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)