Jan Schroers Yale University
Ralf Busch Universitaet des Saarlandes
Nobuyuki Nishiyama RIMCOF-Tohoku University Laboratory
Mo Li Georgia Institute of Technology
Z1: Glass Physics and Deformation Mechanisms
Monday PM, November 26, 2007
Room 202 (Hynes)
9:30 AM - **Z1.1
Models for Deformation and Flow Behavior of Metallic Glasses.
William Johnson 1 Show Abstract
1 Dept of Engineering and Applied Science, California Institute of Technology, Pasadenia, California, United States
10:00 AM - **Z1.2
Thermal Properties of Simple Condensed Matter.
Andrew Granato 1 Show Abstract
1 Physics, University of Illinois at Urbana-Champaign , Urbana, Illinois, United States
One of the most distinctive features of condensed matter is the large entropy of melting [1.2 R for most of the elements of the periodic table – Richards rule (1893)]. There are even larger deviations for a few elements that are systematic with the column in the periodic table. Also, the specific heat of liquids and glasses has many characteristic and universal properties. A quantitative and comprehensive account of these properties is given by the interstitialcy theory of condensed matter.
10:30 AM - Z1.3
Anelastic Deformation of an Al-Rich Metallic Glass.
Kotesvararao Rajulapati 1 , Dongchan Jang 1 , Michael Atzmon 1 2 Show Abstract
1 NERS, University of Michigan, Ann Arbor, Michigan, United States, 2 MSE, University of Michigan, Ann Arbor, Michigan, United States
In a metallic glass subjected to low stress, the strain consists of three contributions: elastic, anelastic and viscoplastic. The anelastic strain is recoverable and time dependent, whereas the viscoplastic strain is permanent and time dependent. Previous studies have shown that the viscosity is strongly dependent on the relaxation state. On the other hand, some authors have suggested that the anelastic strain is independent of the relaxation state. In order to characterize the dependence of anelastic strain on the state of a metallic glass, bending experiments have been conducted to monitor stress relaxation in Al86.8Ni3.7Y9.5. These experiments included both macroscopic and submicron bending, where the latter were conducted using a nanoindenter. The time constants observed for anelastic relaxation range from seconds to hundreds of hours. At room temperature, the time-dependent deformation is predominantly anelastic, whereas at higher temperatures, viscoplastic, permanent, deformation is also observed. Combinations of cold rolling and/or annealing below the glass-transition temperature were used to modify the state of the glass. While relaxation anneals prior to bending do not result in significant changes in the subsequent response to bending, cold rolling and subsequent annealing both lead to significant changes. The trends in the dependence of anelastic deformation behavior on the extent of cold rolling will be discussed. The anelastic deformation behavior of the Al-based alloy will be compared with that of other metallic glasses. This work was supported by the National Science Foundation, Grant DMR-0605911.
10:45 AM - Z1.4
The Underlying Mechanisms of the Anelastic to Plastic Transition in Metallic Glass-Forming Liquids.
Marios Demetriou 1 , John Harmon 1 , Annelen Kahl 1 , William Johnson 1 , Joerg Hachenberg 2 , Konrad Samwer 2 Show Abstract
1 Materials Science, California Institute of Technology, Pasadena, California, United States, 2 I. Physik Institute, University of Goettingen, Goettingen Germany
Some of the earliest efforts to describe the mechanics of deformation and flow of metallic glasses and liquids were carried out by Argon . Inspired by the deformation of soap bubble rafts, Argon argued that deformation of metallic glasses and liquids should be accommodated by plastic rearrangements of atomic regions involving tens of atoms, termed shear transformation zones (STZ’s). Argon further recognized that these plastically rearranging regions were not free but confined within an elastic medium , in accordance with the early insightful theories of Eshelby . As known from the work of Johari and Goldstein , the underlying relaxation mechanisms of liquids and glasses are governed by two kinetic processes: a fast process, termed the β process, viewed as a locally initiated and reversible process, and a slow process, termed the α process, viewed as a large scale irreversible rearrangement of the material. From a potential energy landscape perspective, Stillinger and co-workers  have identified the β transitions as stochastically activated hopping events across “sub-basins” confined within the inherent “megabasin”, and the α transitions as irreversible hopping events extending across different landscape megabasins.In this presentation we will discuss the relevance of the underlying α and β liquid relaxation mechanisms to Argon’s concept of “dressed” STZ’s. We will demonstrate that isolated STZ transitions confined within the elastic matrix are associated with the faster β relaxation processes, while the percolation of these transitions leading to the collapse of the confining matrix and breakdown of elasticity are associated with the slower α process. We investigated these processes by studying the configurational properties associated with the transition from anelasticity to plasticity in a transiently deforming metallic glass-forming liquid. Mechanical, calorimetric, ultrasonic, and strain recovery experiments were performed to probe the instantaneous changes in stress, stored potential energy, isoconfigurational shear modulus, and anelastic strain recovery. The data revealed that the transition from anelastic to plastic response can be separated into reversible and irreversible configurational hopping across the liquid energy landscape, identified with the β and α glass relaxation processes, respectively. More interestingly, the critical stress arising from the transition has been recognized as the effective Eshelby “backstress”, providing a direct link between the apparent anelastic to plastic transition and the collapse of the STZ elastic matrix confinement. A. S. Argon, Acta Metall. 27, 47 (1979). A. S. Argon and L. T. Shi, Acta Metall. 31, 499 (1983). J. D. Eshelby, Proc. R. Soc. A 241, 376 (1957). G. P. Johari and M. Goldstein, J. Chem. Phys. 53, 2372 (1970). P. G. Debenedetti and F. H. Stillinger, Nature 410, 259 (2001).
11:30 AM - **Z1.5
Deformation Mechanisms of Metallic Glasses.
Frans Spaepen 1 Show Abstract
1 , Harvard School of Engineering and Applied Sciences, Cambridge, Massachusetts, United States
This talk consists of two parts. (i) A discussion of homogeneous flow in a stress-temperature regime where shear bands can form. Homogeneous flow can be measured under these conditions by supporting thin films of metallic glass on a compliant polymer substrate. This minimizes the effect of shear band and crack formation on the stress-strain measurement. The yield stress and steady-state flow stress are independent of strain rate at room temperature, which can be understood from the strong stress-dependence of both the strain rate and the disordering rate. (ii) A presentation of experiments on colloidal hard-sphere-like glasses deformed in shear. This technique allows all individual particles, as well as the strain tensor, to be tracked in space and time. The shear deformation is heterogeneous. Shear transformation sites can be identified and their activation volume and energy can be determined and compared to experiments on metallic glasses. The stress field around a shear event induces new events in the vicinity. The shear events are thermally activated, as is apparent from an analysis of thermal fluctuations.
12:00 PM - **Z1.6
Theory of Large-Scale Plastic Deformation in Amorphous Materials: A Progress Report.
James Langer 1 Show Abstract
1 , University of California-Santa Barbara, Santa Barbara, California, United States
The goal of recent shear-transformation-zone (STZ) theories has been to construct a phenomenological description of amorphous plasticity that will be based on physical principles and molecular models, and yet be simple enough to be useful in predicting the performance of real materials. In reporting progress toward this goal, I will focus on the dynamic role played by the effective disorder temperature -- a generalization of the free-volume -- in controlling relaxation rates, determining the values of internal state variables such as STZ densities under nonequilibrium conditions, and predicting nonlinear shear-banding instabilities.
12:30 PM - Z1.7
Condition to Initiate Shear Bands around a Stress Concentration in Metallic Glass.
Corinne Packard 1 , Christopher Schuh 1 Show Abstract
1 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Various fundamental aspects of plastic deformation through shear banding in metallic glasses are not well understood, especially the conditions under which a shear band will or will not initiate. In this work, the onset of plastic deformation during spherical contact has been studied in three bulk metallic glasses to garner data about the initiation of shear bands near a stress concentration. Conventional analyses that assume yield is controlled by the maximum stress at a point in the glass are shown to overestimate the true yield stress by a very wide margin. On the other hand, by recognizing that the yield event occurs only when the yield stress is exceeded everywhere along a viable shear path, we can rationalize the measurements in terms of independent measures of the glass yield stress, and predict a shear band trajectory that is consistent with results from slip-line field theory and experimental observations. This result not only offers new insight on the cooperative nature of shear banding in metallic glasses, but also has special relevance to situations involving stress concentrators such as in composite materials or around cracks.
12:45 PM - Z1.8
Measurements of the Dynamics of Shear Bands in Metallic Glasses
Eun Soo Park 1 , Frans Spaepen 1 Show Abstract
1 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States
Although it is generally known that the shear bands behave like fast-moving shear cracks, only little quantitative information has been collected on their dynamics. Furthermore, the structure and density of these shear bands has been studied mostly after the fact. We have used high-speed observation techniques for studying the nucleation and propagation of shear bands in bend test on thin ribbons. Effects of strain rate, thickness, surface condition and embrittling anneals will be reported.
Z2: Structure, Fragility, Relaxation
Monday PM, November 26, 2007
Room 202 (Hynes)
2:30 PM - **Z2.1
Glasses in Single-component and Binary Metal Systems by Experiment, Computer Simulation and Theoretical Modeling.
Austin Angell 1 Show Abstract
1 Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona, United States
3:00 PM - Z2.2
Connection of Physical Aging and the Slow β-relaxation in Metallic Glasses.
Joerg Hachenberg 1 , Dennis Bedorf 1 , Konrad Samwer 1 , Annelen Kahl 2 , Marios Demetriou 2 , William Johnson 2 Show Abstract
1 I. Physikalisches Institut, Univ. Göttingen, Göttingen Germany, 2 Keck Engineering Laboratories, California Institute of Technology, Pasadena, California, United States
In this contribution, evidence is provided that physical aging can be interpreted in terms of a Johari-Goldstein slow β-relaxation.Metallic glasses are commonly used as model systems for glassy dynamics. This is due to the fact that their interactions can be simplified as such of hard spheres. Special attention has been attracted by recent experimental studies  and computer simulations  revealing the existence of a secondary, slow β-relaxation as a universal feature of the glass transition. Here, heat rate dependant mechanical spectroscopy is used to investigate the connection between this β-relaxation and physical aging. The close dependence of both phenomena can be interpreted as a single relaxation showing up on two timescales (heating rate resp. spectroscopy frequency). It is proposed that both phenomena share a common origin.This work is supported by DFG, Graduiertenkolleg 782 and SFB 602, TP B8. J. Hachenberg, K. Samwer, J. Non-Cryst. Sol. 352, 5110 (2006) H. Teichler, Phys. Rev. E 71, 031505 (2005); M. Zink, K. Samwer, W. L. Johnson, and S. G. Mayr, Phys. Rev. B 73, 172203 (2006); M. Zink, K. Samwer, W. L. Johnson, and S. G. Mayr, Phys. Rev. B 74, 012201 (2006)
3:15 PM - Z2.3
Volume and Enthalpy Relaxation in Bulk Metallic Glasses.
Osami Haruyama 1 , Yoshihiko Yokoyama 2 , Akihisa Inoue 2 , Nobuyuki Nishiyama 3 , Takeshi Egami 4 Show Abstract
1 Physics, Tokyo University of Science, Noda, Chiba, Japan, 2 Materials Research, Tohoku University, Sendai, Miyagi, Japan, 3 , RIMCOF, Sendai, Miyagi, Japan, 4 Materials Science and Engineering, Tennessee, KnoxVille, Tennessee, United States
The kinetics of the structural relaxation has been studied extensively since the discovery of bulk metallic glasses (BMG). At sufficiently lower temperatures than calorimetric glass transition temperature Tg, the structural relaxation is proved to be the process with a spectrum of relaxation times. In the present study, we compare with the kinetics of structural relaxation in Pd42.5Cu30Ni7.5P20, Pd40Ni40P20, Pd45.5Cu35.5P19, Zr55Cu30Ni5Al10 and Zr50Cu30Al10 glasses, where the relaxation process was examined by change in density at room temperature and in-situ enthalpy change during relaxation. In addition, several fundamental concepts in free volume theory, such as the validity of the relation, ΔH=kΔv, where ΔH and Δv are the change in enthalpy and volume of glass with the development of relaxation, are discussed on the base of volume and enthalpy relaxation data.
3:30 PM - Z2.4
The Relation Between Structure and Mechanical Properties, Deduced from Studies of Structural Relaxation in Mg-based Glasses.
Jorg Loffler 1 , Alberto Castellero 1 , Dirk Uhlenhaut 1 , Florian Dalla Torre 1 , Nikolay Djourelov 2 , Bernd Schmitt 3 Show Abstract
1 Materials Science, ETH Zurich, Zurich Switzerland, 2 Subatomic and Radiation Physics, Ghent University, Ghent Belgium, 3 Swiss Light Source, Paul Scherrer Institute, Villigen-PSI Switzerland
When Mg–Cu–Y alloys are produced in bulk form they exhibit high compressive strength but no plasticity. In contrast, when they are rapidly quenched to ribbons or splats they show plastic deformation upon bending for a limited time at room temperature but then undergo a ductile-to-brittle transition within a short time. Corresponding to this time-dependent embrittlement, the Differential Scanning Calorimetry (DSC) curves show a reduction in the relaxation enthalpy that is associated with a structural relaxation (i.e. annihilation of free volume) of the metallic glass. This structural relaxation increases the elastic constants (measured via acoustic excitation) as a function of aging time, with the shear modulus increasing faster than the Young's modulus (i.e., the Poisson ratio decreases as the alloy ages). In fact, the alloy reveals the ductile-to-brittle transition at a critical value of 0.32 , corresponding to earlier results obtained for several families of bulk metallic glasses . Here, this value was verified for one individual sample of a single composition and can be directly related to the relaxation process observed by DSC.To characterize the defect structure in the metallic glass in more detail, we performed synchrotron x-ray diffraction and positron annihilation experiments. The radial distribution functions obtained from the synchrotron experiments show an overall reduction in the interatomic distances during aging at room temperature. In turn, the positron annihilation spectra are split into two sample lifetimes, with the longer lifetime (resulting from larger positron traps) disappearing when the ductile-to-brittle transition occurs . We relate this embrittlement to the amount and extent of free volume which alters upon room-temperature aging, and discuss the interrelation between mechanical properties and the defect structure of metallic glasses.References: A. Castellero, D. I. Uhlenhaut, B. Moser, J. F. Löffler, Phil. Mag. Lett. 87 (2007) 383 – 392. J.J. Lewandowski, W.H. Wang and A.L. Greer, Phil. Mag. Lett. 85 (2005) 77 – 87.  D. I. Uhlenhaut et al., 'Structural analysis of amorphous Mg–Cu–Y during room-temperature embrittlement', Phys. Rev. B (submitted).
3:45 PM - Z2.5
Non-Newtonian Viscosity of Zr41.2Ti13.8Cu12.5Ni10Be22.5, Zr57Cu15.4Ni12.6Al10Nb5 and Pd43Ni10Cu27P20 Bulk Metallic Glass Forming Liquids.
Prashant Wadhwa 1 2 , Christopher Way 2 , Jan Schroers 3 , Ralf Busch 2 1 Show Abstract
1 Mechanical Engineering, Oregon State University, Corvallis, Oregon, United States, 2 Lehrstuhl fuer Metallische Werkstoffe, Universität des Saarlandes, Saarbrücken, Saarland, Germany, 3 Mechanical Engineering, Yale University, New Haven, Connecticut, United States
The viscosity of Zr41.2Ti13.8Cu12.5Ni10Be22.5, Zr57Cu15.4Ni12.6Al10Nb5 and Pd43Ni10Cu27P20 has been measured above the liquidus temperature as a function of temperature and shear rate in a high temperature couette rheometer. All these glass formers show much higher viscosities than monoatomic metallic liquids and the non-Newtonian shear thinning behavior. Zr41.2Ti13.8Cu12.5Ni10Be22.5 exhibits a strong shear thinning behavior on shearing from 0.1s-1 to 250s-1 at the temperatures above the liquidus temperature. This non-Newtonian behavior gets weaker with increasing temperature and the material starts to behave like a Newtonian liquid at temperatures above 1225 K. Zr57Cu15.4Ni12.6Al10Nb5 shows a strong non-Newtonian behavior on shearing from 0.1 s-1 to 100 s-1 at the temperatures between 1130 K and 1330 K. This non-Newtonian behavior disappears at higher shear rates where the viscosity stays constant. The melt viscosity of the Pd alloy shows a lower viscosity than the Zr-based alloys. The non-Newtonian behavior of the alloys is characterized by fitting a power law to the viscosity data as a function of shear rate. Both Zr-based alloys show a stronger shear rate dependence of the viscosity than the Pd alloy which is characterized by a significant difference in the shear thinning exponent.
4:30 PM - **Z2.6
Structure and Thermodynamics of Metallic Glasses.
Takeshi Egami 1 2 3 , Valentin Levashov 2 , Rachel Aga 3 , Jamie Morris 1 3 Show Abstract
1 Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee, United States, 2 Physics and Astronomy, University of Tennessee, Knoxville, Tennessee, United States, 3 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
The atomic structure of metallic glasses is usually described by the atomic pair-density function (PDF) which can be directly measured by diffraction experiments. However, it is not easy to relate the information obtained by the PDF to the properties of interest. For instance the PDF of a metallic glass changes with temperature and composition, and with structural relaxation and mechanical deformation. But we do not possess an effective language to discuss the changes in the PDF in relation to the changes in the properties. In this talk we discuss how the peak height and width of the PDF are related to temperature and structural relaxation, and how the anisotropy of the PDF can be induced by mechanical deformation, in terms of the topological fluctuation of atomic connectivity network. More specifically we show how the local topology defines the local energy landscape through the local elastic deformation energy, and how the local elastic energy changes with temperature and the inherent structure that defines the local fictive temperature. We also show how local shear deformation results in the anisotropic PDF and local anelasticity. Thus the structure and the PDF can be described in the language of local structural distortion and its distribution, as they depend on temperature, thermal history and relaxation, elastic and plastic deformation. This approach can be the basis of more rigorous and effective description of the atomic structure of glasses. This work is supported by the Department of Energy through contract DE-AC05-00OR-22725.
5:00 PM - Z2.7
A Topological Basis for Bulk Glass Formation.
Prabhat Gupta 1 , Dan Miracle 2 Show Abstract
1 Department of Materials Science and Engineerging, The Ohio State University, Columbus, Ohio, United States, 2 Materials and Manufacturing Directorate, AF Research Laboratory, Wright-Patterson AFB, Ohio, United States
The structure of metallic glasses is described as a topologically disordered network of bonds between unlike atoms. Insights into the composition bounds for metallic glass formation and the composition dependence of the glass transition temperature (Tg) are derived from the rigidity of this topologically disordered network, which is taken from the competition between the number of internal constraints and degrees of freedom given by metallic (linear) and covalent (angular) atomic bonds. Optimal stability is achieved when the degrees of freedom are equal to the number of internal constraints. Topology is introduced through atomic coordination and from a qualitative assessment of the metallic or covalent nature of bonding between unlike atoms. Decreasing the degree of covalent bonding decreases bond constraints and hence solute potency, so that metallic glasses are formed with higher solute concentrations relative to topologically similar glasses where covalent bonding dominates. This average bond constraint model provides an estimate of Tg as a function of composition. The shift of bulk metallic glass compositions from the nearest eutectic reaction is rationalized in terms of the variation of Tg with composition near the eutectic. The preference for hypo-eutectic or hyper-eutectic glass compositions is correctly predicted by considering the local topology (coordination number) of competing crystalline phases that bound the eutectic composition. A predicted glass-forming composition range is bounded by a minimum solute concentration for glasses where covalent bonding dominates between unlike atoms, and by an upper concentration for systems with mostly metallic bonding between unlike atoms. The lower curve gives quantitative agreement with experiment and matches predictions from the efficient cluster packing model. The upper curve is not a fixed upper limit for the composition of metallic glasses, but rather represents compositions for which the glass stability is expected to be optimal in glasses dominated by metallic bonding.
5:15 PM - Z2.8
An Analysis of Thermophysical and Mechanical Properties of Glass-Forming Alloys.
Livio Battezzati 1 , Daniele Baldissin 1 , Marcello Baricco 1 , Tanya Baser 1 , Donato Firrao 2 , Paolo Matteis 2 , Giovanni Mortarino 2 Show Abstract
1 Dipartimento di Chimica IFM, Università di Torino, Torino Italy, 2 Dipartimento di Scienza dei Materiali ed Ingegneria Chimica, Politecnico di Torino, Torino Italy
Glass-forming undercooled liquids and glasses are ranked through their thermophysical properties (glass transition, extensive quantities, fragility indexes) using recent correlations  and models describing local minima in potential energy landscapes of the material . The relationships between mechanical properties and some of the above quantities are discussed with the aim of getting insight into the mechanism of the early stages of shear band propagation during mechanical failure when, following up a shear offset event, a local temperature rise occurs  and the shear band matures to give a runaway crack . These considerations will be supported by experimental results on hardness and compression testing of Cu- and Pd-based alloys as well as finite element modelling of the shear band. L. Battezzati, Materials Trans., 46 (2005) 2915. G. Ruocco et. al., J. Chem Phys, 120 (2004) 10666. Y. Zhang et. al., J. Mater. Res., 22 (2007) 419. F. Shimizu, Acta Mater., 54 (2006) 4293.Work performed for “Progetto D23, Bando Regionale Ricerca Scientifica Applicata 2004”.
5:30 PM - Z2.9
An Experimental and Theoretical Evaluation of Structural Changes during Glass Relaxation in a Binary Metallic Glass System: Cu1-xZrx.
M. Kramer 1 2 , M. Mendelev 1 2 , D. Sordelet 1 Show Abstract
1 Ames Laboratory, Iowa State University, Ames, Iowa, United States, 2 Materials Science and Engineering, Iowa State University, Ames, Iowa, United States
Most metallic glasses, like many non-metallic amorphous materials, undergo structural relaxation during heating. While such a phenomenon is easily observed as an exothermic signature during thermal analysis, direct observation of the structural changes are limited. Data acquisition rates for most scattering techniques are too short and detailed structural analysis requires multiple scattering experiments to obtain the partial pair distributions functions. We combine both rapid data acquisition using high energy synchrotron radiation and molecular dynamic (MD) simulations to provide a detailed insight into the structural changes which occur during glass relaxation of a series of Cu1-xZrx metallic glasses formed using rapid solidification. Using a fast acquisition area detector, where high quality data can be obtained in a second or less, we observed measurable changes in diffuse scattering around the glass transition for a series of amorphous CuxZr1-x alloys. The rate of change of the position of the first diffuse peak varies by a factor of 2 at temperatures above the glass transition temperature (Tg) for most alloys from 0.33 < x < 0.645. However, changes in either I(Q) or even the corrected S(Q) do not provide any insight as to how the partial-pair correlation functions (PPCF) change during glass relaxation. Fourier transforming these data provides a means of assessing the total-pair correlation function (TPCF), which suggests that the rate at which the atoms in the first shell are moving apart increases near the Tg. Using a new semi-empirical interatomic potential for the Cu-Zr system we simulated the thermal annealing experiments. The simulated S(Q)s from MD simulations employing this new potential agree very well with experimental data and, therefore, provide a means of both simulating the thermal annealing experiments and interrogating the changes in the partial-pair correlations. While the time scales are very different due to computational limitations, the relative agreements with experiments are consistent. The analysis of the PPCFs shows that the Cu-Cu and Cu-Zr separations decreasing relative to the as-quenched state, while the Zr-Zr distances show a small increase. More importantly, the van Hove correlation function shows that the diffusion mechanism dramatically changes around the Tg, consistent with a glass-liquid transition.
5:45 PM - Z2.10
Correlation Between Thermodynamic and Kinetic Properties of Glass-forming Liquids.
Oleg Senkov 1 , Daniel Miracle 2 Show Abstract
1 Materials and Processes Division, UES, Inc., Dayton, Ohio, United States, 2 Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio, United States
Correlations between three characteristic temperatures: glass transition (Tg), Kauzmann (Tk) and Vogel-Fulcher-Tammann (To), and the strength parameter D of super-cooled liquid were identified from the analysis of several different glass-forming liquids. Both Tk and To are functions of Tg and D and can be expressed as To = Tg/(1+D/16ln10) and Tk = Tg/(1+D/16ln10)0.5. The Tk/To ratio is close to 1 for very fragile liquids and it increases parabolically with an increase in D, so that Tk can be well defined as the geometric mean of Tg and To. The temperature dependences of the excess total entropy, configurational entropy, and vibrational entropy of glass-forming liquids were proposed to explain the correlation between Tk and To.
Jan Schroers Yale University
Ralf Busch Universitaet des Saarlandes
Nobuyuki Nishiyama RIMCOF-Tohoku University Laboratory
Mo Li Georgia Institute of Technology
Z3: Shear Localization, Plastic Deformation
Tuesday AM, November 27, 2007
Room 202 (Hynes)
9:30 AM - **Z3.1
Effects of Plastic Deformation on Metallic Glasses.
A. Greer 1 Show Abstract
1 Materials Science & Metallurgy, University of Cambridge, Cambridge United Kingdom
When metallic glasses are plastically deformed at ambient temperature, it is well known that shear is sharply localized into bands some 10 nm thick. During shear, these bands experience extreme conditions of high strain rate and high heating rate, followed by ultra-rapid cooling. After shear, there are residual effects on the material within the bands, including a decreased resistance to shear compared to the bulk, and possible nanocrystallization and void formation. Heavy plastic deformation by such methods as shot-peening of surfaces can induce changes in larger volumes of material and can give benefits through effects such as compressive residual stress. Recent studies of heavily deformed material, in shear bands and in peened layers, will be reviewed. Peened layers have been studied by X-ray diffraction using synchrotron radiation. These studies reveal structural changes (crystallization and amorphization), as well as deviatoric and dilatational strains. Differential scanning calorimetry has been used to estimate the stored energy in peened layers. The results of these studies are correlated with observations on isolated shear bands, and the structural changes are related to the shear conditions.
10:00 AM - **Z3.2
High Ductility in Small-Volume Metallic Glasses.
Evan Ma 1 Show Abstract
1 Materials Sci & Eng, Johns Hopkins University, Baltimore, Maryland, United States
Monolithic metallic glasses (MGs) generally show no tensile ductility at room temperature and are considered quasi-brittle materials. Under compressive loading, severe plastic instability sets in at the onset of plastic deformation, which appears to be exclusively localized in extremely thin shear bands ~10 nm in thickness. Here we discuss the suppression of such catastrophic strain localization and fracture to bring out the intrinsic ductility in MGs. This is achieved in small-volume MGs, taking advantage of sample size effects on deformation mode and fracture, rather than relying on the multiplication of shear bands or nanocrystallization. The radically different deformation behavior for small-volume monolithic Zr-based and Cu-Zr-Al metallic glasses is demonstrated in situ in a TEM, employing samples with dimensions of the order of 100 nanometers. The in situ tensile experiments were carried out in collaboration with H. Guo and M.L. Sui at SYNL, China, and the in situ nanocompression work was a joint project with Z.W. Shan (Hysitron and LBNL) and J. Li (OSU). Large tensile ductility in the range of 23 to 45% was observed, including significant uniform elongation and extensive necking or stable growth of the shear offset. Under compression, MG nanopillars undergo nearly homogeneous plastic flow, sustained by multiple shear events spaced over a small distance of ~100 nm. Progressive growth of shear offset is also observed without cracking. As such, the MGs apparently can plastically deform in a manner similar to their crystalline counterparts. The sample size effect has important implications for the application of MGs in thin films and micro-devices, as well as for understanding the fundamental deformation and fracture mechanisms of amorphous metals.
10:30 AM - Z3.3
Sample Size-Dependent Deformation of Amorphous Metals.
Cynthia Volkert 1 , Alex Donohue 2 , Frans Spaepen 2 Show Abstract
1 Institute for Materials Research II, Forschungszentrum Karlsruhe, Karlsruhe Germany, 2 , Harvard School of Engineering and Applied Sciences, Cambridge, Massachusetts, United States
Compression tests have been performed on micron-sized samples of amorphous Pd80Si20. The compression specimens were cut in the surface of the amorphous metal using a focused ion beam microscope and then compressed using a flat punch in an nanoindenter. It is observed that columns with diameters larger than 400 nm deform by shear band formation, whereas smaller columns undergo homogenous deformation and softening. The transition in deformation mode with sample size is ascribed to a critical strained volume for shear band formation. When the strained volume is too small, the total stored elastic energy becomes too small to sustain formation of a shear band or the stress concentrations ahead of the incipient shear band become too small to allow propagation. The flow stress of the Pd80Si20 is roughly constant for all column diameters. This fact gives insight into the mechanisms for homogeneous and inhomogenous deformation.
10:45 AM - Z3.4
Bulk and Micro-Scale Compressive Properties of a Pd-based Metallic Glass.
Brian Schuster 1 2 , Qiuming Wei 3 , Todd Hufnagel 2 , Kaliat Ramesh 2 Show Abstract
1 Weapons and Materials Research Directorate, US Army Research Laboratory, Aberdeen Proving Ground, Maryland, United States, 2 , Johns Hopkins University, Baltimore, Maryland, United States, 3 , University of North Carolina at Charlottte, Charlotte, North Carolina, United States
In a number of pure single crystal face-centered cubic (fcc) metals, strength is found to be highly dependent on the specimen size in the micrometer range perhaps because the size of the specimen is smaller than the characteristic length scale of dislocation multiplication. In contrast, plastic deformation in metallic glasses is not controlled by dislocation dynamics. One result of this is that yield strengths of bulk metallic glass specimens approach theoretical limits. A second logical consequence would be that metallic glasses should not show a significant effect of specimen size on strength. We present quasi-static, room temperature compression data for Pd40Ni40P20 metallic glasses for both bulk and micro-scale specimens. At all sizes, plastic flow is localized in shear bands which are accompanied by sudden strain bursts in the micro-scale tests. We show a modest (~9%) increase in the 0.2% offset yield strength in going from bulk specimens to the smallest measured (~2 µm), and attribute this increase to the effect of defects on shear band initiation, rather than to an intrinsic length-scale dependence of plastic deformation. In contrast to bulk specimens, there is no indication of melting during the fracture of micro-posts.
11:30 AM - **Z3.5
Nanostructure and Plasticity of Bulk Metallic Glasses.
Kazuhiro Hono 1 , Golden Kumar 1 , Kallol Mondal 1 , Tadakatsu Ohkubo 1 Show Abstract
1 Magnetic Materials Center, Nat'l Inst Mater Sci, Tsukuba Japan
Typical bulk metallic glasses (BMGs) rupture after elastic limit of about 2%. Recently, many BMGs were reported to exhibit plastic deformation in compression above the elastic limit, which are attributed to localized shear deformation by the propagation of multiple shear bands. Several investigations reported the presence of nanocrystalline particles in the BMGs that exhibit large plastic strain, while some monolithic BMGs were also reported to show plasticity. There are two factors that may influence the plasticity of BMGs; one is microstructural factor such as nanocrystals, compositional heterogeneities or dendritically grown crystalline particles and the other is high Poisson ratios. The former is believed to work as pinning or nucleation sites for shear bands and the latter is believed to facilitate the shear band nucleation. In this work, we have investigated the plasticity of various BMGs and their nanostructures with transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP). Based on the systematic experimental results, we discuss the factors influencing the plasticity of Cu-based and Zr-based BMGs.
12:00 PM - **Z3.6
Length Scales in Deformation and Fracture of Amorphous Alloys.
Upadrasta Ramamurty 1 Show Abstract
1 Materials Engineering, Indian Institute of Science, Bangalore, KA, India
Plastic deformation and fracture in crystalline metals are associated with a hierarchy of structural features that can be categorized into different length scales on the basis of the extent of their operation. Connections between these, i.e. how processes at one length scales affects the next higher level one, etc. is well understood – qualitatively, if not quantitatively. In the context of amorphous alloys, while the consensus is emerging on the length scale of various operating mechanisms, the connections between various length scales, is yet to be agreed upon. For example, how the unitary processes of deformation, shear transformation zones, combine to form shear bands -the raison d être for inhomogeneous plasticity in metallic glasses at relatively low temperatures and high stresses – is not clear. At the meso/macro-scopic level, what controls the minimum spacing between shear bands or the sizes of plastic zones at the crack-tips is also an important question that is yet to be sorted out. These are the key issues not only from the scientific stand-point of view, but also from the technological perspective. For example, an excellent correlation was reported for the toughness and plastic zone in amorphous alloys, thus, if one knows ways and means of enhancing the plastic zone size ahead of a crack-tip, it will be possible to design very tough metallic glasses. In this presentation, I shall first review the length scales of various operating micromechanisms in metallic glasses. Then, I will present results of our recent efforts – experiments as well simulations – for understanding the connections between them. Finally, I shall enumerate questions that remain outstanding within the context of fracture of metallic glasses.
12:30 PM - Z3.7
Characteristic Dimensions of Metallic Glass Shear Band Thermal Profiles.
Dan Miracle 1 , Lindsay Greer 2 , Yi Zhang 2 , Alain (Reza) Yavari 3 Show Abstract
1 Materials and Manufacturing Directorate, AF Research Laboratory, Wright-Patterson AFB, Ohio, United States, 2 Department of Materials Science and Metallurgy, Cambridge University, Cambridge United Kingdom, 3 SIMAP-LTPCM, Institut National Polytechnique de Grenoble, Grenoble France
The characteristic dimensions of the thermal profile behind a moving shear band front are provided. Three distinct zones are identified: (1) a band about 10 nm thick and 0.5-2.5 μm wide within which intense mechanical shear produces significant heating; (2) a molten zone that is 0.3-0.6 μm thick and 30-160 μm wide; and (3) a softened volume heated above the glass transition temperature that is 0.7-2.5 μm thick and 140-1300 μm wide. The thicknesses of the heated zones are small, but the widths extend over structurally significant dimensions, so that size scale effects are anticipated for mechanical properties. Important differences exist between the 8 glasses considered here: Cu60Hf10Zr20Ti10, Cu46Zr46Al8, Ce70Al10(Ni10Cu10), (Fe52Cr4Mn10)Er1Mo12(B6C15), La55Al25(Co5Ni5Cu10), Mg65Y10Cu25, Pd40(Ni10Cu30)P20 and Zr41Ti14(Ni10Cu12.5)Be22.5 (Vitreloy 1). The thermal zone characteristic dimensions depend on 7 distinct material properties, but relationships exist between these properties so that the glass transition temperature, heat capacity, yield stress and shear band velocity exert the most important influence. Fracture toughness shows a modest correlation with the dimensions of the glassy zone. Order-of-magnitude estimates suggest that an important amount of deformation may result from shear within the supercooled liquid and molten zones.
12:45 PM - Z3.8
Change in Activation Volume for Plastic Deformation of Zr-based Bulk Metallic Glass following Annealing.
Jonathan Puthoff 1 , Donald Stone 2 1 Show Abstract
1 Materials Science Program, University of Wisconsin - Madison, Madison, Wisconsin, United States, 2 Department of Materials Science and Engineering, University of Wisconsin - Madison, Madison, Wisconsin, United States
We performed room-temperature nanoindentation creep experiments on a Zr54Cu38Al8 bulk metallic glass (BMG) in an effort to measure the scale of the individual deformation events responsible for plastic deformation. From a nanoindentation creep experiment we can determine V*, the activation volume, which we interpret as the volume of the shear transformation zone (STZ) multiplied by the shear strain undergone by the STZ during thermal activation. For the as-cast alloy we measured a hardness, H, of 5.0±0.1 GPa and V* = 0.098±0.007 nm3. Both V* and H are independent of load between 100 μN and 10,000 μN. We then annealed the alloy at 0.85Tg for 24 hr and retested. After annealing no measurable crystallization was detected. Following annealing, H increased to 7.4±0.1 GPa and V* increased to 0.18±0.02 nm3. We interpret the change in V* as arising from an increase in the number of atoms participating in the STZ. The change in V* is commensurate with a change in medium range order (MRO) in the glass, as shown by Hwang and Voyles on the same samples using fluctuation electron microscopy . This work is supported by the NSF under award CMS-0528073. J. Hwang and P.M. Voyles, this symposium; J. Hwang, J. B. Puthoff, H. Cao, Y. A. Chang, D. S. Stone, and P. M. Voyles, to be submitted.
Tuesday PM, November 27, 2007
Room 202 (Hynes)
3:00 PM - **Z4.2
Nanoimprint of Metallic Glasses for Optical Applications and Patterned Media.
Yasunori Saotome 1 , Akihisa Inoue 2 Show Abstract
1 Osaka Center for Industrial Materials Research, Institute for Materials Research, Tohoku University, Sakai, Osaka, Japan, 2 , Tohoku University, Sendai, Miyagi, Japan
In recent years, forming processes have been recognized as one of the most prominent methods for mass producing nanostructures with controlled quality and low cost. This nanoforming process has been called nanoimprinting. Viscous materials, such as thermoplastic materials and oxide glasses or photosensitive polymers, are usually used for the process. These materials are characterized as amorphous-structured materials. Metallic glasses have intrinsically amorphous structures and exhibit Newtonian viscous flow in the supercooled liquid state above glass transition temperature Tg. They are thus expected to be not only one of the most favorable materials for nanoforming or nanoimprinting but also superior structural and functional materials. This paper introduces a technique for fabricating nanostructures such as reflective interference optical components, a diffraction grating (1 micrometers interval), a hologram and a patterned media for high density data storage by superplastic nanoforging of metallic glass with nanoscale dies fabricated by focused ion beam (FIB), Si-process and by Ni-electroforming with master models fabricated by photolithography of the interference pattern(UV-LIGA process). FIB machining characteristics of glassy carbon and Zr-based metallic glass have been studied. FIB nanomachining of amorphous structure material is useful for fabricating nanostructured dies due to the isotropic homogeneity of their amorphous structure. Pt48.75Pd9.75Cu19.5P22 metallic glass was nanoforged using these dies. The thin foil specimens were heated in a small furnace and compressively loaded in a small vacuum chamber. The effects of contact angle between the working material and the die materials on nanoformability have been observed and should be considered in nanoforging or nanoimprinting. Zr-based metallic glass exhibited good die characteristics for superplastic nanoforging of Pt-based metallic glass. Dies and a die-forged 1-micrometer-diameter microgear and both 800- and 400-nanometer periodic nanostructures for optical applications are demonstrated. These fabrication methods are highly efficient for fabricating various nanodevices such as patterned media for high density data storage. The nano-formability and mechanical and magnetic properties of a Fe-based metallic glass are advantageous in the fabrication of patterned media.
3:30 PM - Z4.3
Thermoplastic Forming on the Nano/micro Scale using Bulk Metallic Glass.
Golden Kumar 1 , Philip Taff 1 , Jan Schroers 1 Show Abstract
1 , Yale university, New haven, Connecticut, United States
In this work we demonstrate that thermoplastic forming (TPF) of bulk metallic glass (BMG) can be used as a precision method to net-shape three-dimensional parts on a nano/micro scale. The TPF processing of BMGs is carried out in the supercooled liquid region where the viscosity falls steeply with increasing temperature enabling the BMGs to be processed like plastics. The processing is carried out in air at comparable pressure and temperatures that are used for plastics. A simple flow model suggests that feature sizes down to 10 nm can be replicated using BMGs. It will be shown that three-dimensional microstructures can be created and erased with BMGs using TPF and subsequent processing steps. This unique process allows the fabrication of various complex geometries with a high precision and smooth surface. The smooth surface is achieved in an additional processing step where surface tension alone is utilized to smoothen the roughness that originated form the roughness of the Si mold. The high precision, high surface finish parts can further be used as a die material for other BMGs and polymers with lower softening temperatures.
3:45 PM - Z4.4
Thermoplastic Forming of Nanoscale Trenches in a Zr-based Bulk Metallic Glass.
Brad Kinsey 1 , Kavic Rason 1 Show Abstract
1 Mechanical Engineering, University of New Hampshire, Durham, New Hampshire, United States
Thermoplastic Forming (TPF) of Bulk Metallic Glasses (BMGs) has been demonstrated as a promising manufacturing process for micro and nanoscale components and systems. As feature sizes are reduced to the nanoscale, capillary pressures are a dominant factor in the success or failure of the TPF process. These capillary pressures are a direct result of the wetting behavior that exists at the interface between the high surface tension molten metal and the mold material. In order to investigate this effect, various mold materials were experimented with in an effort to alter the wetting behavior (and subsequent capillary effects) which tends to dominate the process at the nanoscale. Molds of varying sized trench features and drastically different dielectric properties were employed; Silicon (semiconductor), SiO2 (insulator), and SiO2 coated with gold (conductor). For the SiO2 and SiO2 coated with gold molds, the features varied from approximately 90nm to 340nm wide and 400 nm deep. For the Silicon molds, the trench features were approximately 370nm wide and 500nm deep. A Zirconium based BMG (Zr44Ti11Cu10Ni10Be25) was utilized exclusively in this research. The gold coated molds were expected to provide the best TPF performance; however, no improvement in performance was apparent in comparison to the SiO2 molds. This is presumably due to the fact that the applied layer of gold was too thin to mask the grossly anti-wetting interaction between the BMG and the underlying SiO2. For both of these SiO2 mold cases, features smaller than 500nm could not be replicated via TPF at the processing parameters utilized (i.e. 100 MPa of pressure, 450 C, and 35 seconds TPF processing time). This pressure was used as the mold began to fail in certain areas under this applied pressure. The temperature was selected since it is in the middle of the supercooled liquid state, and the processing time was set to 10% of the predicted crystallization time. Silicon molds offered considerably better performance under identical processing conditions. Aspect ratios of 1.25:1 (470nm depth by 370nm width) were achieved with the Silicon molds, which was the limit of the mold features. Future work will investigate alternative mold materials to further modify the wetting behavior between the BMG and mold materials and smaller features with higher aspect ratios will be attempted. Finally, a theoretical model has been developed to predict the achievable aspect ratio for various processing conditions. A key parameter in this model is the surface tension of the BMG in its supercooled liquid state. Values for the surface tension of the Zr-based BMG used in this research at selected temperatures within its supercooled liquid region were inferred from the geometry of experimentally formed features. The surface tension values deduced are, in general, one order of magnitude higher than that documented at the material’s liquidus temperature.
4:30 PM - Z4.5
Microscale Thermoplastic Forming of Bulk Metallic Glasses.
David Henann 1 , Lallit Anand 1 Show Abstract
1 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States
The unique mechanical properties of metallic glasses combined with their intrinsic homogeneity to the nanoscale (due to the absence of grain boundaries) make them ideal materials to replace silicon in the manufacture of nano/micro-scale component for certain MEMS applications. Thermoplastic forming of metallic glasses, in which the material is formed at temperatures above the glass transition, is a promising process for the manufacture of such components. In this work we (i) experimentally study the homogeneous deformation behavior of a Zr-based metallic glass alloy, Vitreloy 1b, at temperatures ranging from slightly below, to well above the glass transition temperature of the material through a series of isothermal compression experiments; (ii) formulate an elastic-viscoplastic constitutive model describing this behavior; and (iii) implement the constitutive model in a three-dimensional finite element program to be used for the simulation of hot thermoplastic forming processes. The constitutive equations appearing in the theory are specialized to the temperature range spanning from 0.9 Tg to well above the glass transition, and in the range of quasi-static strain rates [10e−4, 10e−2] 1/s. The model is shown to capture the major features of the strain-rate and temperature dependent behavior, especially the transition from non-Newtonian to Newtonian behavior, as well as specific features of the stress-strain response, including the phenomena of stress overshoot and strain softening in monotonic compression experiments. The parameters appearing in the model are estimated from the experimental data for Vitreloy 1b. The finite element capability employing the constitutive model is used to study the deformation response of Vitreloy 1b under a representative thermoplastic forming processes, specifically, the manufacture of micro-patterned metallic glass tools to be used for hot-embossing polymeric substrates. Aspects of results from the numerical simulations are compared against corresponding experiments to validate the process simulation capability.
4:45 PM - Z4.6
Thermoplastic Forming of Metallic Glasses: the Case of Injection Molding.
Aaron Wiest 1 , Marios Demetriou 1 , Gang Duan 1 , Landon Wiest 3 , John Harmon 1 , Robert Conner 1 2 , William Johnson 1 Show Abstract
1 Materials Science, California Institute of Technology, Pasadena, California, United States, 3 Chemistry, Brigham Young University, Provo, Utah, United States, 2 MSEM, California State University Northridge, Northridge, California, United States
Owing to their tendency to soften upon relaxation at the glass transition, metallic glasses have long been thought to be capable of being formed thermoplastically using methods similar to those employed in the processing of polymers . Ideally, a plastically processable metallic glass should possess a viscosity at the processing temperature that is low enough to allow forming at pressures attainable via standard thermoplastic processing apparatuses. The viscosities available for plastic processing a metallic glass are bounded above by the glass transition temperature, Tg, and below by the crystallization temperature, Tx. Low processing viscosity is achieved by a combination of large thermal stability, ΔT = Tx-Tg, and a steep viscosity temperature dependence in the vicinity of the glass transition, known as liquid fragility m.Zr-Ti-based Be-bearing metallic glasses previously designed for large glass forming ability  are now optimized for high ΔT and high m in an aim to explore their thermoplastic forming capabilities. New alloy compositions from this family which exhibit ΔT values in excess of 160C and m values as high as 60 will be presented. Viscosities on the order of 10^4 Pa-s are available with a processing time of~100s. A “figure of merit” parameter derived rigorously from accurate rheological laws  suggests that these newly optimized glasses possess a thermoplastic forming capability superior to all metallic glass compositions known to date. Simple constant-heating-rate squishing experiments performed in the supercooled liquid state for various highly processable glasses reveal that the newly developed glasses are indeed capable of undergoing higher strains prior to crystallization, thereby verifying their superior thermoplastic formability.The ability of these newly optimized glasses to be injection molded has been explored. Injection molded parts processed in the temperature region between Tg and Tx will be presented. The mechanical properties of the molded parts were compared to die cast parts of the same dimension and material in three point bending. The rupture modulus of injection molded and die cast specimens was found to be equal to within statistical error, but the standard deviation of rupture modulus for die cast specimens was found to be 3 times greater than for injection molded specimens. Weibull analysis of the data shows injection molded beams to be much more reliable than die cast beams, suggesting that injection molding gives rise to fewer critical defects. This improvement in part quality is attributed to the viscous flow conditions dominating the injection molding process, giving rise to stable and homogeneous flow which minimizes the production of defects and porosity. W. L. Johnson, JOM. 54/3, 40 (2002). A. Peker and W. L. Johnson, APL. 63, 2342 (1993). M. D. Demetriou, J. S. Harmon, M. Tao, G. Duan, K. Samwer and W. L. Johnson, PRL. 97, 065502 (2006).
5:00 PM - Z4.7
Blow-molding with Bulk Metallic Glass.
Jan Schroers 1 , Adam Bouland 1 , David Knox 2 , Robert Fers 3 , David Henann 1 Show Abstract
1 Mechanical Engineering, Yale University, New Haven, Connecticut, United States, 2 Mechanical Engineering, University of Virginia, Charlottesville, Virginia, United States, 3 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States
The low viscosity that can be accessed for a bulk metallic glass (BMG) in its supercooled liquid region permits forming pressures that can be created with the force exerted by the human lung alone. We used this property for blow-molding of BMG. The absence of external friction during the initial stages of forming allows large forming tangential strains. A simple model suggests strains of up to 10,000% are achievable under suitable processing conditions. The influence of the processing parameters such as temperature, time, and pressure on the achievable strain, strain rate, and minimal feature size that can be replicated are investigated. The results are compared with finite element modeling predictions. Expansion is also carried out against a mold. It was found that complex geometries with micron size features can be precisely replicated. The use of blow-molding of BMG as a precision multi-scale net-shape process will be discussed.
5:15 PM - Z4.8
Novel Thermoplastic Joining Processes using Bulk Metallic Glasses.
Boonrat Lohwongwatana 4 , Jin-yoo Suh 1 , Robert Conner 2 , William Johnson 1 , Daewoong Suh 3 Show Abstract
4 Faculty of Engineering, Chulalongkorn University, Bangkok Thailand, 1 Materials Science, California Institute of Technology, Pasadena, California, United States, 2 Manufacturing Systems Engineering and Management, California State University Northridge , Northridge, California, United States, 3 , Intel Corporation, Chandler, Arizona, United States
5:30 PM - Z4.9
Solid State Joining of a Zr-Based Bulk Metallic Glass.
Nicholas Hutchinson 1 , Justin Bennett 1 , Kathy Flores 1 Show Abstract
1 Materials Science and Engineering, The Ohio State University, Columbus, Ohio, United States
Bulk metallic glasses have excellent mechanical properties, including exceptionally high strength, high toughness, and low damping, which make them well suited for structural applications. A significant barrier to expanding the use of metallic glasses is a lack of well characterized manufacturing processes, particularly joining techniques to create large scale or complex shaped components. The present work focuses on the characterization and optimization of a solid state electro-thermomechanical joining technique . Modified half dog-bone specimens are joined using a Gleeble thermo-mechanical test frame, which resistively heats the specimen into the supercooled liquid regime while simultaneously applying a compressive stress. This results in homogeneous flow at the interface to be joined. An advantage of this approach is that the surroundings remain at close to ambient temperature, enabling rapid cooling of the interface when the current is removed. The effectiveness of the joining process and changes in microstructure at the joint are characterized as functions of temperature, applied stress or strain rate, and the initial surface roughness. The results of these experiments will be discussed in light of other potential joining techniques.1.A.R. Yavari, M.F. de Oliveira and W.J. Botta Filho. “Shaping of bulk metallic glasses by simultaneous application of electrical current and low stress”. Supercooled Liquid, Bulk Glassy and Nanocrystalline States of Alloys. 2000. Boston, MA: Materials Research Society.
5:45 PM - Z4.10
Consolidation of Zr- and Hf-based Amorphous Metal Matrix Composites by Equal Channel Angular Extrusion.
Suveen Mathaudhu 1 , Laszlo Kecskes 1 , K. Hartwig 2 Show Abstract
1 , U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland, United States, 2 Mechanical Engineering, Texas A&M University, College Station, Texas, United States
Bulk metallic glasses (BMGs) have displayed impressive mechanical properties, but the use and dimensions of material have been limited due to critical cooling rate requirements and low ductility. The application of severe plastic deformation by equal channel angular extrusion (ECAE) for consolidation of BMGs and metallic glass matrix composites (MGMC) is investigated. BMGs and MGMCs are produced via ECAE consolidation of low-density zirconium or high-density hafnium based BMGs, and BMG powders blended with crystalline powders of W, Cu, or Ni. Novel instrumented extrusions and a host of material characterization techniques were used to evaluate the effect of processing conditions on material properties. The results show that ECAE consolidation at temperatures within the supercooled liquid region gives near fully dense (>99%) and well-bonded millimeter-scale BMGs and MGMCs. The mechanical properties of the ECAE-processed BMG are comparable to properties of those prepared by casting. In contrast, the mechanical properties of ECAE-processed MGMCs are substandard compared to those obtained from melt-infiltrated composites due to non-ideal particle bonding conditions such as surface oxides and crystalline phase morphology and chemistry. It is demonstrated that addition of a dispersed crystalline phase to an amorphous matrix by ECAE powder consolidation increases plasticity of the amorphous matrix by providing locations for generation and/or arrest of adiabatic shear bands. The ability of ECAE to consolidate BMGs and MGMCs with improved plasticity opens the possibility of overcoming size and plasticity limitations of monolithic BMGs.
Z5: Poster Session I
Wednesday AM, November 28, 2007
Exhibition Hall D (Hynes)
9:00 PM - Z5.1
Experimental Observations on the Failure Mechanisms in a Bulk Metallic Glass Composite.
Ashraf Bastawros 1 , Hui Wang 1 , Bulent Biner 3 1 Show Abstract
1 Aerospace Engineering, Iowa State University, Ames, Iowa, United States, 3 , Ames National Laboratory, Ames, Iowa, United States
9:00 PM - Z5.10
Structural Evaluation in Zr15Al5Ni57Y23 Bulk Metallic Glass Under High Pressure.
Gong Li 1 Show Abstract
1 , Yanshan University, Qinhuangdao China
9:00 PM - Z5.11
Bulk Metallic Glass/metallic Glass Composite Prepared by Mechanical Alloying and Vacuum Hot-pressing.
Yu-Wei Lin 1 , Chih-Feng Hsu 1 , Hong- Ming Lin 2 , Pee-Yew Lee 1 Show Abstract
1 Institute of Materials Engineering, National Taiwan Ocean University, Keelung Taiwan, 2 Department of Materials Engineering, Tatung University, Taipei Taiwan
9:00 PM - Z5.12
Microstructure and Mechanical Properties of FeCrMoVC and FeCrMoVNbWC.
Uta Kuehn 1 , Norbert Mattern 1 , Thomas Gemming 1 , Katarzyna Werniewicz 1 3 , Uwe Siegel 1 , Juergen Eckert 1 2 Show Abstract
1 ICM, Leibniz-IFW, Dresden Germany, 3 Faculty of Materials Science and Engineering, Warsaw University of Technology, Warsaw Poland, 2 Institute of Materials Science, TU Dresden , Dresden Germany
We report about phase formation and mechanical behavior of the steel compositions Fe84.3Cr4.3Mo4.6V2.2C4.6 and Fe85.4Cr3.2Mo1.8V2.2Nb1.8W0.9C4.7 subjected to preparation conditions typically used for fabrication of bulk metallic glasses, i.e. relatively high cooling rate and pure chemical components. Thermodynamical aspects and kinetic limitations on the specific solidification process of phase formation, particularly those, which are strongly dominated by diffusion controlled mechanisms, promote the formation of nonequilibrium phases, such as martensite and complex carbide structures already in the as-prepared state. Composition profiles measured by EELS and EDX show that the chemical composition frequently changes within a short distance, indicating the formation of different very finely dispersed phases, which yields material with highly desirable mechanical properties, such as an ultimate engineering compression strength of more than 4000 MPa favourably combined with a fracture strain of more than 10 %.
9:00 PM - Z5.13
Point Defects, Recovery Kinetics and Ordering in Irradiated Bulk Metallic Glasses.
Yuri Petrusenko 1 , Alexandr Bakai 1 , Valeriy Borysenko 1 , Dmitro Barankov 1 , Oleksandr Astakhov 1 , Michael-Peter Macht 2 Show Abstract
1 , National Science Center "Kharkov Institute of Physics & Technology" , Kharkov Ukraine, 2 , Hahn-Meitner-Institut, Berlin Germany
The problem of the structure properties and structure defects of amorphous solids has still a vital importance. E.g., free volume is usually assumed to be a key structural parameter which controls mechanical and diffusion property of metallic glasses (MG) while vacancies are believed to be unstable there, as some computer simulations show. From the other hand, in polycluster amorphous solids vacancies are stable point defects at least within the locally ordered cluster body. To make clear whether stable point defects exist in MG or not, we have study the accumulation and recovery kinetics of radiation defects in ZrTiCuNiBe and ZrTiCuNiAl bulk MGs irradiated with 2.5 MeV electrons at T~80K. The electrical resistance measurements of the irradiated samples were performed.It is revealed that for the MGs of both compositions the radiation-induced resistance ΔR, linearly depends of the dose but for the Be–containing glass this value increases with dose D, while for MG without Be it decreases. These dependencies ΔR(D) are attributed to both stable point defects generation and changes of the short-range order. For the decrease of resistance the short-range ordering is responsible. This effect, as it occurs, is sensitive to the MG composition.The recovery spectrum of irradiation-induced electrical resistance was obtained for the temperature region 85 –300K. The most important result of the recovery experiments is that they clearly show the annealing stages for the irradiated samples. Two annealing peaks located at T~150K and T~225K are resolved for ZrTiCuNiBe glass. Similar peaks are revealed also for ZrTiCuNiAl but in this case the first peak is located at ~130 K and the second one is at 225K too. From these data one can conclude, that the defects mobility is thermally activated process and that the activation energy is not as large as that for vacancies in crystalline alloys. Thus the data obtained testify in favor of the structure with “perfect” local ordering of atoms. It should be noted that this property is a basic in formulation of the polycluster model of amorphous solids.
9:00 PM - Z5.14
Shear Band Evolution in a Bulk Metallic Glass during Cylindrical Indentation.
Antonia Antoniou 1 , Hui Wang 2 , Ashraf Bastawros 2 Show Abstract
1 Center for Integrated Nanotechnologies, Los Alamos National Lab, Los Alamos, New Mexico, United States, 2 Aerospace Engineering , Iowa State University, Ames, Iowa, United States
Cylindrical indentation provides a stable loading path for in situ monitoring of shear band nucleation, propagation and interaction in Vitreloy-1. The deformation zones from various size indenters were found to be self similar. Beneath each indent multiple shear bands form. The observed shear band patterns are found to compare well with traces of plane strain slip lines for a pressure sensitive material. Both the shear band spacing and angle are monitored with loading for the different radii tested. Within the front of the process zone, the shear bands intersect at an included angle of 78°- 80°, indicating a strong pressure sensitivity of the tested BMG. However, another set of bands that form at a later stage intersect at a higher included angle of 87°. The shear band spacing revealed a consistent dependence on the level of the local homogenous deformation. This indicates that the shear band formation is a bifurcation from the homogeneous deformation path to reduce the local strain energy density. Finally, digital image correlation is used to analyze the in-plane strain fields. The amount of homogeneous deformation prior to localization is measured.
9:00 PM - Z5.15
Temperature and Strain-rate Dependence of Deformation Kinetics in Zr-based Bulk Metallic Glasses.
Alban Dubach 1 , Florian Dalla Torre 1 , Joerg Loeffler 1 Show Abstract
1 Laboratory of Metal Physics and Technology, Swiss Federal Institute of Technology (ETH), Zurich Switzerland
Deformation and flow in crystalline materials is nowadays well understood and is generally explained in terms of the underlying dislocation dynamics. Establishing a corresponding formalism for disordered systems, however, is intrinsically more complicated, and is a hotly-debated major research topic. Disordered systems also include metallic glasses, which usually exhibit excellent strength and elasticity, but unfortunately suffer from low ductility at room temperature due to the formation of highly-localized shear bands.In this study, detailed micromechanical analysis of Zr-based bulk metallic glasses, which even at room temperature exhibit considerable compressive strain, was carried out over a wide range of temperatures (77 to 673 K) and strain rates (3×10-5 to 0.3 s-1) [1, 2]. With regard to inhomogeneous flow kinetics, the temporal and spatial characteristics of shearing were investigated in the context of the resulting information and compared with the ex-situ appearance of shear bands on the sample surface (measured by HR-SEM and laser profilometry). Our results show that serrated flow, typically observed at room temperature, disappears below a critical temperature or above a critical strain rate. Although the deformation remains spatially confined, the disappearance of serrated flow correlates with a change in the strain rate sensitivity from negative to positive values, which indicates a change in the micromechanism of flow. Thus, despite the absence of a dislocation-based deformation mechanism amorphous metals show a serrated flow behavior that has close phenomenological similarities with the dynamic strain aging effect known for crystalline metals. Based on our experimental findings we present a constitutive model which describes the deformation behavior according to a thermally activated (cooperative) motion of shear transformation zones and adjacent, diffusive structural relaxation processes in the distorted structure. F. H. Dalla Torre, A. Dubach, M. E. Siegrist, J. F. Löffler, Appl. Phys. Lett. 89, 091918 (2006). A. Dubach, F. H. Dalla Torre, J. F. Löffler, ‘Deformation kinetics in Zr-based bulk metallic glasses and its dependence on temperature and strain rate sensitivity’, Phil. Mag. Lett. (in press).
9:00 PM - Z5.16
Liquid State / Solid State Phase Separation in Metallic Glasses.
Byung Joo Park 1 , Sung Woo Sohn 1 , Hee Tae Jeong 2 , Won Tae Kim 3 , Do Hyang Kim 1 Show Abstract
1 Department of Metallurgical Engineering, Yonsei University, Seoul Korea (the Republic of), 2 BK21 division of humantronics information materials, Yonsei University, Seoul Korea (the Republic of), 3 Applied Science Division, Chongju University, Chongju Korea (the Republic of)
Microstructural evolution during rapid solidification of Ti-Y-Al-Co, Zr-Y-Al-Co metallic glass system has been studied by using transmission electron microscopy. Ti-Y-Al-Co alloys undergo metastable liquid phase separation in the undercooled liquid state and subsequently solidify into two different Y-rich and Ti-rich amorphous phases. The separated structure varies from inter-connected structure to uniformly distributed droplet structure in a major phase depending on the relative volume fraction of each phase. Zr-Y-Al-Co alloy which was monolithic amorphous phase is decomposed into Zr rich and Y rich amorphous phases in solid state by heat treatment under glass transition temperature. The results of the dynamic mechanical measurements shows two types of low temperature relaxation behaviors and each corresponds to the relaxations of monolithic amorphous phase and two, Zr-rich and Y-rich separated amorphous phases, respectively.
9:00 PM - Z5.17
Correlation Between Fragility and Glass Forming Ability of Metallic Alloys.
Oleg Senkov 1 Show Abstract
1 Materials and Processes Division, UES, Inc., Dayton, Ohio, United States
Analysis of the relaxation time of the glass forming liquids at near liquidus temperatures was conducted, correlation between the critical cooling rate for glass formation, fragility of the glass forming liquid and reduced glass transition temperature was identified, and a new glass forming ability (GFA) parameter was proposed. This new GFA parameter, which increases with a decrease in the critical cooling rate, is a function of the reduced glass transition temperature Trg and a fragility index m, and it varies from ~0 in the case of extremely fragile liquid to 2Trg/(1+Trg) in the case of extremely strong liquid. An exponential relationship between the critical cooling rate for glass formation and the new GFA parameter was identified and verified using available experimental data for metallic and non-metallic glasses.
9:00 PM - Z5.18
Effects of Microalloying with 3d Transition Metals on Glass Formation in AlYFe Alloys.
K. Spence 1 3 , A. Gangopadhyay 1 3 , Z. Marine 1 , T. Kim 1 , Anindita Mukhopadhyay 2 3 , A. Goldman 4 5 , William Buhro 2 3 , K. Kelton 1 3 , A. Sadoc 6 Show Abstract
1 Department of Physics, Washington University, St. Louis, Missouri, United States, 3 Center for Materials Innovation, Washington University, St. Louis, Missouri, United States, 2 Department of Chemistry, Washington University, St. Louis, Missouri, United States, 4 , Ames Laboratory, USDOE, Ames, Iowa, United States, 5 Department of Physics and Astronomy, Iowa State University, Ames, Iowa, United States, 6 LPMS, Université de Cergy-Pontoise, Cergy-Pontoise France
We demonstrated recently that while rapidly-quenched ribbons of Al88Y7Fe5 show glass-like x-ray and transmission-electron-microscopy (TEM) diffraction patterns, they neither show a glass transition nor the expected nucleation and growth peak in isothermal DSC studies. However, the substitution of only 0.5 at.% of the Al by Ti produces a rapidly-quenched ribbon with a glass transition temperature and a nucleation and growth peak in isothermal DSC. Further, microalloying with V at a similar composition gives a new metallic glass with among the largest known supercooled liquid regions for Al-based glasses (ΔT = 80 °C). Here, microalloying with any of the 3d transition metal elements is shown to improve glass stability, but to different degrees; the most effective element is Cr. Primary crystallization is to α-Al for microadditions of the early transition metals Ti, V, Cr, and the late transition metals Co, Ni and Cu. For Mn and Fe, an intermetallic phase forms first; the sharp differential scanning calorimetry (DSC) peak indicates that this phase is closer in composition to the original glass. High-q x-ray diffraction studies and EXAFS measurements made on Al88Y7Fe5 and Al87.5Y7Fe5Ti0.5 show that the microaddition induces structural changes in the liquid/glass structure. The induced short-range order (SRO) differs from that of the primary crystallizing α-Al phase, raising the nucleation barrier; the ordering also changes the atomic mobility in the glass. The SRO is likely similar to that of crystallizing metastable intermetallic compound phases that emerge with microalloying. These points are discussed and preliminary TTT diagrams that reflect DSC measurements of the change in devitrification pathway with microalloying are presented. -Supported by the US Air Force Office of Scientific Research under contract FA9550-05-1-0110 and the National Science Foundation under grant DMR-06-06065.
9:00 PM - Z5.19
Rapidly Quenched Al88Y7Fe5 - Glass or Nanocrystal?
L. Longstreth-Spoor 1 2 , N. Mauro 1 , Debajit Saha 1 , M. Miller 3 , K. Kelton 1 2 Show Abstract
1 Physics, Washinton University in St. Louis, St. Louis, Missouri, United States, 2 Center for Material Innovation, Washington University in St. Louis, St. Louis, Missouri, United States, 3 Materials Science and Technology Division, Oak Ridge National Lab, Oak Ridge, Tennessee, United States
The structural nature of Al88Y7Fe5, a widely studied Al-based rapidly-quenched alloy, is unclear. X-ray and transmission electron microscopy (TEM) diffraction patterns from the as-quenched samples are characteristic of an amorphous material, showing no evidence for crystal diffraction peaks; also, no precipitates are observed in the TEM bright-field images. An exothermic peak is observed in nonisothermal differential scanning calorimetry (DSC) measurements, consistent with glass devitrification to α-Al. Our recent measurements of the changes in electrical resistivity with annealing time show a sigmoidal transformation curve that is also characteristic of glass crystallization. Finally, our TEM measurements show an initial increase in the number of nanocrystals of α-Al with increased isothermal annealing time, suggesting crystal nucleation from the glass. However, DSC nonisothermal scans show no evidence for a glass transition and isothermal DSC studies show a monotonic decrease in the rate of heat evolved with annealing time. The isothermal DSC signature is generally taken to reflect grain coarsening, suggesting that the as-quenched alloys are actually amorphous/nanocrystal composites with such a small crystal grain size that it is undetectable in x-ray or TEM studies. Theoretical studies are also inconclusive. When long-range diffusion during nucleation and growth is taken into account, the predicted DSC peak can occur at such short times as to be hidden by the instrumental transient time, mimicking the monotonic character of the DSC isothermal data. While the increase in nanocrystal density would seem to reflect a nucleation process, this can also be obtained from coarsening calculations based on the LSW theory. To resolve this issue, we present local electrode atom probe (LEAP) measurements of rapidly quenched Al88Y7Fe5 alloys. Based on initial LEAP measurements, the as-quenched alloy is not amorphous but nanocrystalline. *Supported by the US Air Force Office of Scientific Research under contract FA9550-05-1-0110 and the National Science Foundation under grant DMR-06-06065.
9:00 PM - Z5.2
Fabrication of Titanate Nano-mesh on (Ti-Zr-Cu-Pd)Ca Bulk Glassy Alloy by Hydrothermal-Electrochemical Technique.
Naota Sugiyama 1 , HaiYan Xu 1 , Tomoaki Watanabe 1 , Nobuhiro Matsushita 1 , Xinmin Wang 2 , Mikio Fukuhara 2 , Akihisa Inoue 2 , Tsukamoto Masahiro 3 , Nobuyuki Abe 3 , Yuichi Komizo 3 , Takamasa Onoki 1 , Yasuto Hoshikawa 1 , Masahiro Yoshimura 1 Show Abstract
1 Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama Japan, 2 Institute of Materials Research, Tohoku University, Sendai Japan, 3 Joining and Welding Research Institute, Osaka University, Osaka Japan
A series of Ti-based bulk glassy alloys have been extensively studied in recent years due to their excellent mechanical properties like low Young’s modulus and high elastic limit. It is however known that those alloys contain some toxic elements, such as Ni, Al and Be that provide high glass-forming ability. The presence of those toxic elements may restrict their biomedical applications like load bearing dental and orthopedic implants. Recently, Wang et al. have developed Ti-based bulk glassy alloys without any toxic elements1). Those alloys, however, may not be joined directly to human bone because of their high stability and bio-inertness. To enhance the bioactivity ,i.e. bone bonding ability of Ti based implants, various coating techniques have been developed such as plasma spray process, CVD, and micro-arc oxidation. However, those synthetic techniques requires high temperature and/or high vacuum that may cause not only cracks in the fabricated films during cooling process, but also the degradation of excellent mechanical properties of bulk glassy alloys. We have, therefore, developped the hydrothermal-electrochemical method as a low temperature and crack free method to prepare titanate coating on Ti metal and Ti alloy . This method is believed one of the most suitable one for the preparation of bioactive ceramics coatings because no toxic species are employed even in the solution in the process. In the present study, we fabricated bio-active titanate layer on (Ti-Zr-Cu-Pd)Ca bulk glassy alloy, which have excellent mechanical properties, to give the biocompatibility such as appetite-inducing ability. Titanate layers were prepared on these specimens by the hydrothermal-electrochemical treatment at 363K for 2 hours in aqueous solutions of NaOH (1-5N) as an electrolyte. A constant electric current of 0.5 mA/cm2 was applied between (Ti-Zr-Cu-Pd)Ca as an anode (dimension of 10×5×0.07 mm3) and Pt electrode as a cathode. After hydrothermal-electrochemical treatments, the specimens were removed from the aqueous solution, washed with distilled water and then dried. The phase of the produced layers were analyzed by X-Ray diffraction and Raman spectroscopy. The surface morphology of the produced layers was observed by scanning electron microscopy. These evaluations revealed that titanate nano-mesh were fabricated on (Ti-Zr-Cu-Pd)Ca bulk metallic glass. The nano-mesh layer consisted of nano-wires of 20 nm in diameter. Thickness of titanate nano-mesh from 150 nm to 1 um with increasing NaOH concentration in the solution. The immersion test in SBF (simulated body fluid) revealed that the titanate nano-mesh induced the growth of bone like hydroxyapatite confirming the bioactivity of the titanate Nano-mesh prepared on the (Ti-Zr-Cu-Pd)Ca alloy. This study will open a new era for non-toxic bulk glassy alloys (Ti-Zr-Cu-Pd)Ca for medical application by giving bioactivities to bulk metallic glass(s).1) MATERIALS TRANSACTIONS 48 (3): 515-518 MAR 2007
9:00 PM - Z5.20
Phase Separation in Amorphous Metallic Alloys.
Norbert Mattern 1 , Guenter Goerigk 1 , Juergen Eckert 1 Show Abstract
1 , IFW Dresden, Dresden Germany
Phase separation in metallic glasses occurs in alloy systems consisting of element combinations having strong positive enthalpy of mixing. These heterogeneities by the addition of corresponding elements may increase the ductility of bulk metallic glasses. Here, we report the influence of additions of Y and Gd on the microstructure and thermal stability of amorphous Cu-Zr alloys. The structure was analyzed by high energy X-ray diffraction, anomalous small angle X-ray scattering (ASAXS) and transmission electron microscopy. The ASAXS results will be compared with corresponding measurements of phase separated amorphous Ni-Nb-Y alloys. The role of thermodynamic properties on phase separation of metallic glasses will be discussed.
9:00 PM - Z5.21
Electronic Structure and Stability of the Pd-Ni-Cu-P Metallic Glasses.
Daisuke Fukamaki 2 , Tsunehiro Takeuchi 7 1 2 , Kazuo Soda 3 , Masashi Hasegawa 4 , Uichiro Mizutani 5 , Hirokazu Sato 6 Show Abstract
2 Applied Physics, Nagoya University, Nagoya, Aichi, Japan, 7 Eco Topia Science, Nagoya University, Nagoya, Aichi, Japan, 1 Crystalline Materials Science, Nagoya University, Nagoya, Aichi, Japan, 3 Quantum Engineering, Nagoya University, Nagoya, Aichi, Japan, 4 Materials Science and Engineering, Nagoya University, Nagoya, Aichi, Japan, 5 , Toyota Physical and Chemical Research, Nagakute, Aichi, Japan, 6 Physics, Aichi University of Education, Kariya, Aichi, Japan
Trigonal prism clusters with a phosphorus atom in the center are known to exist in Pd-Ni-P metallic glass as a unique structure unit. In the relevant crystals stabilizing in the vicinity of the composition where the metallic glass is obtained, the local atomic arrangements about a phosphorus atom are characterized by the trigonal prism cluster in the same manner as those in the corresponding metallic glasses. By calculating cluster levels, we found that all the electrons in the (Pd, Ni)9P trigonal cluster fill the bonding and non-bonding states, while the anti-bonding states remain completely vacant. This condition leads to highly stable nature of the trigonal prism cluster to play the role of the structure unit of metallic glass and the relevant crystals. One may expect that a partial substitution of Cu for Ni in the Pd-Ni-P metallic glass would increase the Fermi level towards higher energies where the anti-bonding states exist and destabilize the metallic glasses consisting of the trigonal prism clusters. Surprisingly, however, the Pd42.5Ni7.5Cu30P20 metallic glass, which is obtained by the partial substitution of Cu for Ni in the Pd42.5Ni37.5P20 metallic glass, becomes more stable to be widely known as one of the most stable metallic glasses ever discovered. In order to reveal the stabilization mechanism of the Pd42.5Ni7.5Cu30P20 metallic glass, we investigated the electronic structure and the local atomic arrangements by making full use of the relevant crystals, such as Ni2P, Cu2P, and Co2P. We found, as a consequence of the structure analyses on the relevant crystals, that the Cu3P has a characteristic local atomic structure that is fairly different from the ordinary trigonal prism cluster in the Pd-Ni-P metallic glasses. This fact suggests that the Cu atoms provide a variety in the local atomic arrangements in the Pd42.5Ni7.5Cu30P20 metallic glass. It is, therefore, argued that even though the internal energy is slightly increased by the increased electron concentration induced by Cu, the large variety in the local atomic arrangements would reduce the free-energy of metallic glass by significantly increasing the entropy. In the presentation, possible variations in cluster-shapes and inter-cluster connections in the Pd42.5Ni7.5Cu30P20 metallic glass are discussed in terms of electronic structure.
9:00 PM - Z5.22
Micro-scale Medical Device Components from Metallic Glasses.
Cormac Byrne 1 , John Wert 1 , Morten Eldrup 1 , Thorbjorn Andersen 1 , Allan Schroeder Pedersen 1 Show Abstract
1 Materials Research, Risø National Laboratory, Roskilde Denmark
Much of the work on BMGs over the past decade has focused on identifying good glass-forming alloys which allow the production of sections with increasingly larger dimensions. Scaling down of nominally sized BMGs to produce functional components with features at length scales which cannot be easily obtained using crystalline metals, also has commercial appeal. Risø National Laboratory collaborates with several Danish companies and research institutes in a consortium known as MIKROMETAL, which aims to produce functional, micro-scale, medical device components. The consortium’s work encompasses all aspect of product design, from material selection through production process design. An example of the use of these components is in hearing aids. At Risø we focus on the shaping a small number of metallic glass alloys into thin walled sections, tubes, coils and springs. Techniques include pressing, extrusion, surface patterning and creep-forming in the supercooled liquid regime. The challenges involved in understanding the processing science, while effectively delivering prototype components to the industrial partners, will be discussed. Results of mechanical testing and processing studies, particularly related to the ductile-brittle transition during forming of components in the supercooled liquid regime, will be shown.
9:00 PM - Z5.3
Deformation Behavior and the Structural Changes of a Zr-based Bulk Metallic Glass.
Ashwini Bharathula 1 , Benjamin Peterson 1 , Katharine Flores 1 Show Abstract
1 Materials Science and Engineering, Ohio State University, Columbus, Ohio, United States
Bulk metallic glasses have significant potential as structural materials due to their exceptional mechanical properties. However, accelerating the evolution of these alloys from concept to application demands a detailed understanding of the relationship between the glass structure and deformation mechanisms. In the present work, we examine the structural changes associated with homogeneous deformation of a Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 (nominal at%) bulk metallic glass. An electro-thermomechanical test frame takes advantage of Joule heating to rapidly heat and cool the specimen, and is used to characterize the flow behavior over a range of temperatures (300-450oC) and strain rates (0.0001 to 100 s-1). The deformed specimens and failure surfaces are examined using high resolution SEM, and changes to the glass structure associated with homogeneous flow are characterized via DSC and TEM. In addition to the conventional TEM foils, the use of the Focused Ion Beam (FIB) to obtain site-specific foils is investigated. Experimental homogeneous flow results for this complex alloy will be discussed in light of a related molecular dynamics study of deformation in a simpler binary glass.
9:00 PM - Z5.4
Nanometer-scale Structural Relaxation in Zr-based Bulk Metallic Glass.
Jinwoo Hwang 1 , Paul Voyles 1 Show Abstract
1 Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States
We have studied the nanometer-scale structure and relaxation of Zr54Cu38Al8 bulk metallic glass (BMG) using fluctuation electron microscopy (FEM). FEM measures diffraction from medium range order (MRO) in glassy materials using nanometer-resolution dark field imaging in a TEM. Our FEM experiments show that the as-cast BMG contains significant MRO at a length scale of 1.5 nm. That MRO was reduced by structural relaxation induced by annealing at 0.85Tg for 24 hours, meaning that the structure became more homogeneous at the nanometer scale. There was no measurable relaxation-induced change in short range order observed by electron diffraction. These results imply that structural relaxation involves rearrangement of atoms at nanometer length scales, not just annihilation of atom-sized free volume. BMG structural models involving various MRO clusters derived by hand and from reverse Monte Carlo modeling will be discussed. On these same samples, Puthoff and Stone have found a large change in the activation volume of plastic deformation using variable-rate nanoindentation , which may be connected to the change in MRO. This work is support by the NSF under award CMS-0528073. J. Puthoff and D. S. Stone, this symposium; J. Hwang, J. Puthoff, H. Cao, Y. A. Chang, D. S. Stone, and P. M. Voyles, to be submitted.
9:00 PM - Z5.5
Medium-range Order in Zr-based Metallic Glasses Probed by Matching Fluctuation Electron Microscopy to Simulations from Pheonomenological Atomic Models.
Amelia Liu 1 , Shashishekara Adiga 1 , Dean Miller 1 , Daniel Sordelet 2 Show Abstract
1 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States, 2 Materials and Engineering Physics, Ames Laboratory, Ames, Iowa, United States
The character of the medium-range atomic order (MRO) in binary Zr-based melt-spun glasses is investigated using fluctuation electron microscopy (FEM) and simulations generated using phenomenological atomic models. Such simple two-component glasses present ideal systems for identifying structures in metallic glasses that may influence the properties and behaviors of more stable bulk compositions. For example, initial devitrification to the metastable quasi-crystalline (I) phase or to the equilibrium phase is a sensitive function of the method of preparation and the exact glass composition. FEM measures the variance in the scattered electron intensity, V, as a function of the wavevector, k, and is sensitive to MRO. The size and position of features in the V(k) graph gives a strong fingerprint of the degree and character of the MRO present in the amorphous material. Here we simulate the V(k) from small atomic clusters using an extension of the Debye scattering equation and compare such simulations to experimental curves from melt-spun Zr70Pd30. We simulate the V(k) from the Zr-centered icosahedron and the Pd-centred tri-capped trigonal prism that have been identified from reverse Monte Carlo fits to electron diffraction data as principle components of the short-medium range order in the as-spun glass. Such coordination polyhedra arise due to the large undercooling of the material and also the large negative heats of mixing between the constituents, respectively. We also simulate the V(k) from small (1.2 nm) clusters containing the order of the equilibrium C11b phase of Zr2Pd and the “big cube” NiTi2 structure. The “big cube” structure fulfills some of the requirements to be an approximant to the I-phase for this system as it contains some 12-fold coordinated clusters, but lacks the overall icosahedral symmetry of the actual I-phase.Our study suggests that the isolated coordination polyhedra do not account for all the atomic correlations detected in the material from FEM. There is also little evidence for underlying order of the equilibrium C11b phase. The simulated V(k) from the “big cube” structure provides a better match to the experimental curve. This may be evidence for pre-cursor structures to the I-phase quenched in from the melt, providing a structural basis for the initial crystallization to the I-phase. D. J. Sordelet, R. T. Ott, M. Z. Li, S. Y. Wang, C. Z. Wang, M. F. Besser, A. C. Y. Liu and M. J. Kramer. Met. Mat. Trans. A. submitted M. M. J. Treacy, J. M. Gibson, L. Fan, D. J. Paterson and I. McNulty. Rep. Prog. Phys., 68, 2899, (2005) W. E. McBride, D. R. McKenzie, D. G. McCulloch, D. J. H. Cockayne and T. C. Petersen. J. Non-Cryst. Solids, 52, 257, (2005) T. Takagi, T. Ohkubu, Y. Hirotsu, B. S. Murty, K. Hono and D. Shindo. Appl. Phys. Lett. 79, 485, (2001) J. R. Morris, M. Xu, Y. Y. Ye, D. J. Sordelet and M. J. Kramer. Acta Mat. submitted
9:00 PM - Z5.6
Stress Gradient Induced Compressive Plasticity in a Malleable Bulk Metallic Glass.
WenFei Wu 1 , ChunYu Zhang 1 , YongWei Zhang 1 , Yi Li 1 Show Abstract
1 Department of Materials Science and Engineering, National University of Singapore, Singapore Singapore
The authors report a strong geometry dependence of compressive plasticity in a malleable Zr-based bulk metallic glass (BMG). With normal orthogonal geometry, the Zr-based BMG typically fractured with a plastic strain less than 2%. However, by slightly modifying the sample geometry, an apparent large compressive plasticity (>10%) was achieved. Our further finite element analysis (FEA) based on the free volume model has shown a good agreement with experimental observations. It has been revealed that the subtle geometry differences in the BMG sample changed the stress distribution and therefore, the evolution of shear bands. With large stress gradient, the nucleation of shear bands was highly localized and the subsequent propagation of shear bands was constrained thus forcing multiple shear bands