properties. Indeed, it has been observed that as-prepared ion implanted a-Si deforms via plastic deformation but by phase transformation to a high pressure metallic Si phase when it is relaxed . In this presentation, we report on structural relaxation properties of ion implanted a-Si during thermal annealing in the 100-450oC range using Raman spectroscopy and electrical conductivity measurements. Results show that the bond angle distortion as measured by Raman is minimised after annealing at about 350oC, whereas the electrical conductivity appears to reach a minimum value at a significantly lower annealing temperature. Activation energies associated with both the recovery in bond angle distortion and minimisation of electrical conductivity have been measured and found to be quite different (in the range of 0.4-2.5 eV). This behaviour indicates that electrical conductivity and Raman probing of bond angle distortion are sensitive to different atomic recovery processes during relaxation annealing. Furthermore, nanoindentation-induced deformation of a-Si as a function of annealing has been carried out using a 4.3µm spherical indenter. The deformation processes (plastic deformation or phase transformation) have been studied in-situ, by observing the shape of load-unload curves and any pop-out event on unloading , or ex-situ, by Raman spectroscopy to determine the presence of so called high pressure phases of Si. Results show that the mechanical properties are also sensitive to the degree of structural relaxation, with the measured temperature dependence correlating more closely with the electrical conductivity behaviour than the recovery in bond angle distortion. Implications of our results in terms of structural relaxation processes are discussed.  B. Haberl et al., Phys. Rev. B 79, 155209 (2009)  B. Haberl et al., J. Appl. Phys. 110, 096104 (2011)  K. Laaziri et al., Phys. Rev.B 60, 13520 (1999)  B. Haberl et al., J. Appl. Phys. 100, 013520 (2006)
3:45 AM - NN6.04
Medium-range Order of Metallic Glass Thin Films
Li He 1 Yen-Chen Chen 2 Chia-Lin Li 2 Jinn P. Chu 2 Peter K. Liaw 3 Eun Soo Park 4 Ryan Ott 4 Matthew J. Kramer 4 Paul Voyles 1
1University of Wisconsion-Madison Madison USA2National Taiwan University of Science and Technology Taipei Taiwan3University of Tennessee Knoxville USA4Iowa State University Ames USAShow Abstract
We have used fluctuation electron microscopy (FEM) to study the medium-range order (MRO) in thin films of Al90Tb10 and Zr53Cu29Al12Ni6, both deposited by sputtering. Al90Tb10 is a marginal glass former, which, in its rapidly solidified form under primary Al crystallization driven by MRO clusters in the structure consists of pure Al with a nearly face-centered-cubic structure . These Al-like clusters are separated by a Tb-rich network . In a thin film form, the Al-like MRO is significantly reduced, and the Al composition of the Al-Tb network may increase. The Zr53Cu29Al12Ni6 film shows greater order than a melt-spun ribbon of the similar composition, Zr50Cu45Al5 . Zr50Cu45Al5 has two distinct local environments, one of which is icosahedral, and the other is crystal-like, associated with a split first peak in the FEM data . The Zr53Cu29Al12Ni6 film has one broad peak covering the same range of scattering vectors, suggesting a greater diversity of local atomic environments. The films are presumably in a higher energy metastable state (a higher fictive temperature) than even rapidly-quenched ribbons, but these results show that a high energy state does not have a simple structural consequence across different glass systems. We acknowledge support from the National Science Foundation (LH and PMV DMR-0905793 and PKL DMR-0909037, CMMI-0900271, and CMMI-1100080) and the Department of Energy (RO and MJK, DE-AC02-07CH11358).  Y. E. Kalay, I. Kalay, J. Hwang, P. M. Voyles, M. J. Kramer, Acta Mat. 60, 994 (2012).  J. Hwang, Z. H. Melgarejo, Y. E. Kalay, I. Kalay, M. J. Kramer, D. S. Stone, P. M. Voyles, Phys. Rev. Lett. 108, 195505 (2012).
4:30 AM - *NN6.05
Density, Packing Fraction and Atomic Structure of Metallic Glasses
Dan Miracle 1
1AF Research Laboratory Dayton USAShow Abstract
A good structural model should give good estimates of structure-specific properties. Density is one such property, as it relies sensitively on the number and type of atoms in a structure and their separations. The symmetries of crystalline structures, along with the concept of fixed atomic radii, give good estimates of density in ordered materials. However, it is exceptionally hard to estimate metallic glass densities from existing structural models. The uncertainty in atomic size is about +/-3 percent, giving an intrinsic error in predicted density of about +/-10 percent. The efficient cluster packing (ECP) model gives a cluster-based description for the atomic structure of metallic glasses, but cluster-cluster separations can vary by 20-30 percent depending on whether adjacent clusters share common faces, edges or vertices, giving much larger errors. As a result, earlier attempts to predict densities using the ECP model gave only nominal agreement with measured density, with an average error of +/-20 percent. While the ECP model is built on the concept of efficient atomic packing, quantitative estimates of atomic packing, and hence density, have not previously been possible. The purpose of this work is to find new insights into metallic glass structures and how they change with composition. The present work refines the ECP model by quantitative comparison with measured binary metallic glass densities. Physically-based models of structural mass and volume are developed as continuous functions of composition that include both solute-lean and solute-rich structures. Efficient packing between structural clusters is developed in a way that accounts for the unique topologies of the dominant atomic clusters in each structure. Each cluster is different, and the number and orientations of faces and edges is important. The physical adjustments to the ECP model include a more accurate estimate of atomic packing between different-sized atoms in the first shell of atomic clusters and a different treatment for packing of metalloid atoms due to strong angular bonding that discourages metalloid/metalloid contact. As a result of these small adjustments, the ECP structural model gives little or no systematic differences between measured and calculated densities with composition. Density estimates for over 200 binary metallic glasses differ from measured values by an average error well below an expected bound of +/-10 percent, and only one of these predictions has an error greater than 10 percent. The structural adjustments are described and the results are presented and discussed.
5:00 AM - NN6.06
Short and Medium Range Structural Order of Unrelaxed and Relaxed Amorphous Silicon
B. Haberl 1 A. C. Y. Liu 3 J. E. Bradby 1 S. N. Bogle 5 T. Li 5 J. R. Abelson 5 K. B. Borisenko 2 M. J. Treacy 4 Jim S Williams 1
1Australian National University Canberra Australia2University of Oxford Oxford United Kingdom3Monash University Clayton Australia4Arizona State University Tempe USA5University of Illinois Urbana USAShow Abstract
The structure of amorphous silicon (a-Si) has attracted wide interest over several decades as it is a model covalently bonded amorphous system. It has been found that the ‘structure&’ of a-Si is dependent on both the formation method and thermal history. In particular, pure ion implanted a-Si is found to undergo a structural relaxation on thermal annealing, with significant changes observed in both short range order (SRO), interpreted as a reduction in the ‘defect&’ or dangling bond density ), and medium range order (MRO) as measured by fluctuation electron microscopy (FEM) . However, use of such structural data at the short (< 1nm) and medium (1-3nm) range scales to compute robust structural models for a-Si is not straight forward. In this study, pure a-Si layers were prepared under different conditions, including self-ion-implanted material, a pressure-induced material, and their thermally annealed (and thus structurally relaxed) counterparts. SRO out to the second nearest neighbour has been measured by Raman spectroscopy and selected area diffraction electron microscopy. Two variants of FEM have been used to obtain the normalized intensity variance, a parameter that is sensitive to MRO. One method consists of a series of tilted dark-field images , and the other involves collecting nanodiffraction patterns over a square grid in a STEM instrument . The structural order of the different forms of a-Si has then been computed using an experimentally constrained structural relaxation (ECSR) method. In this case, three starting atomic arrangements (single crystal, polycrystal and random) were chosen and experimental selected-area diffraction data and normalized variance data from FEM dark field measurements, together with a Tersoff potential, were used as constraints to guide a Monte Carlo relaxation procedure towards best-fit structural models . Our results show that both ion-implanted and pressure-induced a-Si have distinctly different SRO and MRO but, on relaxation, both appear to converge to a similar structure. However, the different amplitude of variance data, and sample to sample variations for the starting materials, makes definitive structural modelling difficult. In terms of the modelling, despite final self-consistent structural models, particularly for the relaxed cases, we find that the experimental data does not constrain the solution space sufficiently, thus resulting in different best-fit models depending on the starting atom arrangements used for the model calculations. Nevertheless, this study shows that ECSR methods can offer a powerful approach to developing robust structural models for amorphous covalent solids.  K. Laaziri et al, Phys. Rev. B 60,1999,13520  B. Haberl et al, Phys. Rev. B 79, 2009,155209  B. Haberl et al, J. Appl. Phys. 110, 2011, 096104  K. B. Borisenko et al, Acta Met. 60, 2012, 359
5:15 AM - NN6.07
Amorphous and Crystal: Two Quantized States of Matter?
David Porter 1
1University of Oxford Oxford United KingdomShow Abstract
Although we can describe many aspects of amorphous and crystalline materials qualitatively and even quantitatively for structure, we need a way to quantify the state of matter in order to calculate material properties and understand the differences in properties for the different states and their combinations in semicrystalline materials. In particular, the disordered amorphous state has proven difficult to define with a precise quantitative parameter that can be tested by means of some clear property prediction relative to the superficially ‘simpler&’ ordered crystalline state of matter. First, a simple hypothesis is made that the state of matter is determined by a quantized value of zero point energy of vibration of the constituent atoms and molecules. Starting with a potential energy well, the intrinsic vibrational frequency of the atoms,f, can be calculated from the intermolecular stiffness and the mass of the atoms (can be calculated automatically now, even in QM methods). We can thus suggest quantized zero point energy levels, (n+1/2) h f, where h is Planck&’s constant and n = 0 for crystalline and n = 1 for amorphous states to become the starting point for energy at T = 0 K for property predictions using potential energy well calculations. Next, to test this hypothesis, the Born elastic instability criterion (maximum in force between atoms) can be used to predict the energy for onset of mobility of atoms and molecules and define this as a transition point in the material. The two zero point energy values now allow us to define this single transition as the glass transition temperature for amorphous and the crystal melt temperature for crystalline materials, since the thermal energy required to reach this condition will be different for the two states. The change from crystal to amorphous states at the melt transition is discussed briefly as the origin of ‘enthalpy overshoot&’ through the transition as the effective zero point energy changes, further supporting the hypothesis. To validate the hypothesis, predictions are made and compared with observation for transition temperatures for the two important families of materials of polymers and metals, by combining expressions for thermal energy as a function of temperature with the zero point energy values for the examples. An exciting recent application of the model is then presented, where predictions of the glass and melt transition temperatures for water are made using quantum mechanics simulations of the hydrogen bond potential energy well between water molecules, and then extended to water-protein interactions to predict the important denaturation temperature for proteins and the equivalent glass transition for already disordered denatured proteins. Finally, suggestions are made as to how other intrinsic thermo-mechanical properties of the two states of matter can be calculated relatively simply using the potential energy well for the material and the appropriate zero point energy.
5:30 AM - NN6.08
Correlating Local Structure with Heterogeneous Elastic Deformation in a Metallic Glass
Jun Ding 1 Yongqiang Cheng 2 Evan Ma 1
1Johns Hopkins University Baltimore USA2Oak Ridge National Laboratory Oak Ridge USAShow Abstract
Apart from shear localization, in the elastic regime the response of metallic glasses (MGs) is also heterogeneous, due to the wide variation of local structural arrangements. Here we present molecular dynamics simulations on a one-million-atoms sample of the Cu64Zr36 model MG to systematically study the elastic behavior at atomic level and its link with local structure. It was observed that the atomic strain and displacement is non-affine with a wide distribution, exhibiting a clear correlation with short range order (SRO): icosahedral Cu-centered clusters are stiffer, with a narrower distribution for deviation from homogeneous deformation. In addition, anelasticity, at the time scale of our simulation, was qualitatively studied using anisotropic pair distribution function and structural anisotropic coefficient.
5:45 AM - NN6.09
Revealing Ordering and Structural Changes at the Glass-liquid Transition
Michael I. Ojovan 2 1
1IAEA Vienna Austria2Imperial College London London United KingdomShow Abstract
Ordering types in the disordered structure of amorphous materials and structural changes which occur at glass-liquid transition are discussed revealing medium range order and reduction of topological signature of bonding system. Recent direct visualisation (using AFM technique) of twinkling fractal structures near the glass-liquid transition temperatures proved that the glass transition is associated with structural changes in the disordered distribution of atoms [J. Non-Cryst. Solids, 357 (2011) 311]. These findings show that although both liquid and glassy (e.g. solid) phases have topologically disordered structures, there is a significant change in the topology of bonding systems at glass-liquid transition. Namely, it has been revealed that the Hausdorff dimension of bonds changes from 3 in the solid (glassy) state to Df = 2.55 +/- 0.05 in the liquid (molten) state [J. Non-Cryst. Solids, 356 (2010) 2534]. E.g. melts have a fractal geometry of bonds and because of that a liquid-like behaviour, whereas glasses (solid amorphous materials) a 3-D geometry of bonds and because of that they behave like solids. The percolation occurs in the system of broken bonds - configurons with formation near the percolation threshold of dynamic (twinkling) fractal structures. We show here that the average size of clusters formed at temperatures not far from glass-liquid transition is determined by the correlation length xi;(T). The structure of an amorphous material can be hence characterised, depending on range of sizes involved, as follows: SRO -Short Range Ordering with molecular type units such as tetrahedral structures in silicates, at atomic size range; MRO - Medium Range Ordering with fractal structures (dynamic or twinkling which can be frozen in the glassy state) at correlation length range; DS - Disordered Structure which is homogeneous and isotropic at macroscopic sizes. The MRO found in metallic glasses [Nature Materials, 8 (2009) 30] evidences that the glass-liquid transition is a percolation-type phase transition. Moreover both theory and experiment reveal the same dimensionality for remnant clusters in glasses.
NN5: First Principles Calculations on Amorphous Solids
Wednesday AM, November 28, 2012
Hynes, Level 1, Room 101
9:30 AM - *NN5.01
Quantum Mechanical Origin for the Glass Forming Ability in Bulk Metallic Glasses
Wai-Yim Ching 1
1University of Missouri-Kansas City Kansas City USAShow Abstract
Bulk metallic glass (BMG) is a class of non-crystalline metallic system with some outstanding physical and mechanical properties . BMGs are significantly different from crystalline materials due to the absence of long-range order (LRO) and grain boundaries. The ideal BMG for structural applications should have both excellent glass forming ability (GFA) that avoids crystallization and intrinsic ductility that minimizes brittle fracture. A deep understanding of its origin at the atomic and electronic level is the ultimate goal of many researchers. There has been little effort in using first-principles methods to study the electronic structure of BMGs. The main difficulty is that the non-crystalline nature of BMG that requires sufficiently large and accurate structure models that can account for both the short range order (SRO) and medium range order (MRO) and for the evaluation of their physical properties. In this talk, I will present results of ab initio calculations of electronic structure of 17 binary Zr-Cu BMG models with compositions ranging from Zr32Cu68 to Zr54.5Cu45.5. Each model contains 1,024 atoms and was first generated using classical molecular dynamics (MD) with long annealing steps  and then fully relaxed with VASP (Vienna Ab Initio Simulation Package) with high precision. The mass densities of the relaxed models are in excellent agreement with experiment  whereas the MD-derived models underestimate the density. The electronic structures of the 17 ZrxCu(1-x) models are calculated using the first-principles OLCAO method . It is shown that the GFA can be correlated with the calculated total bond order values between all pairs of atoms in the model and to a less extent to the curvature of the DOS curves close to the Fermi level . The specific compositions in ZrxCu(1-x) with greater GFA are in line with experimental observations . Moreover, our calculation shows that interactions beyond those of the basic icosahedral units play an important role to accounts for both SRO and MRO in BMG.  W.L. Johnson, Bulk Glass-forming Metallic alloys: Science and Technology. Mat. Res. Bull. 24:10, 42 (1999).  Data provided by Yunfeng Shi.  Y. Li, Q. Guo, J. A. Kalb, C. V. Thompson. Matching Glass-Forming Ability with the Density of the Amorphous Phase, SCIENCE 322, 1817-1819 (2008).  Wai-Yim Ching and Paul Rulis, Electronic Structure methods for Complex Materials: The Orthogonalized Linear Combination of Atomic Orbitals, Oxford University Press, Oxford (2012).  S.R. Nagel and J. Tauc, Nearly-Free-Electron Approach to the Theory of Metallic Glass Alloys, Phys. Rev. Lett., 35, 380-383 (1975).
10:00 AM - *NN5.02
First Principles Investigation of Transition Metal Based Bulk Metallic Glasses and Their Crystalline Approximants
Michael Widom 1
1Carnegie Mellon University Pittsburgh USAShow Abstract
Transition metal alloys containing early and late transition metals, such as Ni-Ta and chemically similar combinations, form bulk metallic glasses containing a significant degree of local icosahedral order. This order is mimicked by Frank-Kasper type crystalline phases, notably those of prototype Fe7W6 (Pearson type hR13). We will present first principles molecular dynamics simulations of binary, ternary and quaternary alloys in the (Co,Ni)-(Nb,Ta) system, evaluate their local order and elastic properties, and compare these with their crystalline counterparts.
11:00 AM - *NN5.03
Relating Dynamic and Mechanical Properties to Atomic-level Structure of Metallic Glasses
Howard Sheng 1
1George Mason University Fairfax USAShow Abstract
Atomic packing in metallic glasses is not completely random, but displays various degrees of structural ordering. While it is believed that local structures profoundly affect the properties of glasses, a fundamental understanding of the structure-property relationship has been lacking. In this presentation, we provide a microscopic picture to uncover the intricate interplay between structural defects and properties of metallic glasses, mainly from the perspective of computational modeling. Computational methodologies for such realistic modeling will be introduced. Structural ordering at the short-to-medium range scales of prototype metallic glasses will be characterized, with a new focus on geometric measures of subatomic voids. Atomic sites connected with the voids are found to be crucial in terms of understanding dynamic and mechanical properties. Normal mode analysis was performed to reveal the structural origin of the anomalous Boson peak on the vibration spectrum of the glass, and its correlation with atomic packing cavities. Through transition state search on the energy landscape of the system, such structural disorder is found to be a facilitating factor for atomic diffusion, with diffusion energy barriers and diffusion pathways significantly varying with the degree of structural relaxation/ordering. The role of structural defects on mechanical properties, especially the initiation of shear transformations of metallic glasses, will be discussed.
11:30 AM - NN5.04
Influence of Structural Phenomena on Time-of-flight Hole Mobility in Hydrogenated Amorphous Silicon Thin Films
Eric Johlin 1 Christie B. Simmons 1 Nouar A. Tabet 2 Syed Said 2 Jeffrey C. Grossman 1 Tonio Buonassisi 1
1Massachusetts Institute of Technology Cambridge USA2King Fahd University of Petroleum and Minerals Dhahran Saudi ArabiaShow Abstract
Although recognized as a dominant factor governing the overall photovoltaic (PV) efficiency in hydrogenated amorphous silicon (a-Si:H) devices, the influence of film structural properties on hole mobility remains largely uncertain. In this work, we have fabricated a-Si:H cells using a range of deposition conditions, yielding a variety of structural phenomena. Fourier transform infrared (FTIR) spectroscopy was utilized to determine the hydrogen content and bonding configuration (indicative of porosity) of the films, and curvature measurements were employed to determine film stresses. A time-of-flight transient photocurrent technique was used to directly determine the out-of-plane hole drift mobility, and correlations with the film microstructure, bulk properties, and deposition parameters will be presented. We observe a strong dependence of hole drift mobility on deposition parameters (microstructure and stress). These experimental results will be placed in context of recent density function theory (DFT) simulations examining the role of stress on the origins and concentrations of hole traps (and thereby mobility) in a statistical ensemble of a-Si:H structures.
11:45 AM - *NN5.05
On the Non-affine Structural Behavior of Bulk Metalic Glass upon Thermal Expansion and under Plastic Flow
Klaus-Dieter Liss 1 DongDong Qu 2 Kun Yan 1 3 Mark Reid 3 Jun Shen 2
1Australian Nuclear Science and Technology Organisation Lucas Heights Australia2Harbin Institute of Technology Harbin China3University of Wollongong Wollongong AustraliaShow Abstract
In-situ synchrotron high-energy X-ray diffraction studies on a bulk metallic glass reveals a non-linear and non-affine response to both thermal heating and plastic deformation. The 53.0Zr 18.7Cu 12.0Ni 16.3Al (at%) bulk metallic glass, is known for high glass forming ability and to expose locally inhomogeneous regions. Higher near-range ordered clusters, were found to measure 7-8 Å in their pair distribution function, while larger length scales correlate by their coordination shells. Upon heating we found that nearest neighbors expand with 0.5 10-5 , while second-nearest-neighbors move much faster apart with a thermal expansion coefficient of 1.8 10-5. This behavior involves local atomic shear, straightening zig-zag chains. Consequently, shear stresses evolve between the different building blocks, which build up to ~10-2 strain. As they yield, we observe the glass transition, with strictly linear and non-linear expansion for the first and second nearest neighbors, respectively, creating the necessary free volume and giving an atomistic explanation for the glass transition. Upon compression of the same bulk metallic glass, inter-atomic distances first shrink continuously, until a minimum value is reached. Then the material yields at atomic strain of ~10-2 through lateral inter-atomic expansion, sustaining continuous plastic flow with ever increasing lateral dilatation until catastrophic failure. The well accepted free volume theory used to describe atomic displacement mechanisms during plastic deformation, evolves in an anisotropic way. The ultimate closest atomic packing remains constant in the loading direction during further deformation. Furthermore, the lateral movement of atoms accelerates with ongoing plastic deformation, leading to an apparent increase in Poisson's ratio to the maximum theoretical value of 0.5 - at which point failure occurs. The non-affine character of the deformation provides an explanation for the correlation between metallic glasses with high Poisson's ratio and their high plasticity, and in principle, even much larger Poisson's ratio values >> 0.5 are predictable by the present findings. In conclusion, glass transition temperature and mechanical strength are directly correlated. Bulk metallic glass yields locally, while stronger bonds lead to a quasi-elastic behavior and explain the generally high Poisson's ratio of those compounds and relaxation effects like the Bauschinger effect. The atomic deformation and thermal expansion processes are highly non-linear and non-affine.
12:15 PM - NN5.07
Atomistic Level Description of Glass-transition Behavior of Zr-Cu-Al: Structural, Dynamic and Thermodynamic Properties
Rodion Belosludov 1 Yoshiko Yokoyama 1 Dmitrii Louzguine-Luzgin 2 Hiroshi Mizuseki 1 Yoshiyuki Kawazoe 1 Akihisa Inoue 2
1Tohoku University Sendai Japan2Tohoku University Sendai JapanShow Abstract
The amorphous solids are very attractive materials since they exhibit unique mechanical and physical properties attributed to the atomic structure of the amorphous phase. Therefore, the thermodynamic properties of glass formers near the glass transition have been a subject of intensive research, especially after the experimental realization of a number of metallic alloys with high glass forming ability . Despite of significant research advances in this field, there is still much to learn about the nature of the glass transition and so it is an area of intense current interest. The structural, dynamical and thermodynamic properties of metallic glasses have been studied using ab initio molecular dynamics (MD), and lattice dynamics (LD) simulations. This combined approach leads to an accurate and detailed structural, dynamic and thermodynamic description of amorphous ice solids . We studied the Zr50Cu40Al10 and Zr60Cu30Al10 systems. Using the supercell approach the initial model of metallic liquid with random atomic distribution has been constructed. After that, the ab initio MD modeling of vitrification process has been performed. The details of applied methods can be found elsewhere . As a continuation, the dynamic and thermodynamic behavior of Zr-Cu-Al systems during the quenching process has been studied using LD methods. For this purpose, the structures at different temperatures have been selected and the phonon density of states, the Helmholtz and Gibbs free energies of these structures have been evaluated. It has been found that during the quenching process the dynamic properties of Zr-Cu-Al system have been changed and the system became a stable near estimated glass transition temperature. The calculated Gibbs free energies show that beyond glass temperature, the Zr-Cu-Al reach the glassy state characterized as the local energy minima in energy landscape framework. The calculated partial densities of states show noticeable changes in the electronic structures of Zr50Cu40Al10 and Zr60Cu30Al10 compositions towards formation of glassy states. REFERENCES  A.Inoue Acta Mater. 48 (2000) 279.  V. R. Belosludov et al. J. Phys. Chem. 129 (2008) 114507.  D. V. Louzguine-Luzgin, R. Belosludov et al. J. Appl. Phys. 110 (2011) 043519.
12:30 PM - NN5.08
Diamond-like Carbon-metal Nanocomposites for Solar Energy
Georgios Tritsaris 1 Christos Mathioudakis 2 Pantelis C. Kelires 2 Efthimios Kaxiras 1
1Harvard University Cambridge USA2Cyprus University of Technology Limassol CyprusShow Abstract
Solar energy conversion systems are a possible technology for clean energy delivery. Carbon-based materials, including diamond-like carbon (DLC), have been suggested as promising for solar energy harvesting. The physical and electronic properties of DLC may be tuned by the incorporation of transition metal nanoparticles. We use the semi-empirical modified embedded atom method and density functional theory calculations to study the structural, mechanical, and optical properties of DLC with metal nanoinclusions such silver and copper, of varying content in sp3 carbon and metal. We calculate elastic constants and absorption coefficients and we demonstrate the dependence of these macroscopic properties on the atomic structure of the carbon-metal interface in the nanocomposite.
12:45 PM - NN5.09
Relationships between Composition, Structure and Properties of Thermally Stable a-SiBCN Materials for Mechanical and Optical Interfaces
Jiri Houska 1 Simon Kos 2 Vit Petrman 2 Pavel Calta 2 Jaroslav Vlcek 1 2
1University of West Bohemia - NTIS Plzen Czech Republic2University of West Bohemia - Department of Physics Plzen Czech RepublicShow Abstract
Amorphous SiBCN materials provide outstanding thermal stability (up to 2000 °C) and oxidation resistance (up to 1500 °C) combined with stability of useful functional properties (hardness, electrical conductivity or optical transparency). Deep understanding of the complex relationships between composition, short- and medium-range order, spatial homogeneity and properties allows one to tailor a-SiBCN combining the aforementioned properties. We present a detailed discussion on these relationships, using a combined approach of magnetron sputtering and ab-initio calculations. At first, we focus on the role of individual elements in the amorphous networks. The presence of B, bonded almost exclusively to N, leads to converting of some lonepairs of valence electrons associated with N atoms to bonding electrons. Carbon was found to form a high number of multiple CC and CN bonds, contrary to mainly single bonds formed by silicon. The consequently higher network coordination number at high Si/C ratio and at non-zero B content in both cases improves mechanical properties and thermal stability of the coatings. Second, we identify two important cases when the materials are amorphous but no longer homogenous. Presence of B at low or zero N content leads to a segregation of B-rich regions. This indicates an existence of a threshold N content characterizing the transition from a homogeneous to a segregated network, affecting the electrical and optical properties. Presence of implanted Ar atoms (due to sputtering in N2+Ar plasma) leads to a formation of Si-rich regions in the vicinity of Ar. This segregation can explain the experimentally observed material hardening upon annealing, as well as the ability of Si to relieve stress generated by the Ar bombardment (from 3.5 to <1 GPa). Third, we analyze electronic structure of a-SiBCN, link it to the electrical and optical properties, and predict a space-energy correlation in material structures. At high Si/C ratio, decreasing the N content (from 54 to 0 at.%) decreases the electrical resistivity (from 10^8 to 0.2 Omega;m) and the optical gap (from 3.5 eV to zero). In samples with high N content, the electrical conductivity is mainly associated with the C atoms and especially CC bonds. In samples with low N content, B atoms start acting as an efficient dopant. The highest occupied states are on the average localized on shorter (SiN and SiSi) bonds than the lowest unoccupied states. Collectively, the consistent theoretical and experimental results provide a detailed insight into the complex relationships between composition, structure and properties of a-SiBCN, and allow one to tailor synthesis pathways for different technological applications. In parallel, we show methodological aspects which can be useful for studying a wide range of amorphous materials.  J. Houska and S. Kos, J. Appl. Phys. 108, 083711 (2010)  V. Petrman, J. Houska, S. Kos, P. Calta and J. Vlcek, Acta Materialia 59, 2341 (2011)
Yunfeng Shi, Rensselaer Polytechnic Institute
Michael J. Demkowicz, Massachusetts Inst. of Technology
A. Lindsay Greer, University of Cambridge
Despina Louca, University of Virginia
Symposium Support Corning Inc
NN8: Relaxation and Agitation of Glasses
Thursday PM, November 29, 2012
Hynes, Level 1, Room 101
2:30 AM - *NN8.01
Topological Excitations in Liquids and Glasses
Takeshi Egami 1 2
1University of Tennessee Knoxville USA2Oak Ridge National Laboratory Oak Ridge USAShow Abstract
One of the major scientific challenges in the field of liquids and glasses is to explain why the viscosity of a liquid increases as much as 15 orders of magnitude over a relatively narrow range of temperature and results in the glass transition. In contrast much less attention has been paid to the behavior of liquids at high temperatures. According to the energy landscape theory atoms at high temperatures are free to diffuse above the peaks and valleys of energy landscape, and atoms feel the presence of energy landscape only below the landscape crossover temperature, TA. The data show, however, that diffusivity follows the Arrhenius law at high temperatures, so atomic motion is thermally activated and not totally free above TA. In this talk we present a very different view of atomic dynamics in the liquid state. We show that the viscous behavior of a liquid at high temperatures is best described in terms of excitations in the local topology of atomic connectivity. We found that the relaxation time of local topological excitation (TE), tau;(LT), defined as the time to lose or gain one nearest neighbor , is equal to the Maxwell relaxation time, tau;(M) (= eta;/G0, where eta; is viscosity and G0 is the high-frequency shear modulus), at temperatures at T > TA. Above TA tau;(LT) is shorter than the vibrational time-scale, tau;(V), so that TEs cannot communicate with each other. Atoms are not free to move, but TEs are independent of each other, and their time-averaged behavior is the same for all atoms, allowing description by the mean-field model. Below TA TEs interact each other through local phonons and screen each other. Consequently the ratio, tau;(M) /tau;(L)T, increases quickly below TA, leading to the glass transition. Thus it is possible to develop a model of the dynamic behavior of liquids and glasses in terms of local topological excitations, a view which is very different from the conventional one.  T. Iwashita and T. Egami, Phys. Rev. Lett., 108, 196001 (2012).
3:00 AM - *NN8.02
Characterization of Viscous Flow Units in Metallic Glasses
Wei Hua Wang 1
1Institute of Physics, CAS Beijing ChinaShow Abstract
The understanding of the atomic structure of glasses and its relationship to deformation mechanism and glass transition is the challenging issue. Theories and models propose structural heterogeneity like flow defects could act as the flow units, and accommodate the deformation and initiate the glass transition in glasses, but effective experimental evidence for the existence of flow units are still lack. In this study, metallic glasses with distinct β-relaxation behavior and mechanical properties are chosen to identify and characterize the flow units by using both quasi-static cyclic compressions and dynamic tensile tests and dynamical mechanical analyzer. We proposed a viscoelastic model to explain the apparent viscoelastic hysteresis loop in cyclic deformation in elastic regime of the metallic glasses and demonstrate the activation of flow units. We show that the flow units originated from structural inhomogeneity are the basic deformation units and the structural origin of β-relaxation in metallic glasses, and the density of the flow units determine the mechanical properties and relaxations.
3:30 AM - NN8.03
The Effect of Structural Relaxation in a Metallic Glass on Shear Transformation Zones
Michael Atzmon 1 JongDu Ju 2 Dongchan Jang 3
1University of Michigan Ann Arbor USA2University of Michigan Ann Arbor USA3Caltech Pasadena USAShow Abstract
In our recent work , we used anelastic relaxation measurements to determine the properties of shear transformation zones (STZs). We observed a quantized hierarchy of STZs with volume increments of a single atom. The results yielded a volume-fraction size distribution (VFSD) function, which provides an opportunity to explore the microscopic criteria that determine when an atomic cluster is a potential STZ. The measurements have been repeated for structurally relaxed samples. We observe that the STZ properties remain essentially the same, but their density decreases. Based on structural defects that obey Poisson statistics, e.g., free volume, we formulate a criterion for an atomic cluster being a potential STZ. A single-parameter fit of the corresponding expression is consistent with the observed VFSDs and the effect of relaxation. 1) J. D. Ju, D. Jang, A. Nwankpa and M. Atzmon, J. Appl. Phys. 109, 053522 (2011).
3:45 AM - NN8.04
Significantly Improving the Ductility of Bulk Metallic Glasses via Severe Plastic Deformation in a Constrained Volume
Yanbo Wang 1 Xiaozhou Liao 1
1The University of Sydney Sydney AustraliaShow Abstract
In this presentation, we will discuss our recent discovery  that severe plastic deformation in a constrained volume can be used to manipulate the structure of bulk metallic glasses (BMGs) that subsequently improves the ductility of the BMGs. Zr-based and Ti-based BMGs were chosen in this study. Severe plastic deformation was conducted using high-pressure torsion (HPT). Structural characterization and mechanical property testing were carried out using x-ray diffraction, differential scanning calorimetry, micro-indentation, scanning electron microscopy, high-resolution transmission electron microscopy (TEM), and in-situ deformation TEM. Results show that HPT processing increases the free volume of the BMGs and introduces microstructural heterogeneity at the nanoscale through deformation-induced atomic clustering processes. These microstructures effectively reduce shear localization and induce a strain hardening capability, leading to a significant enhancement in their ductility. This finding opens a new and important pathway towards enhanced ductility of BMGs.  Y. B. Wang, D. D. Qu, X. H. Wang, Y. Cao, X. Z. Liao, M. Kawasaki, S. P. Ringer, Z. W. Shan, T. G. Langdon, and J. Shen, Introducing a strain-hardening capability to improve the ductility of bulk metallic glasses via severe plastic deformation, Acta Mater. 60, 253-260 (2012)
4:30 AM - *NN8.05
Radiation-induced Amorphization of Nanostructured Ceramics
Jie Lian 1
1Rensselaer Polytechnic Institute Troy USAShow Abstract
Materials utilized under intensive radiation environments may experience radiation-induced structural modification, disordering, transformation or amorphization, associated with the corresponding materials properties degradation. The radiation-induced defect behaviors including defect production, accumulation and annihilation ultimately determine the structural evolution and amorphization processes. The radiation-induced structural transformation and amorphization are further complexed upon the reduction of the grain size from the micron-metered to the nano-metered scales. Nanostructured materials are generally considered radiation tolerant as the interface and grain boundary behave as effective sinks for defect recovery. In this talk, the correlation among radiation response, microstructure, different length scales, thermodynamics and defect kinetics will be highlighted. Systematic ion beam irradiation studies on model systems of nanostructured oxides and nitrides suggested that nanostructured materials are not intrinsically radiation tolerant. An enhanced vacancy accumulation may occur at the intermediate length scale, and thus nanostructured materials are less resistant against radiation-induced structural transformation or amorphization.
5:00 AM - NN8.06
Tailoring the Optical Properties of Silica Irradiated with Swift Heavy Ions
Antonio Rivera 1 Alejandro Prada 1 Ovidio Yordanis Pena Rodriguez 1 Javier Manzano-Santamaria 2 Miguel Crespillo 3 Jose Olivares 3 4 Fernando Agullo-Lopez 3
1Universidad Politamp;#233;cnica de Madrid Madrid Spain2Euratom/CIEMAT Fusion Association Madrid Spain3Universidad Autamp;#243;noma de Madrid Madrid Spain4Consejo Superior de Investigaciones Cientamp;#237;ficas Madrid SpainShow Abstract
Irradiation with swift heavy ions (SHI), roughly defined as those having atomic masses larger than 15 and energies exceeding 1 MeV/amu, may lead to significant modification of the irradiated material in a nanometric region around the (straight) ion trajectory (i.e., latent tracks). In the case of amorphous silica it has been reported that SHI irradiation originates nano-tracks of either higher density than the virgin material (for low electronic stopping powers, Se < 7 keV/nm)  or having a low-density core and a dense shell (Se > 12 keV/nm) . The intermediate region has not been studied in detail but we will show in this work that essentially no changes in density occur in this zone. An interesting effect of the compaction is that the refractive index is increased with respect to that of the surroundings. In the first Se region it is clear that track overlapping leads to continuous amorphous layers that present a significant contrast with respect to the pristine substrate and this has been used to produce optical waveguides. The optical effects of intermediate and high stopping powers, on the other hand, are largely unknown so far. In this work we have studied theoretically (molecular dynamics and optical simulations) and experimentally (irradiation with SHI and optical characterization) the dependence of the macroscopic optical properties (i.e., the refractive index of the effective medium, n_EMA) on the electronic stopping power of the incoming ions. Our results show that the refractive index of the irradiated silica is not increased in the intermediate region, as expected; however, the core-shell tracks of the high-Se region produce a quite effective enhancement of n_EMA that could prove attractive for the fabrication of optical waveguides at ultralow fluences (as low as 1E11 cm^-2). 1. J. Manzano, J. Olivares, F. Agulloacute;-Loacute;pez, M. L. Crespillo, A. Moroño, and E. Hodgson, "Optical waveguides obtained by swift-ion irradiation on silica (a-SiO2)," Nucl. Instrum. Meth. B 268, 3147-3150 (2010). 2. P. Kluth, C. S. Schnohr, O. H. Pakarinen, F. Djurabekova, D. J. Sprouster, R. Giulian, M. C. Ridgway, A. P. Byrne, C. Trautmann, D. J. Cookson, K. Nordlund, and M. Toulemonde, "Fine structure in swift heavy ion tracks in amorphous SiO2," Phys. Rev. Lett. 101, 175503 (2008).
5:15 AM - NN8.07
Interpreting the Relationship between the Thermophysical and Dynamical Properties of High Temperature Metallic Liquids and Their Glass Formability
James C Bendert 1 Nicholas A Mauro 1 Anup K Gangopadhyay 1 Kenneth F Kelton 1
1Washington University St. Louis USAShow Abstract
Results from a recent systematic survey of the liquid densities in the metallic glass forming Cu100-xZrx alloy system are presented, which show that at high temperatures the liquids that most easily form glasses also have the largest thermal expansion coefficients. Theoretical studies have shown previously that the expansivity correlates with fragility near Tg; the experimental results presented here show that at high temperature they become anti-correlated. From energy landscape arguments this indicates that the temperature at which the expansivity/fragility correlation crosses over, scales inversely with the liquid fragility. These results support the conclusion that structural changes must already be underway near their melting temperatures in the equilibrium glass-forming liquids. This research was partially supported by the National Science Foundation under grants DMR-08-56199, and NASA under grants NNX07AK27G, NNX09AJ19H and NNX10AU19G.
5:30 AM - NN8.08
Rapid Atomic Ordering in Ni-Nb-based Liquids and Glasses -- A Measure of Fragility
Nicholas Mauro 1 2 Mark Johnson 1 2 James Bendert 1 2 Kenneth Kelton 1 2
1Washington University St. Louis USA2Washington University St. Louis USAShow Abstract
High-energy X-ray diffraction data were taken to high momentum transfer for Ni59.5Nb40.5, Ni62Nb38, and Ni60Nb30Ta10 liquids and glasses using the Beamline Electrostatic (BESL) technique. The data were used to obtain the total pair-correlation function, S(q), and total pair-correlation function, g(r). Reverse Monte Carlo (RMC) fits to the S(q) data indicate that in Ni-Nb liquids and glasses ordering is dominated by icosahedral-like short-range order (ISRO). For the glass, the height of the nearest-neighbor peak in g(r) and the amount of ISRO is larger that would be predicted from a linear extrapolation of the high-temperature behavior, indicating an accelerating ordering on approaching the glass transition. This is in stark contrast to the behavior observed in strong Zr-based metallic liquids and glasses, where the ordering increases approximately linearly with decreasing temperature into the glass transition. These results demonstrate that like the commonly used intensive parameters (such as specific heat and specific volume), the rate of structural ordering can be used to distinguish strong and fragile liquids. This research was partially supported by the National Science Foundation under grant DMR-08-56199.
5:45 AM - NN8.09
Residual Stress, Modulus and Hardness of Silicon Oxycarbide Films
Ping Du 1 Xiaoning Wang 1 I-Kuan Lin 1 Xin Zhang 1
1Boston University Boston USAShow Abstract
There is an increasing trend to incorporate silicon carbide into silicon oxides to improve the mechanical properties, thermal stability, and chemical resistance. In this work the silicon oxycarbide (SiOC) films were deposited by RF magnetron sputtering, and rapid thermal annealing (RTA) was applied to tune the mechanical properties. The residual stress, as well as the Young&’s modulus and hardness of the films were measured by film-substrate curvature method and nanoindentation. A thorough microstructural analysis was conducted to investigate the effect of carbon content and annealing temperature on the mechanical properties of SiOC films. SiOC films were deposited on silicon wafer substrates at room temperature by a Discovery 18 RF magnetron sputter. The composition of the SiOC films was controlled by the ratio of RF power applied on the silicon dioxide and silicon carbide, and verified by energy dispersive spectroscopy. The relative low temperature sputtering process rendered the films amorphous. The random bonding model was adopted to depict the chemical structures of SiOC films, which was validated by the SEM micrographs and FTIR spectra. The residual stress was calculated from the film-substrate curvature by using the Stoney&’s equation. For the as-deposited films, the residual stresses are generally compressive, which is associated with “atomic shot peening" mechanism. For the effect of annealing temperature, all five films experience increases in residual stress from compressive to tensile. This is accompanied by a change in the material microstructure, i.e. more orders with fewer defects, and macroscopically film thickness reduction. In addition, the residual stresses of all RTA treated films show monotonic increases with increasing carbon contents. This is due to the transition from flexible Si-O-Si bonding network to the rigid Si-C-Si bonds. The Young&’s modulus and hardness of the SiOC films were measured by nanoindentation. Both the modulus and hardness of as-deposited SiOC films show monotonic increases with increasing carbon contents. For the effect of thermal annealing, both the modulus and hardness generally increase with increasing annealing temperatures. SiCy is more sensitive to annealing than SiOx in both modulus and hardness. The increase of annealing temperature results in the growth of modulus and hardness could be due to the microstructure evolution. Therefore, FTIR absorption spectra of both as-deposited and annealed SiOC films were measured and compared. The blue shift is correlated to the increase of Si-C bond density, and ultimately the modulus and hardness by a constant-plus-linear relation. This work presents a versatile approach to control the mechanical properties of SiOC films, which will provide more opportunities for SiOC to be integrated into the MEMS field. We believe this microstructure-based theory provides insight into the analysis of similar responses of other amorphous materials.
NN7: Structure-property Relation in Bulk Metallic Glasses
Thursday AM, November 29, 2012
Hynes, Level 1, Room 101
9:30 AM - *NN7.02
Stick-slip Shear Banding in Amorphous Materials
Joerg F. Loeffler 1 David Klaumuenzer 1 Peter Thurnheer 1 Eric G. Daub 1 2
1ETH Zurich Zurich Switzerland2Los Alamos National Laboratory Los Alamos USAShow Abstract
The mechanical properties of amorphous materials are strongly influenced by their disordered microstructure. In general, amorphous solids deform inhomogeneously by the formation of thin shear bands at low homologous temperatures. The operation of these bands is intermittent as reflected in serrated flow curves and can be understood in the context of stick-slip, in which extended periods of arrest are followed by rapid slip events. Such processes are also known to occur across macroscopic underlying length scales ranging from granular systems to tectonic faults. In this talk, we will review rate- and state dependent friction laws and identify velocity weakening and static re-strengthening as underlying principles of stick-slip. We will discuss how microstructure and mechanical deformation modes change as a function of strain-rate, time and temperature. By applying these concepts to metallic glasses, we will introduce an effective temperature model, which quantifies time- and rate-dependent structural disordering. In this case, velocity weakening, and thus stick-slip, arises when the strain-rate increase during mechanical deformation is overcompensated by the accompanying increase in effective temperature, thus allowing the applied stress to be lowered. Static re-strengthening follows naturally from the underlying laws. We will also discuss the differences and commonalities of shear-banding among different classes of amorphous materials, and outline in which cases thermally activated stick-slip shear-banding can occur.
10:00 AM - *NN7.03
Effects of Laser Shock Peening on Zr-based Bulk Metallic Glasses (BMGs)
Peter K. Liaw 1 Xie Xie 1 Yunfeng Cao 2 James Antonaglia 3 Bartlomiej Winiarski 4 Gongyao Wang 1 Yung Shin 2 Philip J. Withers 4 Karin A. Dahmen 3
1University of Tennessee Knoxville USA2Purdue University West Lafayette USA3University of Illinois at Urbana Champaign Urbana USA4The University of Manchester Manchester United KingdomShow Abstract
Laser shock peening (LSP), as a modern surface processing technique, can improve the plasticity and increase the strength of BMGs by introducing residual stresses and fine multiple shear bands into their surface. The improvement of both plasticity and strength following laser treatments was confirmed through the compression experiments on Zr-based BMG Vit-105 (Zr52.5Cu17.9Ni14.6Al10.0Ti5.0 in atomic percent, at. %) in this study. To depict the laser effect, a focused-ion-beam (FIB)-based micro-slot cutting (mu;SC) method was employed to map the residual-stress distribution. Meanwhile, a finite element model of LSP was developed to predict the residual-stress distribution using the ABAQUS software, which is comparable with experimental measurements. As a powerful characterization technique, atom probe tomography will be used to probe the existence of nanocrystals induced by laser treatments, and to elucidate the microstructure of the laser-affected layer. Furthermore, a new theory for the statistics of slip avalanches is being developed for the analysis of plastic deformation following laser treatments, expected to unveil the mechanism of plasticity improvement. Acknowledgements: XX, GW, and PKL very much appreciate the financial support of the US National Science Foundation, under DMR-0231320, CMMI-0900271, CMMI-1100080, and DMR-0909037, with Drs. C. Huber, C. V. Cooper, D. Finotello, A. Ardell, and E. Taleff as contract monitors. BW and PJW appreciate the financial support of the Light Alloys Towards Environmentally Sustainable Transport (LATEST) EPSRC Portfolio Project.
11:00 AM - *NN7.04
Bulk-metallic Glasses under Cyclic Loading
Bernd Gludovatz 2 Marios Demetriou 3 William L. Johnson 3 Robert O. Ritchie 1 2
1University of California Berkeley Berkeley USA2Lawrence Berkeley National Laboratory Berkeley USA3California Institute of Technology Pasadena USAShow Abstract
Specific bulk-metallic glasses (BMGs) are now considered as viable candidate materials for many structural applications due to the ease of processing combined with exceptional strength and toughness. However, certain mechanical properties, e.g., their cyclic fatigue resistance, can be very poor, which severely limits their potential use as structural materials. As the fatigue limits of metallic materials are invariably governed by the local arrest of microscopically small cracks at microstructural features, the lack of microstructure in monolithic glasses, coupled with the ease of crack formation in shear bands, can lead to fatigue limits as low as 1/20 of their tensile strength. Recently developed BMG-matrix composites can overcome this limitation by introducing crystalline second-phase dendrites to arrest shear bands and the formation of cracks before they exceed critical size; resulting fatigue limits of ~0.3 UTS makes them comparable with most traditional crystalline alloys. The question that we now pose is whether such fatigue properties can be achieved in monolithic BMGs. Here, we report on a 1.5 GPa strength Pd-based glass with high-toughness which displays similar high fatigue resistance but without the need for a second phase. This monolithic glass has a high bulk-to-shear modulus ratio with the ability to form multiple shear bands which do not readily cavitate into cracks. This proliferation of shear bands with inhibited cavitation efficiently blunts any incipient cracks leading to high fatigue limits (comparable to BMG composites) in a monolithic glass, thereby providing a new strategy to make BMGs far less susceptible to fatigue and premature fracture.
11:30 AM - NN7.05
High-speed Imaging of a Bulk Metallic Glass during Compressive Fracture
Wendelin J Wright 1 Rachel R Byer 2 Xiaojun Gu 1
1Bucknell University Lewisburg USA2Bucknell University Lewisburg USAShow Abstract
High-speed imaging has been performed to capture the serrated flow and fracture events of Zr-based bulk metallic glasses tested in compression. The images of shear band propagation are directly correlated with mechanical measurements of serrated flow. The images and data from a piezoelectric load cell and strain gages applied to the specimens demonstrate that the fracture events in metallic glasses for tests performed at quasistatic strain rates occur much more rapidly than the shear banding events observed during serrated flow. The inferred shear band viscosity decreases with increasing strain until failure is reached, where heating then leads to melting on sample surfaces. Other observations regarding the use of strain gages to record shear band propagation will be presented.
11:45 AM - NN7.06
The Role of Disorder in the Elastic Robustness of Bulk Metallic Glasses
Peter M Derlet 1 Robert Maass 2
1Paul Scherrer Institut PSI-Villigen Switzerland2California Institute of Technology Pasadena USAShow Abstract
Despite significant atomic-scale heterogeneity, bulk metallic glasses well below their glass transition temperature exhibit a surprisingly robust elastic regime and a sharp elastic-to-plastic transition. Here it is shown that, when the number of available structural transformations scales exponentially with system size, a simple thermal-activation model is able to describe these features, where yield corresponds to a change from a barrier energy dominated to a barrier entropy dominated regime of shear transformation activity, allowing the system to macroscopically exit its frozen state (Phys. Rev. B, 84, 220201R (2011)). The work concludes with an application of the theory that re-casts the work of Johnson and Samwer (Phys. Rev. Lett. 95, 195501 (2005)) into an explicit expression for the yield stress as a function of temperature that very well describes experimental data for a large number of different glasses.
12:00 PM - NN7.07
High Strain Rate Mechanical Behavior of a Zr-based Metallic Glass as a Function of Temperature
Weihua Yin 1 Qiuming Wei 1 Chaoli Ma 2
1University of North Carolina at Charlotte Charlotte USA2Beijing University of Aeronautics and Astronautics Beijing ChinaShow Abstract
In this work, the mechanical behavior of Zr55Al10Ni5Cu30 bulk metallic glass is investigated within a wide range strain rates (~10-4 s-1 to ~104 s-1) and temperatures (liquid nitrogen temperature to the supercooled liquid region of the glass), with the focus of interest on the dynamic behavior as a function of temperature. A modified uniaxial compressive Kolsky bar system is used to access the high strain rate regime. A synchronically assembled unit is added to the Kolsky bar system to reduce the cold-contact time during the high temperature test and to ensure the accuracy of the results. The fracture surfaces of the specimens are examined using scanning electron microscopy, in order to reveal the different failure mode as a function of temperature. Transmission electron microscopy is performed to confirm the crystallization, or the lack thereof, in the metallic glass specimens under high strain rate loading at different temperatures.
12:15 PM - NN7.08
Critical Temperature for Ductile-to-brittle Transition for Metallic Glasses
Golden Kumar 1 Jan Schroers 2
1Texas Tech University Lubbock USA2Yale University New Haven USAShow Abstract
A comprehensive analysis of plasticity (and toughness) in bulk metallic glasses (BMGs) is presented. Different effects such as: contribution of shear modulus/bulk modulus ratio, structural relaxation, and cooling rate effect are evaluated for Pt57.5Cu14.7Ni5.3P22.5 (Pt-BMG), Pd43Cu27Ni10P20 (Pd-BMG), and Zr44Ti11Ni10Cu10Be25 (Zr-BMG). We introduce a critical temperature, TC, which is an intrinsic feature of a BMG former above which the BMG does not embrittle. We demonstrate that TC/Tg ratio indicates the embrittlement sensitivity of a BMG due to annealing and cooling rate. This ratio is larger than one for Pd-BMG and smaller than one for the Pt-BMG. As a consequence, Pd-BMG is more sensitive to cooling rate and annealing induced embrittlement. In contrast, Pt-BMG does not embrittle during sub-Tg annealing or at practically achievable slow cooling rates. Study of shear modulus/bulk modulus ratio for Pd-BMG and Pt-BMG does not follow the previously proposed critical value for ductile to brittle transition.
12:30 PM - NN7.09
Local Structure of Sputtered Al-Sm Thin Film and Its Effect on Phase Selection during Devitrification
Eun Soo Park 1 Matthew Besser 1 Matthew Kramer 1 2 Ryan Ott 1
1Ames Laboratory Ames USA2Iowa State University Ames USAShow Abstract
Al90Sm10 metallic glass has been synthesized by both magnetron sputtering and melt spinning. The local structure of the sputtered thin film and its effect on subsequent phase selection during devitrification has been investigated using a combination of difference scanning calorimetry (DSC), scanning transmission electron microscopy (STEM) and simultaneous high-energy synchrotron small- and wide-angle X-ray scattering (SAXS/WAXS). The results suggest the sputtered Al-Sm thin film exhibit a more liquid-like structure compared to the melt-spun ribbon of the same composition. The different chemical and topological order of the as-sputtered Al-Sm alloy plays an important role in phase selection during devitrification. In-situ SAXS/WAXS and STEM results indicate that the melt-spun ribbon transforms polymorphically to a metastable intermetallic as previously reported [1-3], while the sputtered thin film separates into Al-rich and Al-poor amorphous phases at the early stage of annealing. Nano-crystals of fcc-Al initially form in the Al-rich amorphous phase and subsequently Al-Sm metastable intermetallic phases develop in the Al-poor amorphous phase with increasing temperature.  Y.E. Kalay, C. Yeager, L.S. Chumbley, M.J. Kramer, I.E. Anderson, J. Non-Cryst. Solids 356 (2010) 1416-1424.  L. Battezzati, M. Baricco, P. Schumacher, W.C. Shih, A.L. Greer, Mater. Sci. Eng., A179-180 (1994) 600-604.  Y.E. Kalay, L.S. Chumbley, M.J. Kramer, I.E. Anderson, Intermetallics 18 (2010) 1676-1682.
Yunfeng Shi, Rensselaer Polytechnic Institute
Michael J. Demkowicz, Massachusetts Inst. of Technology
A. Lindsay Greer, University of Cambridge
Despina Louca, University of Virginia
Symposium Support Corning Inc
NN11: From Structure-Property Relation to Applications of Amorphous Solids
Friday PM, November 30, 2012
Hynes, Level 2, Room 201
2:30 AM - NN11.01
Zr-based Bulk Metallic Glasses towards Biomedical Applications
Lu Huang 1 Wei He 1 2 Peter K Liaw 1
1The University of Tennessee Knoxville USA2The University of Tennessee Knoxville USAShow Abstract
There is an increasing interest in the biomedical applications of bulk metallic glasses (BMGs) during the last decade, due to their unique properties, including low elastic modulus, high strength-to-weight ratio, high fatigue and wear resistance, good corrosion resistance, and facile thermal plastic formation. The present presentation reviews the recent progress on the biocompatibility/bioactivity of Zr-based BMGs in our group, aiming at its application as hard tissue implants. Corrosion resistance of the Zr-based BMGs was studied by electrochemical polarization in phosphate buffered saline (PBS) at 37 oC with 4 vol.% O2/N2 aeration, in order to simulate the human body environment. Zr-based BMGs exhibit equivalent corrosion resistance to those of crystalline alloys. The initial biocompatibility of Zr-based BMGs was proven through in-vitro cell cultures. Good cell adhesion and viability are determined by fluorescent staining. Cell proliferation behavior is monitored by WST-1 assay. Alkaline phosphatase (ALP) activity and Ca deposition are demonstrated as indicators to investigate the differentiation and mineralization behaviors of MC3T3-E1 cells on the BMG substrates. To enhance the biocompatibility of Zr-based BMGs, pre-immersion treatment was performed on BMG samples in PBS prior to cell experiments. Moreover, surface roughness/patterning effects on cell behaviors were investigated. Based on the initial biosafety, a surface engineering approach is employed to further modify the surfaces of BMGs from bioinert to bioactive in the present study. Effects of calcium implantation on the surface properties and the bioactivity of a Zr-based BMG were investigated. Calcium (Ca) ions were implanted with a fluence of 8 x 10^15 cm-2 at ion energies of 10 keV and 50 keV, respectively. The distribution of Ca and damages induced by the implantation is simulated using the TRIM code. Surface properties and cell culture results were compared among irradiated samples, as-cast BMGs, and Ti-6Al-4V in order to elaborate the Ca implantation effects.
2:45 AM - NN11.02
Amorphous Alloys for High Performance Electrode Materials of Solar Cell
Se Yun Kim 1 Sang Soo Jee 1 Suk Jun Kim 1 Jin Man Park 1 Keum Hwan Park 1 Sang Mock Lee 1 Hyoeng Ki Kim 2 Young Sang Park 2 Min Chul Song 2 Sung Chan Park 2 Ka Ram Lim 3 Won Tae Kim 4 Do Hyang Kim 3 Eun-Sung Lee 1
1Samsung Electronics (SAIT) Yongin Republic of Korea2Samsung SDI Yongin Republic of Korea3Yonsei University Seoul Republic of Korea4Cheongju University Cheongju Republic of KoreaShow Abstract
There are considerable endeavor for making electrodes by low cost processes. Among those low cost processes, a lot of research has been done on the screen printing method because the whole processes are performed in the air. However, the electrode made by screen printing process has lower performance than those by electroplating/deposition processes. To overcome the limitation of the screen printing process, we have applied metallic glasses (MGs), having amorphous structure, in the electrode paste. In general, the conventional electrode paste contains oxide glass (OG) for assisting the reaction between a substrate and electrode materials. However, there is a certain limitation in the OG containing electrode paste, because OG is non-conductive. Since the MGs consist of metal elements, their electrical conductance is approximately 10e14 times higher than that of the OGs. Therefore, it is expected that the electrode performance of the MG containing paste is comparable to that of electroplating/deposition processes. In this study, we have investigated the reaction mechanism of the MG contained electrode paste and wetting behavior of MG in the electrode paste during the firing process. Generally, electrode pastes consist of conductive powder, glass frit, and organic vehicle. In our system, silver and aluminum based MG are used as the conductive powder and the glass frit, respectively. The composition of Al-based MG is Al85Ni5Co2Y8, which has SCL (super-cooled liquid) region between 560 ~ 597 K at heating rate of 40 K/s. Within the SCL region, the Al-MG powder wets between the silver powders and on the silicon wafer surface by capillary force. The wetted MG accelerates Ag sintering reaction and activates Si/Ag adhesion between the electrode and the substrate. Computational Fluid Dynamics (CFD) simulation is used to predict the wetting behavior of MG with various viscosities of MG and different interface energies between MG-Ag and between MG-SiO2. It is clearly shown that the wettability of MG is increased as the viscosity and the interfacial energies of MG are lower. To investigate the paste performance in devices, Interdigitated Back Contact (IBC) solar cells were fabricated using the MG-contained electrode paste at Samsung SDI. The cells have an average energy conversion efficiency of 20.0 %, which is 0.9% higher than the cells using the conventional pastes. This result shows the potential of the MG-contained paste for substituting the electroplating/deposition processes by the screen-printed process.
3:00 AM - NN11.03
Hydrogen Diffusion through Amorphous Metallic Structure
Jin-Yoo Suh 1 Young-Im Wang 1 Min-Hyun Kim 1 Mukta Rani Debnath 1 Eric Fleury 1
1Korea Institute of Science and Technology Seoul Republic of KoreaShow Abstract
Metallic membranes separate hydrogen from a mixture of different gas molecules by selective diffusion of atomic hydrogen through the interstitial sites of a metallic lattice. Palladium and its alloys have long been used for this application. However recent interest in the hydrogen energy prompted the search for an alternative to the precious metal. One of the candidate materials is amorphous metal. In this study, we evaluated the hydrogen flux and diffusivity through Ni-based metallic glass membranes composed of the elemental groups of Ni, Nb, and Zr as major constituents together with minor addition of other elements. Hydrogen flux through the amorphous alloys with different Zr contents was evaluated and the effect of Zr on hydrogen permeability and structural stability were discussed. Also, the effect of Co addition on the structural stability of Ni-Nb-Zr amorphous alloys were studied to conclude that the degradation of hydrogen flux during operation is closely related to the crystallization and its activation energy barrier. The hydrogen diffusivity was measured by the Time-lag method which analyzes the initial transient behavior of the hydrogen diffusion until it reaches a steady state. The hydrogen diffusivity of the amorphous metallic membranes is about one order of magnitude smaller than that of palladium membrane. On the other hand, the amorphous membranes have higher hydrogen solubility to result in hydrogen flux comparable to that of palladium membrane.
3:15 AM - NN11.04
Bond Percolation Effects and Structure Property Relationships in Amorpohous SiC(O,N):H low-k Dielectric Materials
Sean King 1 Ebony Mays 1 Guanghai Xu 2 Li Han 2 Marc French 2 Jeff Bielefeld 3 Brian Daly 4 Shifi Kababya 5 Asher Schmidt 5
1Intel Corporation Hillsboro USA2Intel Corporation Hillsboro USA3Intel Corporation Hillsboro USA4Vassar College Poughkeepsie USA5Technion - Israel Institute of Technology Haifa IsraelShow Abstract
Materials with low dielectric constants (i.e. low-k) are increasingly replacing SiO2 and a-SiN:H as insulating dielectrics in micro/nano-electronic products in order to both reduce resistance-capacitance delays in interconnect wiring and parasitic capacitances in transistor structures. The current industry standard low-k materials are plasma deposited “carbon doped” variations of SiO2 and Si3N4 where carbon has been incorporated primarily as terminal methyl (CH3) groups to disrupt the local SiO2 and Si3N4 network structure. This creates additional free volume (or porosity) in the material and effectively reduces the dielectric constant of the film via averaging the dielectric constant of air with that of the SiO2/Si3N4 network. While effective in reducing the dielectric constant, this approach also affects significant changes in the mechanical, optical, and electrical properties of low-k materials, and provides an excellent platform for studying bond percolation and structure-property relationships in SiO2 and Si3N4 network materials. In this regard, we have utilized a combination of Fourier Transform-Infrared and Nuclear Magnetic Resonance Spectroscopies to study the local bonding structure in a-SiOC:H, a-SiCN:H, and a-SiC:H low-k dielectric materials. These local structure studies have been combined with additional thermal, mechanical, electrical, and optical property measurements to understand both structure property relationships and bond percolation effects in low-k materials. In this report, we will detail the observed process - structure - property relationships for low-k a-SiOC:H and a-SiCN: dielectrics and specifically demonstrate that a remarkable range in dielectric constant (< 3 - > 7), Young&’s Modulus (< 5 - > 200 GPa), and thermal conductivity (0.09 - 4 W/mK) can be achieved in a-SiC:H. We will additionally demonstrate that the observed structure property relationships can be easily understood based on bond / rigidity percolation concepts.
3:30 AM - NN11.05
The Effect of Self-induced Electric Fields on the Catalytic Properties of Supported Amorphous Metal Nanoparticles
Sergey A Gurevich 1 Vladimir M. Kozhevin 1 Denis A. Yavsin 1 Tatiana N. Rostovshchikova 2 Ekaterina S. Lokteva 2
1Ioffe Institute St. Petersburg Russian Federation2Lomonosov Moscow State University Moscow Russian FederationShow Abstract
External electric fields are known to affect the properties of heterogeneous catalyst . In particular, significant enhancement of the catalytic reaction rate can be achieved if the field intensity is high enough, comparable with intramolecular field. Application of such strong external fields may cause, however, unwanted heating or even breakdown in the reaction area. The reasonable solution would be concentration of strong electric field in close vicinity of the catalyst surface. We consider a catalyst system consisting of supported metal nanoparticles. If the particle density on the support surface is high enough, electrons can tunnel between the neighboring particles creating a pair of positive and negative charges. In case of conducting support tunneling can also proceed between the particles and the support. Being thermally stimulated these processes result in appearance of fast-fluctuating charge dipoles . Associated with these dipoles electric field is localized in the gap between the particles or in the gap between the particle and the support. Estimations show that the peak intensity of such self-induced electric field can be up to 10^7 V/cm^2 while the life span of the field fluctuations is very broad (from 10^-8 s to 10^-13 s). Entering the area of the field the reactant molecule will exert strong excitation. This consideration suggests that the catalytic property of supported metal nanoparticles will essentially depend on the particle surface density. One can expect such dependence because in a given system the particle density determines both the field strength and the possibility for the reacting molecules to enter the field area. The experiments were performed with the catalysts comprising amorphous metal nanoparticles fabricated by laser electrodispersion technique . Due to amorphous state of the metal these particles reveal enhanced coagulation stability which opens the possibility to thoroughly trace the catalytic activity in a broad range of particle density. At certain particle densities, when the field effects were expected to be most pronounced, maximum catalyst performance with record-high specific catalytic activity was observed with different metals in different reactions. This clearly shows that the impact of self-induced electric fields on the properties of supported metal nanocatalyst is very general phenomenon.  P. Deshlahra, et.al., J. Phys. Chem. A, 113 (2009) 4125.  S.A. Gurevich, et.al., Physico-Chemical Phenomena in Thin Films and at Solid Surfaces, L. I. Trakhtenberg, S. H. Lin, O. J. Ilegbusi, eds. Amsterdam: Elsevier, v. 34 (2007) 726.  V.M. Kozhevin, et.al., J.Vac.Sci.Techn. B, 18, (2000), 1402.
3:45 AM - NN11.06
Structural and Electrical Characterization of Amorphous Ta-W-Si-C Metal Films for CMOS Applications
Jiaomin Ouyang 1 Ranida Wongpiya 1 Melody Grubbs 1 Mike Deal 2 Bruce Clemens 1 Yoshio Nishi 2
1Stanford University Stanford USA2Stanford University Stanford USAShow Abstract
Metal gate electrodes have replaced polycrystalline silicon gates in CMOS technology to eliminate poly gate depletion and to enable integration with high-k dielectrics. However a rising concern with polycrystalline metal gates for MOSFETs is that device variability could become a problem as the gate dimensions scale and become comparable to the grain size due to the grain orientation work function variation. Zhang has shown that work function variation is the dominant factor in threshold voltage variation at 22nm technology node . Amorphous metal gates have been investigated in order to reduce work function variability in nano-scale MOS devices. Grubbs has shown that TaWSiC alloys remain amorphous at temperatures as high as 1120oC, are stable on HfO2 and have suitable work functions for NMOS applications . In this study we investigate the effect of composition on the crystal structure, thermal stability, and electrical properties, including work function. In order to accomplish this, the following Ta-W-Si-C compositions: Ta45W45Si5C5, (TaxW(1-x))80Si10C10 (0
NN10: Structure-Property Relation of Nanostructured Glasses
Friday AM, November 30, 2012
Hynes, Level 2, Room 201
9:30 AM - *NN10.01
Using Artificial Microstructures to Understand Microstructure-property Relationships in Metallic Glasses
Jan Schroers 1
1Yale University New Haven USAShow Abstract
Materials science seeks to correlate microstructure with (mechanical) properties. This has been successfully shown in some limited cases, however, for most technological relevant materials the microstructures and their fabrication are too complex for a systematic study. Most challenging is the interconnection of microstructural features. For example, when varying the grain size of a microstructure through the cooling rate, essential all other microstructural features such as volume fraction, spacing, shape, chemical composition, dispersity, etc are also affected. “Virtual experiments” through molecular dynamics simulations have been widely used to investigate structure-property relationships but are limited by today&’s available computing power, which limits the system size and simulation time. As a consequence, on the length scale required for microstructural investigations mainly continuous modeling is feasible and the complex constitutive equations for plastic deformation limits the modeling to elastic deformation. As a novel approach, in between real microstructures and virtual experiments, we propose to study microstructure-property relationship with artificial microstructures. This approach allows us to individually and independently vary parameters and thereby determine their individual effects on mechanical properties. The artificial microstructure are fabricated through a two-step process; silicon lithography is used to fabricate the mold and thermoplastic forming to replicate the mold into a bulk metallic glass structure. A vast range of shapes comprising of length scales ranging from 10 nm to millimeters can be fabricated. Examples where this approach is useful include toughening mechanism in metallic glasses, transition from plastic deformation to elastic buckling in metallic glass heterostructures, and flaw tolerances of microstructures.
10:00 AM - *NN10.02
Size Matters: Fabrication and Deformation of Nano-sized Metallic Glass Structures
Julia R Greer 1 2 Dongchan Jang 1 Robert Maass 1 David Chen 1
1CALTECH Pasadena USA2Caltech Pasadena USAShow Abstract
When microstructural (intrinsic) or external material dimensions are reduced to nano-scale, they have been shown to exhibit unique behavior. We fabricate nano- and micro-scale metallic glass samples ranging from nano- and micro-pillars, nano tensile specimens, and micro-trusses by using 3-dimensional patterning, templated electroplating, the Focused Ion Beam. Their tensile and compressive mechanical properties are measured in a custom in-situ mechanical deformation instrument, SEMentor, and in the nanoindenter. Our experiments reveal that 1.6-micron diameter Zr-based metallic glass samples do not fail after 40 million cycles when subjected to compressive fatigue cycling; while the same material with larger dimensions under identical loading fails after 10,000 cycles. We further show that the metallic glass fatigue endurance limit increases over the bulk yield strength by 10% under compression-compression and up to 90% of yield strength under bending, when tensile stress components are present. We also show that metallic glasses exhibit strength increase and ductility when reduced to nano-scale in monotonic compression and in tension. We attribute these unique findings to the presence of free surfaces and their effect on shear band formation. Finally, we demonstrate the fabrication of extremely lightweight metallic glass structures formed into micro-truss architectures, with individual truss dimensions below plastic zone size of the glass.
11:00 AM - *NN10.03
Structure-property Relationships in Metallic Glass-crystalline Composites
Nicholas Hutchinson 1 Anupriya Agrawal 1 Wolfgang Windl 1 Katharine M. Flores 2
1The Ohio State University Columbus USA2Washington University St. Louis USAShow Abstract
The unique atomic structure of metallic glass alloys presents a number of challenges for describing and controlling their mechanical behavior. While monolithic metallic glasses exhibit near theoretical strengths and large elastic deflections, their lack of extensive tensile ductility limits their structural applications. Microstructural control has long been the materials scientist&’s chief tool for improving material properties. By creating metallic glass-crystalline composites, we introduce microstructural features and gain control over the initiation and distribution of plastic deformation. One family of composites utilizes ductile crystalline dendrites which precipitate from the melt prior to vitrification of the matrix. Our work to quantitatively characterize this microstructure and its role in the resulting mechanical behavior will be discussed. Full field strain mapping measured from in situ digital image correlation reveals that at relatively low strains, the glassy matrix acts as an isotropic “buffer layer” between crystals of different orientations, permitting the crystals to deform unimpeded along their preferred slip system. The role of the interface in transmitting the deformation between the crystalline and glassy phases is of particular interest. To address this, we have studied the mechanical behavior of a model metallic glass/crystalline metal composite system using Molecular Dynamics simulations with Embedded Atom Method potentials. Deformation of the composite system was studied under different applied stress states, crystal orientations with respect to the interface, and crystalline volume fractions. For the system studied, deformation initiates first in the glassy phase, where shear localization is observed and is correlated with the absence of icosahedral short range order. After significant deformation of the glass, the deformation moves into the crystalline phase, where stacking faults bounded by partial dislocations form at the interface and propagate into the crystal. Efforts to determine how deformation in the glassy phase precipitates defect formation in the crystalline phase are ongoing.
11:30 AM - NN10.04
Phase Stability and Boundary Structure of Unique Nanoparticle-reinforced Iron-based Amorphous Metal Composites: Processing and Modeling
Olivia A. Graeve 1 Michael S. Saterlie 1 Paulo J. Colmenares 1 Scott T. Misture 1 Hoorshad Fathi 1 Linda E. Jones 1
1Alfred University Alfred USAShow Abstract
Recently, several alloy compositions of Fe-based metallic glasses with outstanding corrosion resistance have been produced and characterized. In this study, fundamental phase stability and boundary structure of two such alloys have been determined. The specific materials in question are Fe48Mo14Cr15Y2C15B6 and Fe49.7Cr17.7Mn1.9Mo7.4W1.6B15.2C3.8Si2.4, both of which have been modified by the inclusion of yttria and tungsten nanoparticles, with the purpose of enhancing the toughness in these materials. Processing of the materials included high-energy ball milling, for mixing of the amorphous alloy powders with the nanoparticles, and subsequent spark plasma sintering of the mixed powders to obtain dense bulk specimens. Spark plasma sintering allowed the formation of dense metallic glass compacts without losing their amorphous character, since consolidation was completed at relatively low temperatures and in shorter times as compared to conventional sintering methods. Evaluation of consolidation and divitrification behavior of our materials during spark plasma sintering, with respect to processing parameters such as temperature and pressure, was completed. Boundary structure and coupling between the nanoparticles and the amorphous metal matrix was determined from scanning and transmission electron microscopy. A detailed X-ray diffraction analysis was completed that allowed us to determine, in a very accurate way, the percent crystallinity in these materials as processing parameters were modified. Details of this analysis will be presented. In addition, thermodynamic modeling using FactSage was used for determining the glass forming ability, equilibrium phases, and eutectics, of the alloys as the base compositions were modified. Specifically, the base alloys were modified by the incorporation of additional tungsten as an alloying element.
11:45 AM - NN10.05
Nanomechanics of a Zr-based Bulk Metallic Glass
Jeffrey Martin Wheeler 1 Rejin Raghavan 1 Johann Michler 1
1EMPA - Swiss Federal Laboratories for Materials Science and Technology Thun SwitzerlandShow Abstract
Bulk metallic glasses (BMGs) exhibit high yield strengths akin to ceramics, fracture toughness equivalent to metals and formability like polymers, which are unique combinations of mechanical properties placing them in the desirable regimes of Ashby maps. Plastic deformation in BMGs occurs by the formation of localized ‘shear bands&’, which are observed as offsets or surface steps at small length scales, high strain rates and low temperatures or under constrained loading cases. A transition in the deformation mechanism from heterogeneous shear banding to Newtonian viscous homogeneous flow is observed at low strain rates and elevated temperatures. One excellent methodology to study plastic deformation in brittle or quasi-brittle materials like BMGs is compression of micro-pillars machined by focused ion beams (FIB) in displacement-controlled mode. This has been shown to have superior accuracy over ‘bulk&’ testing for determination of the intrinsic yield stress [1, 2]. Much of the work using this technique has been concerned with a ‘size effect&’ where the yield strength or deformation mechanism is observed to change with decreasing sample size. However, here small scale, in situ testing (micro-pillar compression) of BMGs is used at elevated temperatures to study the change in deformation mechanics and mechanisms as a function of strain rate and temperature in a size range which is representative of bulk behavior . Previous work  has shown that a deviation from macro-scale behavior is observed during micro-scale testing: the stress at which general yielding was observed remained constant at ~2 GPa with temperature until the glass transition temperature, Tg. This is thought to be due to micro-scale testing preventing premature brittle failure. Homogeneous deformation was observed at temperatures above Tg. This suggests that the shear band propagation/reactivation stress for this type of BMG remains invariant with temperature below Tg. This relationship is further explored here in terms of strain rate using constant strain rate and strain rate jump testing. References  Q. Zheng, S. Cheng, J.H. Strader, E. Ma, J. Xu, Scripta Materialia, 56 (2007) 161-164.  Y.-Y. Zhao, E. Ma, J. Xu, Scripta Materialia, 58 (2008) 496-499.  A. Dubach, R. Raghavan, J.F. Löffler, J. Michler, U. Ramamurty, Scripta Materialia, 60 (2009) 567-570.  J.M. Wheeler, R. Raghavan, J. Michler, Scripta Materialia, 67 (2012) 125-128.
12:00 PM - NN10.06
Variation of Hardness and Modulus across the Thickness of Zr-Cu-Al Metallic Glass Ribbons
Zenon H. Melgarejo 1 Joseph Jakes 2 Jinwoo Hwang 1 Eren Kalay 3 Matthew Kramer 3 Paul Voyles 1 4 Donald Stone 1 4
1University of Wisconsin-Madison Madison USA2Forest Product Laboratory Madison USA3Iowa State University Ames USA4University of Wisconsin-Madison Madison USAShow Abstract
The mechanical properties of bulk metallic glass (BMG) ingots are known to vary through the thickness, with hardness and modulus being lowest near the surface of the ingots and highest in the center. Property variation is attributed to differences in free volume caused by differences in rate of cooling, with highest rate of cooling and highest free volume found near the surface. In a melt-spun ribbon the side that contacts the wheel cools quickest, so by analogy with property variation in an ingot, the wheel side of the ribbon should have the lowest hardness and modulus. To test this idea we measure through-thickness hardness and modulus of Zr50Cu45Al5 ribbons (Tg = 676K). Because of their thinness, the ribbons are challenging to measure, so we designed the nanoindentation experiments to remove artifacts caused by ribbon flexing and edge effects. We observe that hardness and modulus vary approximately linearly across the thickness but, unlike bulk ingots, the side of the ribbon that cooled most quickly had the highest hardness and modulus. We propose this “inverse” variation is caused by the fast-moving solidification front, which pushes free volume out in advance of the front. Annealing near Tg causes both properties to increase and become more uniform across the thickness.
12:15 PM - NN10.07
Synthesis and Microstructure of Nanometal-particles-dispersed Amorphous Silica
Yumi H Ikuhara 1 Tomohiro Saito 1 Seiji Takahashi 1 Yukichi Sasaki 1 Tsukasa Hirayama 1
1Japan Fine Ceramics Center Nagoya JapanShow Abstract
Amorphous silica compose of nanopores in their structures where silica tetrahedral networks form connected pores with less than 1nm. The addition of transition metals to silica has been effective to enhance the ability of composite materials such as catalysis, gas separation membrane, or gas adsorbing materials. Gas adsorption sites located predominantly at interfaces between catalytic metal particles and the silica matrix. To increase the number of interfaces, or surface of the catalytic metal and hence adsorption sites, composites need to be prepared in which the metal particles is highly dispersed in the form of nanoparticles in the silica matrix. Because the formation of nanoparticles in the composite is strongly influenced by the synthesis route of the precursor, it is necessary to investigate the many factors affecting particle precipitation so that they can be adequately controlled to produce the optimum homogeneous microstructure for the application. Here, the chemical and microstructural changes that occur in solution-derived precursors during fabrication of Ni-containing amorphous silica was intensively studied A stable metal-organic precursor solution of the ternary Si-Ni-O system was prepared and the transition from the Si-Ni-O precursor to the nanoparticle-Ni-containing amorphous silica powder was investigated. During heat treatment, nickel oxide nanoparticles are found to form simultaneously with the Si-O network, since decomposition of the alkoxy ligand and nucleation of NiO nanoparticles both occur at around 300oC. The formation of ionically-bound Si-Oδ--Niδ+ units plays a key role in anchoring the NiO in the Si-O network and impeding further crystalline growth. Moreover, the formation of the Si-O2--Ni2+ interactions in the composite of dispersed NiO in SiO retard the reduction of NiO, resulting in a characteristic microstructure consisting of highly dispersed metallic Ni nanoparticles within a Si-O matrix.
12:30 PM - NN10.08
A Novel Methodology to Determine the Mechanical Properties of Amorphous Materials through Instrumented Nanoindentation
Marcos Rodriguez 1 Jon M. Molina-Aldareguia 1 Carlos Gonzalez 1 2 Javier LLorca 1 2
1IMDEA Materials Institute Madrid Spain2Polytechnic University of Madrid Madrid SpainShow Abstract
A novel methodology  based on instrumented nanoindentation has been developed to determine the parameters (elastic modulus, compressive yield strength and friction angle) which control the elasto-plastic deformation of amorphous materials with a cohesive-frictional behaviour. The approach is based on the concept of a universal hardness equation for cohesive-frictional materials which results from the assumption of a characteristic indentation pressure which is proportional to the hardness. The actual universal hardness equation was obtained from a detailed finite element analysis of the process of sharp indentation for a very wide range of material properties. Based on this universal hardness equation, the inverse problem (i.e. how to extract the material parameters, namely elastic modulus, compressive yield strength and friction angle from instrumented indentation) was solved, and the applicability and limitations of the new method analyzed. It was found that if the deformation under the indenter is mainly elastic, the pressure sensitivity can only be determined if the yield compressive strength is known in advance. If plasticity controls deformation under the indenter, all the parameters can be extracted from a single pyramidal indentation test, provided the contact area is measured independently (for instance, by atomic force microscopy). If the actual contact area is not measured independently, the constitutive behavior can only be determined if one of the three parameters (elastic modulus, compressive yield strength or friction angle) is known in advance. The method was validated experimentally in a number of amorphous materials (metallic and ceramic glasses as well as polymers) which exhibited a cohesive-frictional behavior and covered a very wide range of elastic and plastic properties.  M. Rodríguez, J. M. Molina-Aldareguía, C. González, J. LLorca.. Acta Materialia, 60, 3953-3964, 2012.
12:45 PM - NN10.09
A Laser-assisted Combinatorial Approach for Designing Metallic Glass Alloys
Peter Tsai 1 2 Katharine M. Flores 2
1The Ohio State University Columbus USA2Washington University in St. Louis St. Louis USAShow Abstract
To date, the identification of metallic glass alloys with high glass-forming ability has been accomplished primarily through trial and error involving conventional melt spinning or casting of individual alloy compositions. This is a costly and time-consuming process, particularly for multi-component systems, motivating the development of a systematic, combinatorial approach to alloy design. Although experimental methods such as sputtering have been reported to create compositionally-graded binary and ternary alloy specimens, the tendency for vitrification via these methods is not representative of glasses produced from an alloy melt. For example, graded specimens of Cu-Zr synthesized via sputtering exhibits glass-formation across nearly the entire range of possible compositions, while the glass-forming range accessible via casting is much more limited. Laser deposition in which the feed-powder composition is varied in situ offers the capability of producing compositionally graded alloys from the melt. In addition to the rapid heating and cooling rates inherent to laser processing, which may be advantageous for identifying even more marginal glass formers, this method is also highly efficient, enabling the rapid evaluation of a wide range of compositions using a relatively small amount of raw material. In this work, compositionally-graded specimens of Cu-Zr and Cu-Zr-Al specimens are produced via a laser deposition process. The microstructures of the graded deposits are characterized via optical microscopy, SEM and TEM in order to identify glassy or partially glassy regions. The glass transition behaviors of glassy and partially glassy compositions are examined via DSC. The local mechanical behavior is characterized via nanoindentation and is correlated with the composition and microstructure. Future efforts to combine this novel experimental methodology with a set of physics-based design criteria will also be discussed.