Amit Misra Los Alamos National Laboratory
T. John Balk University of Kentucky
Hanchen Huang Rensselaer Polytechnic Institute
Maria Jose Caturla Universitat d'Alacant
Christoph Eberl University of Karlsruhe
EE1/NN1: Joint Session: In-situ Nanomechanics
Monday PM, December 01, 2008
Room 200 (Hynes)
9:00 AM - EE1.1/NN1.1
SEM In situ Compression of Silicon Nanowires.
William Mook 1 , Rudy Ghisleni 1 , Karolina Rzepiejewska-Malyska 1 , Samuel Hoffmann 1 , Laetitia Philippe 1 , Johann Michler 1 Show Abstract
1 , Swiss Federal Laboratories for Materials Testing and Research (EMPA), Thun Switzerland
Silicon nanowires have created a great deal of interest for over a decade due to their enhanced electrical properties when compared to bulk values. These structures also show size-dependent mechanical properties and since they can determine the reliability of many nanodevices and nanosystems, it necessary to quantify their elastic and plastic mechanical response to externally applied loads. This, however, is experimentally challenging due to the length scales involved. Therefore uniaxial compression experiments have been performed in situ with a high resolution scanning electron microscope (SEM) on single crystal silicon nanowires with diameters ranging from 100 nm to 1 μm. A flat-punch indenter can be positioned above the structure of interest with nanometer precision without modifying it due to deformation from mechanical scanning. Compressions are then run under displacement-control at 1 nm/s. By observing the deformation and contact area throughout the experiment engineering stress-strain curves can be extracted from the load-displacement data. Nanowire compressive strength is compared to silicon micropillar compression experiments and to silicon wires in bending.
9:15 AM - EE1.2/NN1.2
SEM In situ Micropillar Compression - Room Temperature Ductile to Brittle Transition of Si, GaAs, InP Semiconductors.
Johann Michler 1 , Fredrik Oestlund 1 , Karolina Rzepiejewska-Malyska 1 , William Mook 1 , Klaus Leifer 2 , Rudy Ghisleni 1 Show Abstract
1 , Swiss Federal Laboratories for Materials Testing and Research (EMPA), Thun Switzerland, 2 Electron Microscopy and Nanoengineering, Department of Engineering Science, Uppsala University, Uppsala Sweden
Current fabrication technology is capable of producing micro- to nano-meter scale structures. The mechanical response of such structures has been shown to depend upon length scales such as pillar diameter. These findings contradict the classical laws of mechanics which assume that mechanical properties are independent of sample size. This contradiction has fostered an increasing number of investigations into mechanical size effects in order to accurately design and fabricate devices at these scales. In an effort to characterize and understand the mechanical behaviour dependence on the size, an investigation on single crystal semiconductor micropillars is presented. Single crystal silicon, gallium arsenide, and indium phosphide micropillars were fabricated by a focused ion beam (FIB) technique. The diameter of the pillars ranged from 200 nm to 10 μm with a length to diameter aspect ratio of three. The micropillars’ mechanical response was investigated by uniaxial compression tests performed with a diamond flat punch using an in situ SEM nanoindenter instrument.Engineering stress-strain curves as a function of pillar diameter are presented. The results show that all the investigated semiconductor materials exhibited a brittle to ductile transition with a decrease in pillar diameter. The deformation mechanism that is responsible for the plasticity is shown to be the formation of Shockley partial dislocations. The decrease of the projected pillar diameter on the crystal slip plane below the equilibrium distance (proportional to the stacking fault energy) between the leading and trailing partial dislocation controls the transition from a brittle to ductile behavior.
9:30 AM - EE1.3/NN1.3
In-situ Investigation of Nano-scale Plasticity in Cubic and Tetragonal Crystals via Homogeneous Deformation of Nano-Pillars.
Julia Greer 1 , Ju-Young Kim 1 , Steffen Brinckmann 1 Show Abstract
1 Materials Science, California Institute of Technology, Pasadena, California, United States
Strength of crystalline materials at reduced dimensions is important for fabrication and reliability of devices at nanometer scales such as MEMS and NEMS, bio-cell sensors, and fuel cells. Plastic flow stress of crystals, a size-independent property for bulk, is found to strongly depend on sample size as it is reduced to nano-scale. To investigate plasticity under homogeneous deformation, we have developed an in-situ micro-deformation methodology, where nano-pillars are mechanically deformed in a one-of-a-kind instrument, SEMentor, which merges the strengths of SEM and Nanoindenter, and offers the advantage of measuring mechanical response of nano-scale materials while capturing video frames throughout the deformation process. We present for the first time results of compression and tension tests performed in-situ inside SEMentor, where load-displacement data is correlated with real-time slip step formation on the surface of the deforming specimens. We perform a new robust technique for stress-strain calculation based on load-displacement data. We also report mechanical strengths of uniaxially-deformed single crystalline nano-pillars with different crystallographic structures (Au, Al, In, Mo) and compare them with one another. Our experiments demonstrate pronounced differences in the behavior of individual structures, and possibly plasticity mechanisms are discussed. We find that although all crystals show an increase in flow stress with decreasing diameter in a power-law fashion, the slopes of these size effects vary with the material, and the ratio between the observed maximum flow stress and the theoretical strength vary significantly.
9:45 AM - EE1.4/NN1.4
Quantitative In Situ Tensile Testing of 1D Nanostructures.
Daniel Gianola 1 , Reiner Moenig 1 , Oliver Kraft 1 2 , Cynthia Volkert 3 Show Abstract
1 Institute for Materials Research II, Forschungszentrum Karlsruhe, Karlsruhe Germany, 2 , University of Karlsruhe, Karlsruhe Germany, 3 Institut für Materialphysik, Universität Göttingen, Göttingen Germany
Plasticity in extremely small volumes is fundamentally different than in large materials; the law of averages gives way to discrete processes that dominate the response. Probing the mechanical response and uncovering the underlying deformation mechanisms of diminishingly small structures at the micro- and nanoscale requires new strategies and approaches that circumvent difficulties associated with handling, gripping, loading, and measuring small specimens. The need for in situ experiments that give a one-to-one correlation between mechanical response and deformation morphology is exacerbated by the fact that electron optics are needed to image and manipulate nanostructures. Tensile experiments are the preferred modality at larger scales since they apply a homogeneous stress state and are less sensitive to boundary conditions, easing interpretation. Meanwhile, results obtained using the popularly employed techniques at the nanoscale (e.g. nanoindentation, micro-compression testing) are clouded by these unresolved issues. Here we describe quantitative in situ tensile experiments on 1D nanostructures in a dual-beam scanning electron microscope (SEM) and focused ion beam (FIB). Specimen manipulation, transfer, and alignment are performed using an in situ manipulator, independently-controlled positioners, and the FIB. Gripping of specimens is achieved using electron-beam assisted Pt deposition. Local strain measurements are obtained using digital image correlation of SEM images taken during testing. Examples showing results for single-crystalline metallic nanowires and nanowhiskers, having diameters between 30 and 300 nm, will be presented in the context of size effects on mechanical behavior, the theoretical strength of crystals, and the influence of defects on the accommodation of plasticity in small volumes.
10:00 AM - **EE1.5/NN1.5
Observation of Size-Dependent Plasticity by In Situ SEM and TEM.
Gerhard Dehm 1 2 Show Abstract
1 Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Leoben Austria, 2 Materials Physics, University of Leoben, Leoben Austria
The continuing trend of miniaturizing materials in many modern technological applications has led to a strong demand for understanding the complex mechanical properties of materials at small length scales. This talk focuses on the recent understanding of the size-dependent plasticity in face-centered cubic metals with dimensions of several microns down to some tens of nanometers. At that length scale sophisticated measurement approaches are required with the advantage of in situ microscopy techniques providing both, control of the deformation experiment and insight in the underlying deformation mechanisms. Size effects of the flow stresses are compared for “wires” and thin films on compliant or stiff substrates. The interpretation of the results is based on recent insights on dislocation nucleation, glide band formation, and dislocation mobility stemming from in-situ SEM and TEM studies of single-crystalline and polycrystalline samples. The results are discussed with the attempt to explain the size effects in straining experiments at small length scales.Acknowledgement: Significant contributions from D. Kiener, R. Pippan, S.H. Oh (Leoben), M. Legros (Toulouse), and P. Gruber (Stuttgart) are acknowledged.
10:30 AM - EE1/NN1:joint
11:00 AM - EE1.6/NN1.6
In Situ Examination of Nanoscale Deformation of Thin Film Bridges within a Scanning Electron Microscope.
Erik Herbert 1 , Arnold Lumsdaine 1 , R. Brian Peters 1 , Warren Oliver 1 Show Abstract
1 , Agilent Technologies, Oak Ridge, Tennessee, United States
Using a high precision nanoindentation head, a new technique has recently been developed to determine the elastic modulus and residual stress of a free-standing doubly-clamped thin film bridge . A desire to examine the impact of certain anomalies in the experimental results (possibly occurring due to misalignment of the tip of the nanoindentation head with the surface of the bridge or due to adhesion of the tip with the surface of the bridge) motivates an examination of the experiment within a scanning electron microscope (SEM). A linear feedthrough mechanism has been developed to position the indentation head within the SEM chamber for precise targeting of the thin film bridge sample (placed on the SEM sample stage). This configuration also allows for the examination of the multi-dimensional deformation state of the bridge when the indentation head contact occurs offset from the center of the bridge.E.G. Herbert, W.C. Oliver, M. P. de Boer, and G.M. Pharr, “Measuring the Elastic Modulus and Residual Stress of Free-Standing Thin Film Bridges by Nanoindentation,” 2007 MRS Spring Meeting, 2007.
11:15 AM - **EE1.7/NN1.7
A New Perspective on Nano-Mechanics: Quantitative Deformation Test in the TEM
Zhiwei Shan 1 , A. Minor 2 3 , J. Nowak 1 , S. Syed Asif 1 , O. Warren 1 Show Abstract
1 , Hysitron Inc., Eden Prairie, Minnesota, United States, 2 , Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 , University of California , Berkeley, California, United States
It is often a challenge to measure accurately the mechanical properties of nanostructures and nanomaterials on account of their extremely small physical dimensions. Recently, we have developed an in situ TEM nanomechanical testing apparatus. This device enables one to acquire quantitative mechanical data while simultaneously recording the microstructural evolution of the materials during deformation, developing a one-to-one relationship between an imposed stress and an individual deformation event. In this talk, we report on the current progress in the application of this in situ TEM device for measuring the mechanical behavior of individual single crystal nickel and metallic glass (MG) pillars. Prior to the deformation tests, the Focused Ion Beam (FIB) fabricated nickel pillars were observed to contain a high density of defects. However, quite unexpectedly, the defects density was observed to decrease dramatically during the deformation process and, in some cases, even resulted in a dislocation-free crystal. The phenomena which we termed as “mechanical annealing” is the first direct observation of the dislocation starvation mechanism and sheds new light on the unusual mechanical properties associated with submicron- and nano- scale structures (Shan et al, Nature Materials, 2008). The compression tests on Cu-Zr-Al MG pillars revealed the intrinsic ability of fully glassy MGs to sustain large plastic strains, which would otherwise be preempted by catastrophic instability in macroscopic samples and conventional tests (Shan et al, PRB, 2008).
11:45 AM - EE1.8/NN1.8
In- Situ Observation of Deformation Characteristics in Nanotwinned Copper Pillars.
Vinay Sriram 1 , Jia Ye 2 3 , Andrew Minor 2 3 , Jenn-Ming Yang 1 Show Abstract
1 Department of Materials Science and Engineering, University of California Los Angeles, Los Angeles, California, United States, 2 National Centre for Electron Microscopy, Lawrence Berkeley National Lab, Berkeley, California, United States, 3 Department of Materials Science and Engineering, University of California Berkeley, Berkeley, California, United States
The semiconductor industry is currently moving towards integration of “air gap” technology beyond the 32nm node. A major concern in the introduction of air gap technology is the mechanical integrity, reliability and stability of the vias and interconnects line structures. A new method based on In-situ Transmission Electron Microscopy nanocompression testing of copper pillars which are of the same size scale of vias, interconnects will be presented. Copper pillars having nanotwined boundaries and nanocrystalline grains were tested by this technique. We show direct evidence that twin boundaries can withstand extensive plastic deformation and still retain their structure when compared to regular grain boundaries. The deformation mechanisms of twin boundaries predicted by Molecular Dynamic (MD) simulations has been verified by real-time TEM analysis. Quantitative in-situ stress measurements for deformation twinning are in close agreement with those reported by first principle based calculations.
12:00 PM - EE1.9/NN1.9
In Situ TEM Nanocompression Testing of Gum Metal.
Elizabeth Withey 1 , Andrew Minor 2 , Jia Ye 2 , Shigeru Kuramoto 3 , Daryl Chrzan 1 , John Morris 1 Show Abstract
1 Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States, 2 , National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 , Toyota Central R&D Laboratories, Inc., Nagakute Japan
“Gum Metal” is a newly developed β-Ti alloy that, in the cold-worked condition, has exceptional elastic elongation and high strength. The available evidence suggests that Gum Metal does not yield until the applied stress approaches the ideal strength, and then deforms by mechanisms that do not involve conventional dislocation plasticity. To study its behavior, submicron-sized pillars of solution-treated and cold-worked Gum Metal were compressed in situ in a quantitative compression stage in a transmission electron microscope. Quantitative load vs. displacement data was correlated to real-time images to determine a pattern of deformation that agrees with previous results from ex situ nanoindentation.
12:15 PM - EE1.10/NN1.10
Thermal Behavior of Gold Nanoparticles on Pyrite and Arsenopyrite Surfaces.
Niravun Pavenayotin 1 , Qiangmin Wei 2 , Yanbin Chen 1 , Lumin Wang 1 2 Show Abstract
1 Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, Michigan, United States, 2 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Diffusion of gold nanoparticle on pyrite (FeS2) and arsenopyrite (FeAsS) surfaces was studied under in-situ TEM. The gold nanoparticles were deposited onto the surfaces by sputtering of gold TEM grids by ion milling. The gold nanoparticles have a uniform size of approximately 2 nm in diameter. The samples were heated up using a hot stage in the TEM to 100, 200, 300, 400, 500 and 550°C. The movement and characteristics of the nanoparticles were monitored by in situ TEM. The gold nanoparticles coalesce and grow by Oswald’s ripening as the temperature rises. At 500°C, pyrite starts to decompose into amorphous Fe and S. Gold particles on the decomposed surface are driven together and form particles as large as 30 nm in diameter while at the same temperature the gold particles in arsenopyrite are less than 20nm in diameter. Some of the solid gold nanoparticles on pyrite surface also melt and form a film-like morphology. Arsenopyrite does not decompose until 550°C. The gold particles that reside on top of the amorphous decomposed region are immobile. The particles on the crystalline surface grow at a fast rate and visibly mobile on the surface. The differences in the diffusion behavior of gold nanoparticles on two different pyrites will be explained.
12:30 PM - **EE1.11/NN1.11
Revealing the Deformation Processes Responsible for Controlling Mechanical Properties.
Ian Robertson 1 Show Abstract
1 Materials Science and Engineering, University of Illinois, Urbana, Illinois, United States
Transmission electron microscopes have played a critical role in building our knowledge base regarding dislocations, dislocation-obstacle interactions and microstructural evolution as a function of deformation history. Past usage yielded primarily snapshots of the microstructure, leaving the pathways by which it was attained to be deduced. Development of straining stages for use in electron microscopes, cameras for capturing the reaction dynamics and computer technology and software for image processing have enabled observation of dislocation processes in real time and at the spatial resolution of the instrument. Provided the impact of the thin film geometry and the stress state are appreciated, this technique can and has been used to provide visual and quantitative information about dislocation reactions and processes. The results of these studies are now being incorporated into physically-based models for predicting the mechanical properties. Recent developments in stage design provide the capability to measure the macroscopic response and to simultaneously observe the deformation behavior, thus, providing the opportunity to correlate microscopic processes with a macroscopic property. In this talk, examples of applications of standard and new straining stages will be used to illustrate how these tools have advanced our understanding of dislocation reactions and processes and how this insight has been used to yield new models.
EE2: Anomalous Nanomechanical Behavior
Monday PM, December 01, 2008
Room 200 (Hynes)
2:30 PM - **EE2.1
Stress-coupled Grain Boundary Migration in Nanocrystalline Thin Films.
Kevin Hemker 1 , John Sharon 1 , Timothy Rupert 2 1 , Daniel Gianola 3 1 Show Abstract
1 Mechanical Engineering, Johns Hopkins University, Baltimore, Maryland, United States, 2 Materials Sci & Eng, MIT, Boston, Massachusetts, United States, 3 , KIT, Karlsruhe Germany
Stress-coupled grain boundary migration has recently been manifest in observations of stress-assisted room temperature grain growth in nanocrystalline metals. Of particular interest to this study are micro-tensile experiments involving nanocrystalline thin films that highlight the fact that the mechanical behavior of these films is not only different that bulk materials but dynamic as well. The observation of stress-assisted grain growth in these nanocrystalline thin films cannot be described by traditional grain growth driving forces and appears to be a direct experimental observation of stress-coupled grain boundary motion. This stress-coupled grain boundary motion appears to be highlighted in nanocrystalline films because of their unusually high flow stresses.
3:00 PM - EE2.2
Diffusion Induced Stresses in Nanostructured Materials for Energy Conversion and Storage.
Yang-Tse Cheng 1 2 , Mark Verbrugge 1 Show Abstract
1 , GM R&D Center, Warren, Michigan, United States, 2 Department of Chemical and Materials Engineering, University of Kentucky, Louisville, Kentucky, United States
3:15 PM - EE2.3
First Observation of Hollow Silver Nanotubes.
Maureen Lagos 1 2 , Fernando Sato 1 , Jefferson Bettini 2 , Varlei Rodrigues 1 , Douglas Galvao 1 , Daniel Ugarte 1 2 Show Abstract
1 Applied Physics, State University of Campinas, Campinas, Sao Paulo, Brazil, 2 , LNLS, Campinas, Sao Paulo, Brazil
Metallic nanowires (NWs) have been object of many theoretical and experimental studies in the last years. In nanostructures, the atomic arrangements may be different from macroscopic matter, mainly due to the role of surface energy. In fact the stretching of metal junctions has revealed quite surprising metastable structures such as suspended atomic chains and helical wires. In this work we report the first experimental observation of the formation of the smallest possible square cross-section hollow silver nanotubes during elongation of Ag nanocontacts as revealed by real time atomic resolution transmission electron microscope (HRTEM). Silver NWs were produced inside the HRTEM (JEM-3010 URP 300kV, 0.17 nm point resolution). Firstly, holes are opened in a self-supported metal film by focusing the electron beam and, nanometric bridges are formed between neighboring holes. These constrictions evolve, elongate and break. This process is recorded with a high sensitive camara (Gatan 622SC, 30 frames/s). We have analyzed the NW structural stability by total energy ab initio calculations using the well-known SIESTA code. We have used norm-conserving pseudo potentials (PP)and atomic orbitals as a basis set. We have used the PP built on Troullier-Martins scheme in a local density approximation (LDA). The theoretical analysis suggests that the formation of these metastable hollow structures requires a combination of minimum basis size and high gradient stress generating a soft structure capable of absorbing a huge tensile deformation when high stress is applied.
3:30 PM - **EE2.4
Unusual Mechanical Behaviors Of Nanocrystalline Metals - Effect Of Size And Heterogeneity Of Microstructure.
Jagannathan Rajagopalan 1 , Taher Saif 1 Show Abstract
1 Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Nano-grained thin metal films exhibit substantially different mechanical behaviors compared to their bulk, coarse-grained counterparts due to their unique dimensional and microstructural constraints. We have experimental evidence showing that nanocrystalline freestanding thin metal films (aluminum and gold films, grain size of 50-80 nm) recover a large fraction of their plastic strain after unloading, under macroscopically stress-free conditions. This recovery is time and temperature dependent with three distinct activation energies, possibly linked to three impurities in the films. The same films with larger grain sizes (200 nm) show strong Bauschinger effect during unloading, even at large overall tensile stresses. At an intermediate grain size (150 nm), they exhibit both Bauschinger effect and strain recovery upon unloading. We hypothesize that these unusual and seemingly unrelated phenomena are the consequence of the interplay between the small grain size of these films and the microstructural heterogeneity (size and texture variations of individual grains) – a paradigm that has not been explored in much detail. We carry out in situ Transmission Electron Microscopy (TEM) of these films while their macroscopic stress-strain response is measured, using a novel micro mechanical stage. TEM analysis shows first evidence of a dislocation mediated, and thermally assisted, mechanism for both strain recovery and Bauschinger effect in nano-grained thin films.
4:30 PM - **EE2.5
Time-Dependent Deformation of Thin Aluminum and Gold Films.
Richard Vinci 1 , Xiaojun Yan 1 , Walter Brown 1 , Yuan Li 2 , John Papapolymerou 2 Show Abstract
1 Materials Science and Engineering, Lehigh University, Bethlehem, Pennsylvania, United States, 2 Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Time-dependent deformation of thin metal films can lead to failure in a number of microelectromechanical devices. Even at room temperature, creep and stress relaxation can be rapid and large, and can continue for months. In this regime, small changes in temperature can have significant effects on the rate and extent of deformation. In an effort to characterize the time-dependent behavior of thin metal films we present experiments and models addressing the stress relaxation observed in micrometer-thick sputtered Al and electroplated Au membranes at room temperature. It is shown that relaxation can be due to both inelastic processes (creep plasticity) and to anelastic processes (fully elastic but with delayed response). However, under certain conditions it is possible to induce a state in which the behavior is purely anelastic, allowing separation of the two contributions to the more general case. Linear viscoplastic and viscoelastic models are presented that enable prediction of the film’s mechanical behavior under monotonic relaxation conditions and under complex cyclic loading, both for the purely anelastic state and for the mixed inelastic-anelastic state.
5:00 PM - EE2.6
Creep Deformation Behavior of Nanocrystalline Copper Doped with Antimony.
Rahul Rajgarhia 1 , Ashok Saxena 1 , Douglas Spearot 1 Show Abstract
1 , University of Arkansas, Fayetteville, Fayetteville, Arkansas, United States
Nanocrystalline materials are prone to premature grain growth causing the loss of its superior mechanical properties attributed to its low grain size. Recent theoretical and molecular dynamics simulation results have shown promising results of stabilizing the microstructure by including impurity atoms (dopants) at the grain boundaries. In this study, nanocrystalline copper doped with 0.2 and 0.5 at.% antimony is studied under the influence of elevated temperatures and mechanical stress. At these low concentrations, Sb completely segregates to grain boundaries in Cu without forming precipitates or secondary phases. The Cu-Sb alloy is prepared by casting in an induction arc furnace followed by equal channel angular extrusion process (ECAE) that causes grain size refinement. The TEM images of heat treatment analysis at 250 C for 1 hour show that the microstructure of the nanocrystalline samples with Sb remain unchanged whereas extensive grain growth is observed in pure Cu. Also, the combined effect of stress and temperature, using creep deformation tests, on the microstructure of the nanocrystalline Cu-Sb and pure Cu materials are discussed.
5:15 PM - EE2.7
Void Formation Mechanism of Nanocrystalline Cu Alloys During Uniaxial Relaxation Test.
Junya Inoue 1 , Yosuke Fujii 1 2 , Toshihiko Koseki 1 Show Abstract
1 Materials Engineering, The University of Tokyo, Tokyo Japan, 2 , SONY, Miyagi Japan
Void formation in nanocrystalline Cu alloys during uniaxial relaxation experiment was quantitatively studied to clarify the effect of alloying elements to the void formation process in nanocrystalline Cu. Cu alloy films (Cu-Sn and Cu-Ag) with various thickness were deposited on heat-resistant polyimide substrates by electron beam physical vapor deposition (EB-PVD) method. On the top and bottom surfaces of the Cu layer, Ta layers were deposited to suppress the surface diffusion of Cu during the relaxation test. The Cu alloy films were annealed after the deposition to achieve complete grain growth to the size equivalent to the film thickness at the temperature of 460K in a vacuum of 1.0x10-5Pa for 12h. A uniform uniaxial tensile strain was induced in the films by applying a constant radius of curvature to the polyimide substrate. Isothermal microstructural evolution as well as void formation in the films was studied at elevated temperature, 460K.No apparent initial voids were found in EB-PVD films after annealing the samples at 460K, while stress-induced voids were found initiated at the grain boundary triple points at various levels of applied strain below that equivalent to the maximum ultimate strength of nanocrystalline Cu with identical grain size reported by others. The voids were found to nucleate without incubation time, and the nucleation rate was highly dependent on the applied strain. No severe plastic deformation, such as dislocation pile up or mechanical twinning, could be found in the films with a thickness less than 300nm even at the stress level almost equivalent to the tensile strength. The results indicate that the microvoiding is the predominant accommodation process in this range of grain sizen, stress and temperature. By introducing an approximate void formation model, the void growth and the effect of stress relaxation to the void nucleation were examined. Resulting model shows a reasonable agreement with the experimentally observed number density and area fraction of voids for various strain levels and grain sizes. The application of the model to the void formation process in Cu alloys revealed the effect of alloying elements to the void nucleation rate and void growth rate separately. From the microstructural investigation, the difference in the segregation behavior of alloying elements was found responsible to the observed trends in the suppression of the void formation process by the alloying elements.
5:30 PM - EE2.8
Strain Recovery Driven by Grain-boundary Diffusion for Nanocrystalline Thin Films.
Xiaoding Wei 1 , Jeffrey Kysar 1 Show Abstract
1 Mechanical Engineering, Columbia University, New York, New York, United States
Plastic strain in metals is typically considered to be irreversible. However plastic strain in nanocrystalline materials can be recovered over a period of time via diffusive mechanisms. In this study, free-standing nanocrystalline Cu films of submicron thickness with an average grain size of about 40 nm are mechanically loaded via a plane-strain bulge test in order to obtain the stress-strain behavior. During loading, the specimens are deformed with a significant plastic strain after which the specimens are left at room temperature. The specimens recover their plastic strain in a period of time that can range from a few minutes to a few days depending upon the magnitude of the induced plastic strain as well as the number of strain recovery cycles. We will present results of multiple cycles of plastic deformation and recovery as well results of the rate of recovery of the plastic strain. There are two characteristic strain rates that occur during plastic strain recovery. A model of the plastic strain recovery that invokes various diffusive mechanisms will be presented as a possible explanation for the phenomenon.
5:45 PM - EE2.9
Mechanical Behavior Associated with Heterogeneous Grain-boundary Diffusion and Sliding in Nanocrystalline Materials.
Yujie Wei 1 , Allan Bower 1 , Huajian Gao 1 Show Abstract
1 Engineering Division, Brown University, Providence, Rhode Island, United States
To improve our understanding of deformation mechanisms in materials with small characteristic microstructures, like nanocrystals and thin films, we consider a mesoscopic continuum model of a two-dimensional assembly of grains separated by sharp grain boundaries. In this model, the crystal deforms plastically by grain boundary diffusion, grain-boundary sliding, as well as grain interior dislocation activities described by single crystal plasticity. Some of the phenomena found in this study include:(a) If the diffusivity changes abruptly in the grain-boundary network, the transient stress distribution in response to an applied load develops crack-like stress concentrations. The stress concentrations develop at a rate determined by the fast diffusion coefficient, and subsequently relax at a rate determined by the slow diffusion coefficient .(b) Creep deformation by heterogeneous grain-boundary diffusion is partially recoverable , which could explain the recent experiments published in Science by Rajagopalan, Han, and Saif .(c) Coble creep changes character under heterogeneous grain-boundary diffusion exists .(d) There is a transition from sliding and diffusion dominated creep in relatively small grain sized nanocrystals at low strain rates to plasticity dominated flow in nanocrystals with larger grain size deformed at higher strain rates  and .Refereces: Wei, Bower, Gao, J. Mech. Phys. Solids, 2008;56:1460.  Wei, Bower, Gao, Scripta. Mat., 2007;57:933.  Rajagopalan, Han, Saif, Science 2007;315:1831.  Wei and Gao, Mat. Sci. Eng. A 2008;478:16.  Wei, Bower, Gao, Acta Mater.2008;56:1741.
Amit Misra Los Alamos National Laboratory
T. John Balk University of Kentucky
Hanchen Huang Rensselaer Polytechnic Institute
Maria Jose Caturla Universitat d'Alacant
Christoph Eberl University of Karlsruhe
EE3: Fatigue, Fracture & Tribology
Tuesday AM, December 02, 2008
Room 200 (Hynes)
9:30 AM - **EE3.1
Fatigue Deformation of Nanocrystalline and Ultrafine-grained Ni Alloy.
Sheng Cheng 1 , Peter Liaw 1 , Xun-li Wang 2 , Alexandru Stoica 2 , Hahn Choo 1 Show Abstract
1 Dept of Materials Science and Engineering, University of tennessee, Knoxville, Tennessee, United States, 2 Neutron scattering science division, Oak ridge national laboratory, knoxville, Tennessee, United States
Grain size of metals plays an important role in their deformation mechanism. It also applies to fatigue deformation. However, the fatigue deformation of nanocrystalline and ultrafine-grained metals was not extensively studied, and many issues remain unsolved. We have recently performed fatigue studies on a range of materials with grain size from nanocrystalline to ultrafine-grained to conventional coarse-grained samples under both tensile and compressive loading modes. We used in situ and ex situ neutron diffraction and synchrotron X-ray diffraction to study the lattice strain as well as the intergranular strain evolutions during fatigue tests. From the lattice strains as well as intergranular strain evolution, we show a clear grain size-related mechanism in fatigue. Different influential mechanisms will be discussed. This work is supported by the National Science Foundation International Materials Institutes (IMI) Program (DMR-0231320) and Major Research Instrumentation (MRI) Program (DMR-0421219) with Dr. C. Huber and Dr. C. Bouldin as the Program Directors, respectively.
10:00 AM - EE3.2
Effect of Cyclic Loading and Microhardness Indentation on the Microstructural Stability of Nanotwinned Cu Samples with Highly Aligned Twin Interfaces.
C. Shute 1 , B. Myers 1 , S. Xie 1 , A. Hodge 2 , T. Barbee 3 , Julia Weertman 1 Show Abstract
1 , Northwestern University, Evanston, Illinois, United States, 2 , University of Southern California, Los Angeles, California, United States, 3 , Lawrence Livermore National Laboratory, Livermore, California, United States
The effects on the microstructure of cyclic loading and of microhardness indentation on high purity nanotwinned Cu samples were examined. Samples were made by a laminate process using magnetron sputtering. The foil consists of long micron-width columnar grains containing nanotwins spaced about 40-50 nm apart with their interfaces aligned perpendicular to the growth direction. Transverse cuts of the thick foil samples (170 um) by FIB technique showed that the twins were remarkably stable under tension-tension fatigue except under the highest stress tested (450 MPa maximum stress amplitude, in the LCF regime). HREM pictures showed the appearance of some incoherent regions on the twin interfaces, development of high internal strains and dislocations in the twin interiors. Indentation of a foil surface caused significant disruption of the columnar grain boundaries, appearance of a shear band, large incoherent regions on twin interface boundaries, and extensive regions of very high internal strain. The highly aligned nanotwins, unlike equiaxed nanocrystalline or UFG Cu, present quite a stable microstructure.*The TEM, FIB and SEM work was performed in the EPIC facility of NUANCE Center, supported by NSF-NSEC, NSF-MRSEC, Keck Foundation, State of Illinois, and Northwestern University.
10:15 AM - EE3.3
Damage Behavior of Al Thin Films Fatigued at Ultra-high Frequencies.
Diana Courty 1 , Chris Eberl 1 , Werner Ruile 2 , Oliver Kraft 1 3 Show Abstract
1 izbs, University of Karlsruhe, Karlsruhe Germany, 2 , EPCOS AG, Munich Germany, 3 IMF2, Forschungszentrum Karlsruhe, Karlsruhe Germany
Components used in radio frequency communication are typically stressed at frequencies in the GHz-regime. We perform fatigue tests at ultra-high frequencies in metallizations using testing devices that reach cycle numbers up to 10^14 in one day to investigate the reliability of such thin film devices. Fatigue effects, such as the formation of voids and extrusions that result in a frequency shift or in short circuits, respectively, are observed in the polycrystalline metallization.In more detail, we have used patterned Al/Cu thin films of a thickness of approximately 200 nm on a piezoelectric substrate, which are subjected to an AC electric field that leads to cyclic strains in the film/substrate system. Transmission Electron Microscopy is used to study the possible role of dislocations and to determine the mean grain size, which is in the order of 100 nm. No long range dislocation structures were observed in the fine grained material. Extrusions and voids that form in the stressed metal film are analyzed quantitatively by Scanning Electron Microscopy and are correlated to the frequency shift which represents an easily accessible fatigue criterion. For a quantitative correlation, the modeling focuses on the influence of damage on the frequency. By means of finite element simulation, the influence of void size, density and position in the metal conductor lines is explored. These simulations are part of a physically based life time model which is currently being developed.
10:30 AM - EE3.4
Study the High-cycle-fatigue Behavior of a Nano-precipitate Strengthened Alloy by In-situ Neutron-diffraction Experiments.
E-Wen Huang 1 , Wei-Ren Chen 2 , Michael Hofmann 3 , Chih-Pin Chuang 4 , Sven Vogel 5 , Peter Liaw 1 , Lee Pike 6 Show Abstract
1 Materials Science & Engineering, University of Tennessee, Knoxville, Tennessee, United States, 2 Neutron Scattering Sciences Division, Spallation Neutron Source, Oak Ridge National Laboratory , Oak Ridge, Tennessee, United States, 3 Spec-Stress, Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II), Garching Germany, 4 Department of Engineering and System Science, National Tsing Hua University, Hsinchu Taiwan, 5 Los Alamos Neutron Science Center, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 6 Technology Engineering, Haynes International Inc., Kokomo, Indiana, United States
In this study, a theoretical model of the nano particle deformation under a tension-tension high-cycle-fatigue experiment and a tension test is proposed for investigating the structural properties of precipitation-strengthened metallic alloys using the in-situ neutron-diffraction technique. The model is based on the macroscopic stress-strain curves incorporating with atomic-scale responses measured by the in-situ neutron-diffraction experiments. A newly-developed corrosion-resistant nickel-based alloy, HASTELLOY® C-22HSTM, is selected to demonstrate the applicability of this model and a complementary texture investigation is used to gauge its validity. The alloy contains the strengthening nano precipitates, which are treated as a set of polydisperse hard ellipsoids and their spatial arrangement, given by the inter-precipitate structure factor, is calculated by a stochastic phenomenological approach using small-angle neutron-scattering experiments accompanied by TEM measurements. In comparison with the fracture surfaces, the different deformation mechanisms of the nano precipitates under two types of tensile loadings have been characterized by the evolution of the lattice strain and peak-profile, extracted from in-situ neutron-diffraction profile fitting.ACKNOWLEDGMENTSThe National Science Foundation (NSF), International Materials Institutes (IMI) Program (DMR-0231320), supports this research. Dr. Carmen Huber is the Director of the IMI program. The Department of Energy’s Office of Basic Energy Science funds the Lujan Neutron Scattering Center at Los Alamos Neutron Science Center (LANSCE). The Los Alamos National Security LLC under the DOE Contract DE-AC52-06NA25396 operates the Los Alamos National Laboratory. Haynes International provides the alloys.
10:45 AM - EE3.5
Suppression on Fatigue Crack Growth in Carbon Nanotube Composites.
Nikhil Koratkar 1 , Catalin Picu 1 , Wei Zhang 1 Show Abstract
1 Mechanical Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Fatigue is one of the primary causes for catastrophic failure in structural materials. In this study, we report an order of magnitude reduction in the fatigue crack propagation rate for an epoxy system with the addition of ~0.5% weight of multiwalled carbon nanotube (MWNT) additives. The fatigue suppression performance of the nano-composite was found to be the most effective at low stress intensity factor amplitudes. Using fractography analysis and fracture mechanics modeling we show that the crack suppression is caused by a crack bridging phenomena which results in an effective crack closing stress due to the frictional pull-out of nanotube-fibers in the wake of the crack tip. Using this model, we show that the suppression of crack growth can be further optimized by reducing the nanotube diameter, by increasing the number density of nanotubes that bridge the crack and by increasing the pull-out length of the nanotubes. Control experiments with nanotubes of different diameters, lengths and dispersion quality confirmed the model predictions. These results demonstrate that carbon nanotubes can significantly enhance the fatigue-life of structural polymers that are susceptible to damage in the form of cracks. One of the limitations of fatigue suppression using carbon nanotube additives is that the crack growth rates can be effectively suppressed only at low values of the stress intensity factor. We show that the reason for this is the shrinkage of the nanotube (fiber) bridging zone in the wake of the crack tip which occurs at the high stress intensity factors. We demonstrate that this problem can be overcome by using amine-functionalized multiwalled carbon nanotubes (A-MWMT). The reason is that the A-MWNT interact covalently with the epoxy matrix which prevents pull-out of the nanotubes. Instead we find that the A-MWNT additives change the structure/morphology of the local epoxy in the interphase zone which encapsulates the A-MWNT. The modified epoxy within this interphase zone has a relatively lower cross-link density compared to the bulk and is prone to 'Crazing' which significantly enhances the energy dissipation and dramatically slows the crack growth rates across the full range/spectrum of applied stress intensity factors. We report over 20-fold reduction in crack growth rates for the A-MWNT/epoxy composite compared to the neat epoxy over the full range of stress intensity factor amplitudes. In this way addition of A-MWNT to the epoxy fundamentally changes the material's fatigue behavior due to the altered molecular scale morphology of epoxy within the interface zone (that encapsulates the nanotubes) as compared to the bulk, which allows crazing to take place in an otherwise brittle material.
11:30 AM - **EE3.6
Effects of Grain Refinement on Lifetime and on Damage Evolution under Repeated Contact in Nanostructured Metals.
Ruth Schwaiger 1 2 Show Abstract
1 IMF2, Forschungszentrum Karlsruhe, Karlsruhe Germany, 2 IZBS, University Karlsruhe, Karlsruhe Germany
In structural applications, materials may be subjected to conditions in which cyclic contact loading inevitably occurs. This may lead to contact fatigue effects which are critical to assess the overall usefulness of nanocrystalline materials but have not yet been investigated systematically on the nanoscale.This talk will describe our studies on sliding contact and normal contact fatigue behaviors of nanocrystalline metals, with average and total range of grain sizes well below 100 nm. Cyclic contact deformation was induced in Ni samples having different grain sizes by repeatedly indenting the surface of the specimens and by repeatedly sliding a tip over the surface. The mechanical behavior as well as the evolution of damage in nanocrystalline Ni during repeated contact were systematically compared with ultrafine-crystalline and microcrystalline materials in order to gain a broader perspective on the effects of grain size on contact fatigue.
12:00 PM - EE3.7
Tribology of a Nanocrystalline Alloy across the Hall-Petch Breakdown.
Timothy Rupert 1 , Christopher Schuh 1 Show Abstract
1 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States
Nanocrystalline alloys exhibit generally desirable wear properties, but the extreme conditions produced during sliding or abrasive wear can lead to evolution of the nanostructure and properties. This talk addresses the tribological response of nanocrystalline Ni-W alloys across a range of grain sizes spanning the Hall-Petch breakdown. Both cyclic sliding and abrasive wear are studied in electrodeposited coatings with grain sizes ranging from 3 to 40 nm, to understand the influence of microstructure and mechanical properties on wear resistance. The experiments reveal evidence of a dynamic nanostructure where grain boundary character evolves under sliding loading, which in turn alters the mechanical response. The effect of a grain size gradient on tribological response is also discussed.
12:15 PM - EE3.8
Connecting Plasticity and Fracture in Brittle Materials.
Douglas Stauffer 1 , Aaron Beaber 1 , William Gerberich 1 Show Abstract
1 Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States
12:30 PM - EE3.9
Brittle and Ductile Failures of Silicon Nanowires under Tension Simulation.
Keonwook Kang 1 , Wei Cai 1 Show Abstract
1 , Stanford University, Stanford, California, United States
We performed molecular dynamics study of tension simulations of small diameter silicon nanowires (D < 10nm) grown along  direction and found that their brittle or ductile failure behavior depends on their size and temperature. A dislocation dynamics model is developed to explain the size and temperature effects on the failure behavior of nanowires observed in atomistic simulations. Results are also compared with recent tensile experiments on silicon nanowires.
12:45 PM - EE3.10
Deformation Mechanisms in Nanocrystalline Pd and Pd-Zr Thin Films.
Rudolf Baumbusch 1 , Patric Gruber 1 , Tanja Ulyanenkova 2 , Tanja Filatova 2 , Stephen Doyle 3 , Anna Castrup 4 , Juergen Markmann 5 , Joerg Weissmueller 4 , Tilo Baumbach 2 3 , Horst Hahn 4 , Oliver Kraft 1 Show Abstract
1 Institut für Zuverlässigkeit von Bauteilen und Systemen, University of Karlsruhe, Karlsruhe, Baden-Württemberg, Germany, 2 Laboratorium für Applikationen der Synchrotronstrahlung, University of Karlsruhe, Karlsruhe, Baden-Württemberg, Germany, 3 Institut für Synchrotronstrahlung, Research Center Karlsruhe, Eggenstein-Leopoldshafen, Baden-Württemberg, Germany, 4 Institut für Nanotechnologie, Research Center Karlsruhe, Eggenstein-Leopoldshafen, Baden-Württemberg, Germany, 5 Fachrichtung 7.3 -Technische Physik, Universität des Saarlandes, Saarbrücken, Saarland, Germany
EE4: Thin Films, Multilayers and Nanocomposites: Mechanics and Radiation Effects
Tuesday PM, December 02, 2008
Room 200 (Hynes)
2:30 PM - **EE4.1
Deformation and Fatigue of Nanoscale Multilayer Metallic Composites.
David Bahr 1 , Nicole Overman 1 , Cory Overman 1 , Ioannis Mastorakos 1 , Firas Akasheh 2 , Hussein Zbib 1 Show Abstract
1 Mechanical and Materials Engineering, Washington State University, Pullman, Washington, United States, 2 , Tuskegee University, Tuskegee, Alabama, United States
Multilayer metallic thin films, where film thicknesses are on the order of 10 to 100 nm, have been shown to have substantially elevated strengths over their constituent materials. This presentation focuses primarily on Nb/Cu at thicknesses of 20/20 nm, and the testing methodology used to compare the effects of uniaxial and biaxial stress states on the deformation and fatigue of the systems. The films are deposited via sputtering on silicon wafers, and form discreet layer structures. The mechanical response of the films were tested using two complementary techniques; nanoindentation and bulge testing. Bulge testing was used to determine the residual stress in these films, approximately 30 MPa. The composite elastic modulus was measured using both techniques, and found to correlate well between the two systems. The plasticity of these films was assessed using both techniques. The hardness of the Nb/Cu films was approximately 5 GPa. Alterations in the bulge test geometry, ranging from rectangular membranes that produce effectively uniaxial strain conditions, to square membranes that form biaxial stress states, but have significant stress concentrations, and more complex tailored shapes selected using high strain finite element modeling of the structures that localize the stress concentrations away from the etched window structures used to form the test specimens will be compared with an etched “dimple” structure that leads to a stress concentration in the center of the window. This presentation will discuss and compare the experimental techniques required to assess the properties of these extremely strong films. Plastic deformation is shown to occur during bulge testing either at regions of microstructural variations, such as an anomalously large grain, or these controlled stress concentration sites. With a best practices condition for non-unaxial bulge testing, low cycle, high strain fatigue testing using the bulge test is shown to be a viable method for biaxial loading conditions. Evidence of strain softening in these conditions is presented. Finally, the experimental conditions will be related to simulations of deformation using dislocation dynamics of the multilayer structures.
3:00 PM - EE4.2
Deformation in Multilayers at Elevated Temperatures.
Sandra Korte 1 , David Dunstan 2 , William Clegg 1 Show Abstract
1 Materials Science and Metallurgy, University of Cambridge, Cambridge United Kingdom, 2 Physics Department, Queen Mary, University of London, London United Kingdom
Multilayers made of InGaAs are of interest due to their use in optoelectronic devices. Formation of dislocations can result in failure of the entire device and therefore an exact understanding of the onset of yield in these systems is important.Nanoscale superlattices of heteroepitaxially grown InxGa1-xAs of different compositions were characterized by means of nanoindentation and micropillar compression in order to identify the pressure at the onset of yield. Using a new nanoindentation system under protective atmosphere to prevent oxidation this was done over a range of temperatures from room temperature to 500°C. In layered systems the mechanical properties are partly determined by whether or not flow is confined to indiviual layers or occurs continuously over a number of layers. In order to identify the overall deformation mechanism in InGaAs superlattices, in-situ micropillar compression and indentation inside a transmission electron microscope was used to observe flow within and across layers during deformation.In order to predict yield in heteroepitaxial multilayers, the system parameters layer thickness, coherency strains and mechanical properties of the constituent layers need to be known. Nanoindentation at room and elevated temperatures was employed to obtain mechanical data on the solid solution InGaAs over the relevant compositional range and incorporated into a model predicting the measured multilayer properties.
3:15 PM - EE4.3
Radiation Damage and He Solubility at Coherent and Incoherent Interfaces.
Dhriti Bhattacharyya 1 , Osman Anderoglu 1 , Michael Demkowicz 2 , Igor Usov 2 , Richard Hoagland 2 , Amit Misra 1 Show Abstract
1 MPA-CINT, Los Alamos National Laboraotry, Los Alamos, New Mexico, United States, 2 MST-8, Los Alamos National Laboraotry, Los Alamos, New Mexico, United States
3:30 PM - **EE4.4
Elasto-plastic Transition in High Strength Nanocomposite Wires and Composite Micro-pillars Studied by In-situ Deformation under X-rays.
Ludovic Thilly 1 , Robert Maass 2 , Marc Legros 4 , Steven Van Petegem 2 , Pierre-Olivier Renault 1 , Vanessa Vidal 5 , Jean-Baptiste Dubois 1 3 , Florence Lecouturier 3 , Helena Van Swygenhoven 2 Show Abstract
1 PHYMAT, University of Poitiers, Futuroscope France, 2 , Paul Scherrer Institute, Villigen Switzerland, 4 , CEMES, Toulouse France, 5 , CROMeP, Albi France, 3 , LNCMP, Toulouse France
Copper-based high strength nanocomposite wires are prepared by severe plastic deformation (Accumulative Drawing and Bundling process) for the windings of high pulsed magnets. The process leads to a multi-scale Cu matrix containing up to N=854 (52.2 106) continuous parallel Nb filaments or tubes with diameter down to few tens nanometers. After heavy strain, the multiscale Cu matrix is nanostructured and the Nb reinforcing phase is strongly refined. The resulting macroscopic strength is in excess from rule of mixture predictions calculated from bulk coarse-grained counterparts: an ultimate tensile strength up to 2 GPa is reached at 77K.In-situ tests have been performed under synchrotron radiation on nanocomposite wires containing Nb nanotubes. The evolution of elastic strains and peak profiles versus applied stress evidences the co-deformation behavior with different elasto-plastic regimes: the Cu matrix exhibits size effect in the finest channels while the Nb nanotubes remain elastic up to the macroscopic failure, with a strong load transfer from the Cu matrix onto the Nb nanotubes. During multiple loading-unloading cycles, the macroscopic stress-strain curve evidences strong hysteresis that could be independently followed in the different phases via the measurement of elastic strains: internal stresses are building-up because of large yield stress mismatch in the nanocomposite structure. This Bauschinger effect can be related to the dislocation storage in the different phases (APL 90 (2007), 241907). The elasto-plastic transition is also studied in the different phases, in particular the microplastic to macroplastic transition with respect to microstructure size.In addition, in-situ compression of Cu-Nb micro-pillars (containing a single Nb filament surrounded by Cu grains) has been performed under synchrotron micro-beam: the evolution upon loading of the Cu and Nb Laue peaks gives additional information on the elasto-plastic transition and the effect of interfaces on the plasticity of these nanomaterials.
4:30 PM - EE4.5
Effect of Ion Irradiation on Mechanical Deformation of Low-Density Nanoporous Materials.
K. Lord 1 , Sergei Kucheyev 1 , J. Satcher 1 , A. Hamza 1 Show Abstract
1 , Lawrence Livermore National Laboratory, Livermore, California, United States
Mechanical properties of porous solids exhibit a strongly superlinear dependence on the material density. Hence, ultralow-density nanoporous solids have poor mechanical properties, and this remains the major factor limiting many potential energy-related applications of these materials. For example, silica aerogels with densities below ~100 mg/cm3 typically have a very low Young’s modulus of ~1 MPa. Here, we use ion irradiation to manipulate mechanical properties of low-density nanofoams. In particular, we study dose dependencies and effects of electronic and nuclear energy loss on the mechanical properties of nanoporous silica monoliths (aerogels) with densities below ~350 mg/cm3. We use depth-sensing indentation with various indenter geometries (spherical, pyramidal, and flat punches) to measure Young’s modulus, crushing pressure, fracture toughness, and brittleness of nanofoams. Our results show that ion bombardment could be used to improve the foam connectivity and, hence, the mechanical response of low-density nanoporous solids. This work was performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344.
4:45 PM - EE4.6
Modeling of Effects of Ion Energy and Mass on Self-Organized Nanostructure Formation During Ion-beam Implantation.
Kun-Dar Li 1 2 , Alejandro Perez-Bergquist 2 , Qiangmin Wei 1 , Lumin Wang 1 2 Show Abstract
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Nuclear Engineering and Radiological Science, University of Michigan, Ann Arbor, Michigan, United States
Ion beam is an important technique for achieving controlled modification of surface and near-surface regions. One of the recent developments in ion beam processing has focused on the studies of self-organized nanostructure created after ion-implantation. Various surface morphologies have been produced by focused ion beam (FIB), e.g. ripple, cellular, nanodot or nanofiber patterns. In addition to material properties, temperature and other experimental parameters, e.g., ion energy and ion mass play a crucial influence on the formation of self-organized nanostructures. In this study, a 3D theoretical model has been developed to study the effects of ion energy and ion mass on the formation of these structures during ion-beam bombardment. We adopted a phase field approach which integrates the production and elimination rates of vacancies considering ion sputtering, recombination with interstitials and redeposition of sputtered atoms with the free energy of mixing and interfacial energy as the driving force for vacancy diffusion. The depth profile of vacancy production along the depth of an irradiated matrix is considered as approximately Gaussian in shape according to the Monte-Carlo calculation by SRIM code. With increasing ion energy while all other parameters treated as constant, the number of vacancies produced and the depth of the peak vacancy concentration will increase. The simulated surface morphologies transformed from cellular nano-holes into a nanofiber structures. While the implanted ion mass increases, the number of vacancies produced increases and the depth of the peak vacancy concentration decreases. The simulated morphologies change from nanofibers, nanowall-like structures into open and/or embedded hole structure. The simulated morphologies are consistent with experimental observations achieved under comparative experimental conditions. Our model provides a distinct numerical approach to predict results or direct experimental designs to form a desired nanostructure by ion beam technology.
5:00 PM - EE4.7
Nanospire Formation by Heavy Ion Beam Irradiation of Oxide Multilayer Surfaces.
Cynthia Reichhardt 1 , Marylin Hawley 1 , David Devlin 1 , Kurt Sickafus 1 , Igor Usov 1 , James Valdez 1 , Youngqiang Wang 1 Show Abstract
1 T-12, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
5:15 PM - EE4.8
Nano Nickel Composites for High-Temperature LIGA/MEMS-Applications.
Manel Haj-Taieb 1 , Klaus Bade 2 , Siriyara Jagannatha Suresha 3 , Kevin Hemker 3 , Jarir Aktaa 1 Show Abstract
1 intitute for Matirials research II, Forschungszentrum Karlsruhe GmbH, Eggenstein-Leopoldshafen Germany, 2 Institute for Microstructure Technology , Forschungszentrum Karlsruhe GmbH, Eggenstein-Leopoldshafen Germany, 3 Department of Mechanical Engineering, Johns Hopkins University, Baltimore, Maryland, United States
For high temperature LIGA/MEMS applications the often applied ED-Nickel is limited by its remarkable microstructural instability connected with a loss of mechanical strength even at intermediate temperature (300-400 °C). To increase strength at higher temperatures (> 300 °C) a second phase could be introduced which shall hinder the recrystallization during high temperature exposures. Here, the objective is to fabricate such materials with added phases: Ni-Al2O3 ODS (oxide dispersion strengthened) and Ni-Al composites by electrodeposition and characterize them by microtesting experiments. As electrolytes two ammonia citrate electrolytes with different chemical compositions and dispersed nano-sized alumina or aluminium particles (nm or µm) were used.The deposited Ni-Al2O3 layers were investigated by SEM and FIB to verify the incorporation of the oxide particles into the nickel matrix. The chemical composition of the electroplated material was determined by EDX. The latter investigations demonstrated the successful incorporation of the Al2O3 nano particles into the layers with ~3 weight-%. In addition the micro-hardness of the as-deposited Ni-Al2O3 measured on the cross-section increases up to ~500 HV which is larger than the micro-hardness of pure Nickel (<300 HV), which confirms the strengthening effect. Ni-Al2O3 layers on copper-substrate have been annealed in the temperature ranges of 300-600 °C for two hours. The micro-hardness remains unchanged at intermediate temperatures (300 and 400 °C). At higher temperatures, e.g. 600 °C, the micro-hardness decreases to a value still higher than the value of pure nickel in the as-deposited state. These enhancements are probably due to the pinning of the grain boundaries and dislocations by the nano particles increasing the microstructural stability improving mechanical properties. These effects have been studied more closely with microtensile tests carried out on LIGA fabricated ODS-microspecimens at different temperatures. TEM investigations will be presented in order to show the localization of the particles in the microstructure.The nickel layers with the nano-sized aluminium particles are compact without voids and also with lower roughness whereas depositions with µm-sized particles yielded non compact layers. At the moment electrodeposited layers have been heat-treated at different annealing temperatures and durations in order to achieve a fine scale γ’-precipitation in the nickel-matrix. The content of the γ’-phase have been determined using XRD. The microstructure of the composite has been characterized using TEM. The results of these investigations available so far will be presented in addition.
5:30 PM - EE4.9
Bauschinger Effect in Unpassivated Freestanding Metal Films.
Jagannathan Rajagopalan 1 , Jong Han 1 , Taher Saif 1 Show Abstract
1 Mechanical Science and Engineering Department, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
The mechanical behavior of unpassivated freestanding metal films, subjected to uniaxial tension, was measured during several loading-unloading cycles. The results show that the stress-strain response of these films deviates substantially from linear elastic behavior during unloading, even at large overall tensile stresses. For example, in aluminum films (thickness 210-400 nm, grain size ~ 200 nm) the deviation from elastic behavior, indicative of reverse plasticity, starts at stresses as high as 150 MPa during unloading. As a result of this Bauschinger effect, the plastic strain in these films after unloading is often less than 50% of the expected value. Similar trends are seen in gold films, but the effect is less pronounced. The current understanding is that only passivated metal films, where reverse deformation is assisted by back stresses from dislocation pile-ups at the film/passivation layer interface, should exhibit Bauschinger effect. The Bauschinger effect observed in these unpassivated films thus suggests an alternate mechanism, which appears to result from the coupling of small grain size and heterogeneity of the microstructure. During loading, relatively larger grains deform plastically and relax their stresses whereas the stresses in smaller elastically deforming grains keep increasing, leading to a build up of internal stresses. During unloading, the internal stresses induce reverse plastic deformation in the larger grains leading to Bauschinger effect. This alternate mechanism of Bauschinger effect is supported by observations from in situ straining experiments in TEM.
5:45 PM - EE4.10
Kinetics of the Plastic Deformation of Gold during Nanoindentation.
Vineet Bhakhri 1 , Robert Klassen 1 Show Abstract
1 Mechanical & Materials Engineering, The University of Western Ontario, London, Ontario, Canada
Two stage indentation tests, involving a constant-loading rate stage, where the indentation shear strain rate γ*ind is very high, followed by a constant-load stage, where the γ*ind is low were performed on annealed and 20% cold-worked Au at 300 K to investigate the effect of indentation depth and initial dislocation density on the time-dependent plastic deformation process. Indentation data were analysed in terms of an obstacle-limited dislocation glide mechanism. The apparent obstacle activation energy ΔGo during low γ*ind associated with the constant-load stage was of the order of 0.16 μb3 and was neither a function of initial indentation depth, from h0 = 200 to 2000 nm, nor degree of cold-work. The apparent activation volume, V*, of the deformation process follows the same dependence upon dislocation density and effective indentation stress regardless of h0 or the degree of cold-work. The data from the indentation tests display linear dependence of inverse apparent activation area,1/Δa*, upon effective indentation shear stress,τind_eff, however the mechanical activation work, ΔW, which is the reciprocal of the slope of the 1/Δa* versus τind_eff trend, is larger during the high γ*ind constant-loading rate stage than during the low γ*ind constant-load stage of the tests. We observe also that ΔW is not a function of the initial depth, over the range from 200 nm to 2000 nm, or the material condition (i.e. the degree of cold-work). This suggests that more mechanical energy must be applied to the mobile dislocations to overcome obstacles in the microstructure during the high γ*ind compared to during low γ*ind.The dislocation structure within the indentation plastic zone of “crept” and “uncrept” indentations of h0 = 400 nm was investigated with TEM performed on foils cut from beneath the indentations using FIB micromachining. TEM observations indicate that during the high γ*ind stage (i.e. the “uncrept” indentation) of the test individual dislocations are visible throughout the indentation plastic zone while the dislocations recover into well developed cells during the subsequent one-hour low γ*ind stage (i.e. the “crept” indentation) of the test. The combined results of this investigation indicate that larger mechanical activation work, ΔW, is dissipated during constant loading rate indentation due to the large amount of work-hardening of the material in the rapidly expanding indentation plastic zone. The plastic zone expands at a much slower rate during constant-load indentation and the dislocation structure has the opportunity to recover by rearrangement into lower energy dislocation cell configurations.
EE5: Poster Session: Nanomaterials, Mechanical Behavior and Radiation Effects
Wednesday AM, December 03, 2008
Exhibition Hall D (Hynes)
9:00 PM - EE5.1
Microstructure, Mechanical, and Chemical Properties of Nanoporous Gold Films.
Dongyun Lee 1 , Jinwoo Ahn 1 , Hae-Sung Kim 1 Show Abstract
1 Nanomaterials Engineering, Pusan National University, Busan Korea (the Republic of)
Nanoporous thin film of pure gold, which is comprised of nanosized pores and ligaments regularly distributed across the volume of the film, was fabricated by dealloying technique. To obtain this film, multi-layers of two pure metals (gold and silver) with buffer layers (Cr or Ti) were deposited on silicon substrate by several deposition methods, thermal evaporation, e-beam evaporation, or sputtering system, and then the films were heat treated at elevated temperatures to attain homogeneous solid solution of Au-Ag alloy films. Dealloying was then performed to accomplish fabricating nanoporous gold films on silicon substrate. Dealloying was achieved in either concentrated/diluted nitric acid, HNO3 or controlled electrolyte with various chemical substances, like Perchloric acid, HClO4, or HNO3. Pore size and regularities of the films were verified under Scanning Electron Microscopy (SEM) equipped with Energy Dispersive X-Ray (EDX). Crack-free nanoporous gold films on hard substrate (Si) were fabricated by well controlled electrochemical dealloying process in specific range of alloying compositions. Mechanical properties of the films were investigated by nanoindentation; the primary results of mechanical tests show the elastic modulus and hardness of nanoporous gold films on Si substrate are about 10 GPa and 200 MPa, respectively. Implications for the performance of the nanoporous gold under certain environments containing sulfur, hydrogen, or carbon dioxides are discussed.
9:00 PM - EE5.10
Crystallization-induced Stress in Phase Change Random Access Memory.
Minghua Li 1 , Jianming Li 1 , Qiang Guo 2 , Luping Shi 1 , Yi Li 2 , Tow Chong Chong 1 3 Show Abstract
1 , Data Storage Institute, Agency for Science, Technology and Research (A*STAR), 5 Engineering Drive 1, Singapore 117608 Singapore, 2 Department of Materials Science and Engineering, National University of Singapore, 7 Engineering Drive 1, Singapore, 117576 Singapore, 3 Department of Electrical and Computer Engineering, National University of Singapore, Singapore, 119260 Singapore
GeSbTe (GST) alloys are of much interest as non-volatile Phase Change Random Access Memory (PCRAM) elements, based on their fast and repeatable switching performances. The structure reversible phase change provides the two logic states, which are represented by the crystalline phase (low resistivity) versus the amorphous phase (high resistivity). In addition to the huge resistivity change, the phase change process is accompanied by a significant volume change due to the difference of the density between the amorphous and the crystalline phases.This in turn results in large stresses during operation of the PCRAM device. The stress can generate defects and directly affect the p