We present a mechanism—recently discovered through atomistic modeling—whereby nano-scale cracks in polycrystalline solids undergo complete healing under externally applied mechanical loading. This mechanism occurs as a result of stress-induced grain boundary migration and may be explained through the mechanics of disclinations. Crack healing through this mechanism may occur under both mode I and mode II loading. The opportunity for designing fracture-resistant microstructures based on this mechanism will be discussed.

The relationship between grain size dependence of strength and the effect of irradiation defects on strength is investigated in polycrystalline metals with a crystal plasticity formulation. Concerns surrounding nuclear waste storage container degradation and improvements in nuclear reactor design highlight the need for a more comprehensive understanding of the strength evolution of irradiated metal polycrystals. Using Voronoi Tessellation geometries, a 3-D triple junction three grain crystal plasticity model of FCC irradiated copper is implemented in ABAQUS. Strain hardening is incorporated into the crystal plasticity formulation through a Taylor-type relation between shear strength and immobile dislocation densities. The mean free glide path length is a function of the densities of both statistically stored dislocations and geometrically necessary dislocations. Geometrical necessary dislocation densities are calculated from the strain gradient, with designations for both edge and screw dislocations. Radiation defects couple with dislocation densities to further inhibit the mobile dislocation motion. Defects from an initial irradiation exposure are included at the continuum level as a contribution to yield strength in the crystal plasticity model. We examine the compounding effects of irradiation defects on strain hardening patterns in a simple three grain model: increased strength near grain boundaries and other areas of high strain gradient are expected in the presence of irradiation defects. Results of these three grain crystal plasticity simulations are expected to show an increase in yield stress and strength of polycrystalline copper under increasing radiation doses.

In recent years, the use of computational materials science has drastically expanded due to increasing computing capability and new analytical approaches. Molecular dynamics (MD) simulations in particular allow the appropriate time and length scales to study the phenomena of interest in nanomaterials. Inorganic nanostructures such as nanowires are important morphologies of great scientific interest for future technological innovation. Nanowires have mechanical, electrical and optical properties that could make them useful in small-scale sensing and photonic applications. We have focused our research on the structure and properties of silica nanowires. We have performed MD simulations in accelerated computer environments to investigate the structure and mechanical properties of amorphous silica nanowires. We have integrated, developed and implemented a novel graphics processing unit (GPU)-based computing platform with high-speed computational research capabilities. This study was performed using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) code and the BKS empirical interatomic potential developed for silica glass. We have applied MD simulations to study the response of silica nanowires to elevated tensile and compressive loads. Oxide nanowires ranging in diameter from 5-15 nm are investigated. We derived the elastic modulus of the nanowires from stress-strain curves and examine their dependence on nanowire diameter. We have also quantified structural transformations via local ring size distributions in these nanowires. Results are compared with previous experimental and simulation findings as well as theoretical predictions. This investigation advances the field by enhancing the understanding of oxide nanowires and their mechanical properties, influencing applications and design of nanoscale devices, with implications in nanotechnology and photonics. Our computational environment is notably the first of its kind to be used in materials science research. This novel environment provides the opportunity to enhance materials research while promoting interdisciplinary collaborations.

The mechanical behaviour of steels is strongly related to their underlying atomistic structures which evolve during processing or thermal treatment and during their life cycles. In copper-alloyed α-iron, precipitates form on a relatively large time scale within the iron matrix, especially when operated at higher temperatures of above 300°C, yielding a change of the tensile properties. In order to model this complex material behaviour, we will present a sequential multiscale coupling scheme applying Kinetic Monte-Carlo (KMC), Molecular Dynamics (MD), Phase Field (PF) and Dislocation Dynamics (DD) simulations focusing on the transfer parameters between the respective simulation methods.
Nucleation, growth and early stage coarsening are simulated with KMC while further coarsening is accounted for with PF simulations. MD simulations provide the critical resolved shear stresses (CRSS) depending on the particle radius as well as structural information of the Cu precipitates which do not remain perfectly on bcc lattice sites when reaching diameters of several nanometers. The combination of CRSS values of the dislocation-precipitate interaction together with the grown precipitate arrangements form the basis for larger scale DD simulations yielding macroscopic tensile behaviour at different stages of thermal ageing. In this way, a computational modelling of tensile tests depending on a realistic precipitate distribution throughout the ageing process of copper-containing iron is achieved.

In the dislocation assisted plastic deformation of crystalline materials the strength is derived from the mobility, multiplication and interaction of dislocations with other dislocations and microstructure defects such as nanometric obstacles, secondary phase precipitates and grain boundaries. In body-centered cubic (bcc) materials the dislocations with Burgers vectors of b = ½a0<111> lying in {110} or {112} slip planes constitute the main slip systems. The intersection of dislocations in these slip systems may result in the formation of binary junctions with b = a0<100> that contribute to the strengthening of the material through forest hardening. In this study we use molecular dynamics simulation to study the formation of this binary junction due to the interaction of two ½<111> dislocations of edge and screw character in bcc-Fe. It appears that the <100> binary junction is formed in the glide plane of the edge dislocation and is oriented along <101> direction consisting edge character. In the formation of the binary junction, however, no constrain is imposed from the three fold glide planes of the screw dislocation. This is due to the emission of kinks along its line from the contact point of its line with the edge dislocation. The unzipping process of the binary junction is assisted by the formation of screw arms that may constitute screw dipole along the edge dislocation Burgers vector. Effects of temperature and strain rate on the formation and destruction of this junction was quantified by the critical release stress of the moving edge dislocation from the screw dislocation. Temperature decreases the critical dipole length and subsequently the critical stress needed to destruct the binary junction. With increasing strain rate, i.e. dislocation speed, the critical dipole length and release stress is increased. Comparison of the <100> binary junction with different nanometric defects shows that it induces a comparable strengthening effect to that of nanometric voids, Cr and Cu precipitates and different dislocation loops depending on their size at temperatures from 10 to 300 K.

Disordered ferroelectric (FE) systems including relaxors, morphotropic FE-FE-boundaries and FE-antiferroelectric remain Terra Incognitae of modern physics. While macroscopic behaviors are well explored and in fact utilized in multiple applications, it is the mechanisms underpinning unique properties of these materials and their emergence from atomic to mesoscopic scales that presents a challenge. In particular, many of these systems exhibit spatially organized structures on the intermediate length scales, including micron-sized clusters controlling electromechanical nonlinearity in the ferroelectric thin films with sub-50 nm grains, ~200 nm periodic structures in ferroelectric relaxors with sub-5 nm polar nanoregions, etc. In this presentation, I will summarize the results of our recent studies of structure and functionality of disordered ferroelectric systems using the synergy of piezoresponse force microscopy (PFM) and spectroscopy with macroscopic scattering techniques and phase field modeling. To get insight in the factors controlling these systems, we have explored phase evolution and dynamics model BiFeO3 based ferroelectrics in the bulk in the vicinity of FE-AFE boundary. We report the interplay between the domain structures corresponding to one order parameter and formation of mesoscopic domains of the complementary phase that give rise to slow time dynamics, intermittency of local spectra, and are amenable for direct observation. We are also using time-resolved synchrotron diffraction to investigate the details of electric-field induced structural transition at the boundary. These behaviors are directly compared with phase field modeling of material responses to localized and global stimuli. The universality of these behaviors for other systems with multiple order parameters and role of spatial confinement on these behaviors is discussed.
Research supported (SVK) by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division and partially performed at the Center for Nanophase Materials Sciences (SVK), a DOE-BES user facility

Combining metals at the nanoscale allows for tuning the activity and selectivity of catalysts. For Fischer-Tropsch synthesis, cobalt-copper composites increase the formation of oxygenated products without compromising the catalyst activity. However, copper and cobalt are practically not miscible on the macroscale. At the nanoscale, cobalt and copper can be intimately mixed by preparing core-shell structures, which opens new avenues to study their interplay in a catalytic reaction. So far, morphology evolution of these advanced structures has scarcely been studied, although it raises paramount questions about the stability of the nanoalloys under reaction conditions.
Here, monodisperse core-shell Cu-Co nanoparticles were exposed to syngas at 250C. The morphology evolution was studied with STEM-EDS and HRTEM. These techniques showed segregation of the two metals during the reaction, giving rise to copper-rich and cobalt-rich nanoparticles. This phenomenon was explained using in-situ core-level spectroscopy (ambient-pressure XPS and high-pressure XAS) conducted at the Advanced Light Source, Berkeley, CA. In these experiments, the nanocomposite was exposed to reactive gas mixtures (carbon monoxide, oxygen, hydrogen, syngas) in conditions relevant for catalytic studies (up to 300°C and 1 bar). XPS and XAS spectra were collected in situ. They highlighted the synergistic role of carbon monoxide (as a strong ligand for cobalt) and hydrogen (as a reducing gas) in the segregation process.

Materials exhibit unique properties at the nanoscale and the study of size-dependent melting phenomena is critical in understanding the thermodynamics of these systems. We have developed an ultra-sensitive, fast-scanning thin film nanocalorimetry device for the measurement of metal nanoparticles, polymer thin films and self-assembled lamellar crystals. Magic size formation occurs during indium deposition due to the stability of clusters with complete atomic shells. These magic sizes exhibit discrete size-dependent melting points. For self-assembled lamellar crystals of silver alkanethiolates (AgSR), incremental changes in the lamellar thickness are achieved by changing the number of carbons in the alkanethiol. A new synthesis method allowed us to control the number of layers on the AgSR lamella and we have achieved growth of monodisperse 1-layer and 2-layer AgSR. For single layer crystals, the melting point depends on the number of carbons in the alkanethiol. There is also a significant melting point depression between multilayer and single layer AgSR with discrete melting points dependent on the number of layers. These results highlight the importance of surface and interfacial free energies in the melting behavior of this layered lamella.

Quantum capacitance arises from the electronic interaction at the metal-insulator interfaces and only becomes apparent at the nanoscale. Experimentally, it has been reported that quantum capacitance is the dominating factor to the net capacitance in Cu-Cu2O-C co-axial nanowires and thin Au-BN-Au films. In the former system, the capacitance is enhanced by a factor of two, compared to the prediction by electrostatic theory. First principles calculations and continuum models offer insights into the electronic origin and engineering designs of quantum capacitors. Studying the interfaces and the exchange-correlation contributions reveals novel keys to enhance the net capacitance.

Size effects in mechanical properties of mostly metallic materials have been already thoroughly investigated. However, how the coupling of materials properties is related to size effects is a relatively new field.
This paper will focus on two case studies. The first describes the effect of external dimension on the maximum attainable elastic strain in semiconductors such as Si and Ge and how this is coupled to the electronic properties of the semiconductors. The second focuses on the effect of defect spacing on the mechanical and optical properties of intermetallic phases with special focus on purple Al2Au on thin film form. Both are then correlated to film thickness.

Near or below a critical length-scale, the classical laws governing mechanical behavior and the underlying mechanics deviate from the bulk and subsequently breakdown. Similarly, the thermal and electrical properties also breakdown, because the specimen dimension becomes comparable to the mean free path of current and heat carriers (electrons and phonons). While the current trend is to understand the mechanics behind such breakdown in single (mechanical, electrical or thermal), the present study aims to create a new direction by pointing out the striking ‘overlap among multiple domains&’ at the nanoscale.
To study the length-scale induced coupling among thermo-electro-mechanical domains, it is imperative to perform simultaneous characterization of all these domains while varying the stimuli. However, even single domain studies are challenging at the nanoscale. Hence the literature does not have any technique to study multi-domain physics simultaneously. To address this shortcoming, we present the design and microfabrication of a chip capable of performing mechanical, electrical and thermal characterization of ultra-thin films of any material that can be deposited on a conventional substrate. Here, integrated micro actuators apply mechanical load in freestanding tensile specimens. The specimens are also integrated with four micro-electrode, which allow four-point electrical and thermal (also known as the 3-omega) probing. In addition to quantitative studies, the technique also allows direct visualization through vistually all forms of microscopy. The 3 mm x 3 mm size of the chip results in the unique capability of in-situ testing in analytical chambers such as the transmission electron microscope (TEM). The basic concept is to ‘see&’ the micro-mechanisms while ‘measuring&’ the deformation and transport properties of materials and interfaces.
We present strain-thermal conductivity relationship for 100 nm thick (25 nm grain size) aluminum films. Under classical physics, there should be no such coupling because metal has no bandgap and hence mechanical strain does not appreciably alter electron scattering. For the first time, we present evidence of mechanical strain dependence on thermal conductivity in metallic thin films. In particular, we show about 50% reduction in thermal conductivity at about 1.5% strain. To explain such unprecedented coupling, we point at the unambiguous TEM observation of breakdown of the mechanical deformation mode at this grain size (average 30 nm) from 1D (dislocation) to 2D (surface diffusion and grain boundary) type. This results in order of magnitude increase in electron scattering. Since the metal grains also rotate as a function of strain, a strong strain dependence evolves. In bulk aluminum, mechanical deformation is accommodated by dislocations (weak electron scatterers) and also the grains do not rotate - hence there is no strain dependence of thermal conductivity.

Mechanoresponsive materials convert mechanical stimuli into optical, electrical or chemical signals. In this contribution we introduce cationic, fluorescently labeled polyelectrolyte brush, in which brush compression results in fluorecence quenching as a novel approach towards mechano-optical coupling. Sensitivity and lateral resolution of these systems are gauged using the so-called soft colloidal probe technique [1]: A soft elastomeric bead is attached to an AFM cantilever, such that it can be integrated into a combined AFM-Confocal Laser Scanning Microscopy (CLSM)system. The in situ combination with CLSM permits to correlate local pressure in the beads contact area with local fluorecence. Thus a fluorecence-response function for a range of pressures can be determined. We find a high pressure sensitivity in the order of (1 kPa) and a lateral resolution better than 1 µm [2], making these systems very attractive for studaing pressure distributions in complex contact or adhesion scenarios. We discuss perspectives in the field of bioinspired adhesion.
[1] Erath, J.; Schmidt, S.; Fery, A., Soft Matter 6, (2010) 1432.
[2] Bünsow, J.; Erath, J.; Biesheuvel, M.; Fery, A.; Huck, W. T. S., Angewandte Chem. Int. Ed., 50, (2011) 9629-9632

High quality epitaxial thin films of J_eff = 1/2 Mott insulator Sr₂IrO₄ with increasing in-plane tensile strain have been grown on SrTiO₃ (001) substrates. Increasing the in-plane tensile stress up to ~0.3% drops the c/a tetragonality by 1.2%. X-ray absorption spectroscopy detected a severe reduction of the linear dichroism upon increasing in-plane tensile strain towards a reduced anisotropy in the electronic structure. While the most relaxed film shows a consistent dependence with previously reported single crystal measurements, electrical transport reveals an energy gap reduction of ~ 200% for the thinnest and most epitaxy-distorted film. We argue that reduced tetragonality plays a major role in the electronic reconstruction, which reflects in the transport properties. This work sets up the stage for exploiting epitaxial strain as a systematic tool for both structural and functional manipulation of 5d correlated oxides. Doping of these epitaxy-distorted thin films is now being explored.

: Nanomaterials can exist in many different forms in macroscopic and microscopic scales. Well-designed (either by nature or artificially) nanomaterials are generally known to have highly enhanced properties. The key parameters in the material design process in many length scales would be the geometric size and the internal microstructure of the materials. Since the surface and interface effects become dominant in small length scale, it has to be systematically considered to predict the material properties in many length scales. In this study, we considered design parameters such as macroscopic size (thickness) and microstructures (from crystalline to amorphous) to address the macroscopic properties of inorganic nanomaterials. Classical molecular dynamics was employed to investigate the mechanical properties (modulus, toughness) of the nanomaterials, 2-dimensional nanofilms in particular. Some electrical properties of polycrystal semiconductors were also investigated for the grain size effect. It is shown that the material properties in nano length scales are dependent on the thickness and the internal microstructure as well. The numerical results are shown well fitted with analytical predictions for nanofilms by introducing relevant surface effects in small length scale.

Freestanding nanocrystalline Platinum thin films have been tensile tested in-situ in a transmission electron microscope. These films have a notch at the mid section in order to have stress concentration. Essence of reported work is to understand the material behavior under stress concentration. Grains sitting at the notch tip are found to show significant grain boundary motion and grain growth. There is some growth slightly away from the notch tip also. Some of the grains are found to grow 3 times their original area. From quantitative measurements we find that rapid grain growth occurred at ~350 MPa of far field stress and ~0.14% elongation. Growing grains are found to have remnant dislocations as well as steps from grain boundary migration. This behavior correlate very well with the dislocation model of grain boundaries resulting in grain boundary motion under applied stress.

In this presentation, we report on the plastic deformation mechanisms in Cu-Nb lamellar nanocomposites processed via Severe Plastic Deformation as a function of decreasing layer thickness. We utilize Accumulative Roll-Bonding (ARB) to process bulk Cu-Nb nanolamellar composites from 1 mm thick high-purity polycrystalline sheet down to layer thicknesses of 10 nm. This processing technique has the advantage of producing bulk quantities of nanocomposite material, and also exposes the interface and bulk constituents to large strains (1000&’s of percent). These extreme strains result in rolling textures, interfacial defect structures, and deformation mechanisms very different from those seen in nanolamellar composites grown via Physical Vapor Deposition methods. For instance, deformation twinning is observed in Cu in ARB material as opposed to PVD material. Evolution of preferred interfacial structures during processing will be linked to the structure, energetics, and kinetics of a given interface type. Mechanical properties and behavior will be discussed in terms of the effects of interfacial content on deformation processes at diminishing length scales, and defect/interface interactions at the atomic scale.

In this work the influence of Al solute content on the mechanical properties of Cu and binary Cu solid solution is investigated using strain rate controlled nanoindentation testing [Maier et. al. JMR 2011]. Used for this study are Cu and CuAl alloys [An et. Al. Scripta Mat. 2011] in single crystalline and nanocrystalline condition as obtained after severe plastic deformation by high pressure torsion. It is well known that small additions of Al lead to a strong reduction in the stacking fault energy in Cu. The reduced stacking fault energy, influencing dislocation cross slip is thought to be the main reason for the reduced grain size obtained after severe plastic deformation of these crystals. During nanoindentation testing of the SXs, a strong indentation size effect is observed, which moreover depends on the alloying content. Higher solute concentrations lead to an over proportional increase in the hardness at small indentation depths. Moreover strong serrations are observed during indentation of the highest solute content, indicating a Portevin le Chaterlier effect caused by dynamic strain aging. The observed strain rate sensitivity of these crystals is with the resolution limit of the indenter. The nc-materials exhibit a much higher strength compared to their SX counterpart, with increasing strength levels at increased solute content. The nc- structure leads also to an enhanced strain rate sensitivity, which also depends on the solute content. Interestingly, the strain rate sensitivity of largest solute content is the smallest, even though the material exhibits the smallest grain size. Besides grain size also other factors governed by solid solution strengthening are contributing to the strain rate sensitivity of the nc-alloys.

The effectiveness of a second phase in metallic glass structures like foams and composites to improve mechanical properties varies widely. Unfortunately, methods to fabricate such structures have insufficient control over the arrangement of microstructural features. The strength and purpose of our method is the ability to precisely vary individual microstructural features completely independent, and determine the effect on mechanical response [1]. It has been hypothesized that shear bands can be stabilized as long as the second phase spacing is less the critical crack length [2]. Conventional methods do not allow testing of this synopsis.
Our results revealed three critical aspects of a metallic glass heterostructure that controls mechanical behavior. The first one confirms the previous findings that difference between the critical crack length and the second phase spacing should be bigger than zero to attain global tensile ductility. However, this criterion alone is not sufficient in predicting performance of metallic glass heterostructures due to the effect of stress concentrations on the deformation mechanism. In addition, we present that the ratio of diameter to spacing of the second phase structures is another important criteria for toughness and tensile ductility optimization. We also argue that effectiveness of these heterostructures also depends on the size of the sample when compared to second phase spacing. This novel approach provides us with a versatile toolbox to manipulate the properties of the metallic glasses in a controlled manner, which leads to understand and improve existing heterostructures, and design novel structures with predictable properties.
[1] B. Sarac, J. Ketkaew, D. Popnoe, and J. Schroers, "Honeycomb Structures of Bulk Metallic Glasses," Adv. Funct. Mater., vol. 22, pp. 3161-3169, 2012.
[2] D. C. Hofmann, J. Y. Suh, A. Wiest, G. Duan, M. L. Lind, M. D. Demetriou, and W. L. Johnson, "Designing metallic glass matrix composites with high toughness and tensile ductility," Nature, vol. 451, pp. 1085-U3, Feb 28 2008.

This study reports on the influence of sputter-deposition conditions on the structural, electrical properties and chemical composition of refractory alloy (WTi) thin films for two thicknesses: 180 and 10 nm. WTi thin films have been deposited using a planar DC Magnetron sputtering apparatus from WTi alloyed target (70:30 At%) in pure Ar working gas, under working pressure ranged from 0.14 to 1.4 Pa, at constant power discharge, without substrate bias and external heating. Body-centered cubic W(x)Ti(1-x) solid solution thin films have been obtained, with x in the range 0.75

Soft contact lenses are fabricated from thin sheets of hydrogel materials, typically with maximum thickness around 300 mu;m and containing up to 65% water. There are a number of conditions that can affect the eyes of contact lens users and a number of these are believed to be associated with the relative elastic constants of the eye and the lens material. Conventionally, the mechanical properties of contact lens hydrogels are tested by cutting thin strips and testing in a tensometer. There is considerable interest in using point probe techniques such as tapping mode AFM or nanoindentation to characterise these material in order to determine whether the mechanical properties of the lenses changes with time in service or varies with location across the lens. Here we investigate the mechanical properties of a number of contact lens materials and also a model poly(HEMA) specimen with a range of cross-link densities prepared with specimen thickness from 50 - 1500 mu;m using nanoindentation under fully hydrated conditions. Data has been analysed using both a fully elastic model based on the contact stiffness obtained during unloading and also using a viscoelastic time dependent model for specimen loading and relaxation.
Using the elastic unloading model, a significant influence of specimen size was found for hydrogels with thickness < 300 mu;m. However, it is possible to correct for this using an empirical exponential function and use this to normalise the influence of thickness across a number of contact lenses of different optical strengths and hence radial height profiles. More remarkably, the correction factor is shown to be applicable to a number of different polymer composition hydrogels and water contents. Using this analysis the mechanical properties of all the hydrogel contact lens material;s studied were found to be very close to those reported by the manufacturers for bulk tests. The contact lenses all showed a small reduction in elastic modulus with radial distance from the lens centre but this did not correlate with lens thickness.
The nanoindentation data was also analysed using a viscoelastic deformation model based on a step integral method [1]. This analysis produced higher values of elastic modulus (both instantaneous and relaxed) than were found with the elastic unloading analysis. However, this analytical method produced much greater experimental scatter than was found with the data after elastic analysis, suggesting thta the viscoelastic analysis used may be sensitive to initial conditions. This experimental scatter made it impossible to determine whether there is any significant size effect observed with this analysis. The relaxation time constants identified appear to show no significant dependence on specimen thickness or indenter radius, which would be expected if these hydrogels deformed by a porelastic mechanism.

It is well established that sample dimensions are important in the mechanical and fracture behavior of engineering materials. Size-dependent mechanical properties become increasingly important as materials are used in devices and technologies that are scaled to smaller length scales. While well known in metals, this size-scale effect is often overlooked or unknown in the soft organic materials ubiquitously used in emerging technologies. In particular, thin polymer membranes used in flexible electronics, batteries, and fuel cells present particular challenges regarding their mechanical behavior and tearing resistance when confined in such small structures. For example, polymer proton exchange membranes in fuel cells are constrained by rigid O2/H2 gas channels. When tested as unconstrained films, these membranes exhibit extensive viscoelastic flow that often extends without limit into the plane of the membrane. However, when used in small device structures, the associated mechanical constraint significantly limits flow and reduces fracture resistance. Moreover, the length scales over which these effects occur are often very different to those observed for metals.
In this research we discuss the role of mechanical constraint and resulting size-dependent fracture behavior for a number of different polymer membranes. Fracture behavior was quantified in terms of the “tearing energy” measured using a modified form of an out-of-plane tearing test. Mechanical constraint was imposed over a wide range of length scales defined with respect to the membrane thickness. Membranes composed of a low tearing resistance perfluorosulfonic acid (PFSA) polymer, a common proton exchange material, and a much higher tearing resistance low density polyethylene (LDPE) polymer, were characterized under varying levels of constraint. We demonstrate that without constraint, the tearing resistance increases monotonically with tear propagation as viscoelastic flow increases in the membrane. On the other hand, with increasing levels of mechanical constraint, we show that the tearing resistance decreases dramatically and stabilizes at a constant tearing resistance dependent on the level of constraint. The role of constraint is discussed in terms of the tearing mechanism and the form of molecular relaxation and flow. The added effects of environmental and temperature parameters that may relate to the operating environment of the membrane are also explored. We describe how the role of such in-situ mechanical constraint is integral in characterizing the mechanical and fracture behavior of polymer membranes in emerging device technologies.

Micro- and nanofibers are important structural elements in many functional materials where one of their main tasks is providing mechanical stability. Characterizing these small-scale fibers requires highly advanced techniques beyond standard methods for macroscopic materials.
In our contribution, we focus on nanoscale bending of free-standing fibers using the Atomic Force Microscope. It is a versatile approach that is suitable for a wide variety of one-dimensional fiber systems. We present bending experiments perpendicular and parallel to the substrate plane and discuss major advantages of these two bending modes, for example validation of boundary conditions, direct integration of optical methods and detailed investigation of the mechanical properties beyond linear elastic deformations. For the interpretation of the data, we use analytical as well as finite element models.
In particular, we investigate supramolecular 1,3,5-Benzenetrisamides (BTAs), which offer unique possibilities in terms of structural control: They allow combining the complementary advantages of bottom-up and top-down techniques, since they form well-defined fibers by self-assembly and melt electrospinning. We show that the morphology of self-assembled BTA fibers can be tailored via the trisamide substituents. Using bending experiments, we establish mechanical structure-property relations and distinguish between size and material contributions. Furthermore, we compare self-assembled and electrospun fibers from the same BTAs and demonstrate that regardless of the preparation pathway, the fibers possess a remarkable mechanical stiffness. This is a striking result, since only supramolecular interactions and no intermolecular covalent bonds are present in these fibers.

Bio-inspired materials have been center of attraction by researchers in the last years due to their intricate and regular microscopic architectures with nanometer detailed morphologies such as those found diatoms. Diatoms are unicellular algae that feature intricate micro-size cell walls made of amorphous silica (SiO2). Due to the nanopatterned cell wall design of their structures, and the possibility of being created in the lab at ambient conditions with minimal energy usage, diatom bio-structures have been widely study and suggested for potential nanotechnology applications. In particular, diatom structures contain specie-specific regular shapes and symmetric porous distributions which suggest promising applications as drug delivery carriers, optical sensors, actuators, catalytic components and self assembled devices. In this work, we have calculated the mechanical response of bio-inspired diatom structures by performing Atomic Force Microscopy (AFM) mechanical measurements and Finite Element Method (FEM) simulations. We have chosen two diatom species based on well-established biological classification (based on their symmetric and pore arrangement) and different conditions for this study. These diatom structures had pores diameters ranging from 0.3 to 6 mu;m and their contours from 20 to 30 mu;m. AFM nanoindentation tests were performed to calculate the Young&’s modulus of the diatom structures. FEM simulations were also performed on 3D diatom structures (using the Young&’s modulus obtained from AFM results) to correlate their mechanical behavior on different geometric variables (e.g. number of pores, pore diameter and pore arrangement). We then calculated von Mises stress and displacement distributions to analyze the effect of loading conditions. Results were compared with recent AFM experiments and relevant simulations performed on similar silica structures. This research contributes to improving the understanding of the mechanical response of bio-inspired materials such as diatoms and it represents a step towards their future applications in photonics, self-repair devices and templates for nanotechnology applications.

In this talk, I will report our recent research progress on probing the mechanical behavior of submicro-sized single crystal metals. It was found that prior to the compression tests, the <111> orientated single crystal face centered cubic (FCC) nickel pillars fabricated through Focused Ion Beam (FIB) contained a high density of defects. However, quite unexpectedly, the dislocation density was observed to decrease dramatically during the deformation process and, in some cases, even resulted in a dislocation-free crystal. The phenomenon, which we termed as “mechanical annealing”, is the first direct observation of dislocation starvation mechanism and sheds new light on the unusual mechanical properties associated with submicron- and nano- scale structures. Similar phenomena were also observed soon later in in FCC structured Al and Cu metals. However, because of the dislocation core structure difference between the screw and edge dislocations in body centered cubic (BCC) metals as well as the weaker size strengthening behavior, it was believed that BCC metals are incapable of mechanical annealing. By fabricating the Mo single crystal pillars into an unprecedented smaller scale and employing in situ compression technique inside a transmission electron microscope, we demonstrate that significant mechanical annealing does occur in BCC Mo. In addition, there exists a critical size (~ 200 nm for Mo at room-temperature) below which the strengthening exponent in Hall-Petch like regression increases dramatically to that similar to FCC metals. Thus, a new regime for size effects in BCC is discovered that converges to that of FCC, revealing deep connection in the dislocation dynamics of the two systems. We attribute the observed phenomena to the diminishing importance of lattice friction at high stresses, when the size-enhanced flow stress exceeds a single screw dislocation&’s lattice friction. Further, by applying cyclic tensile straining on a submicro-sized aluminum single crystal, we demonstrate that along with the increase of the cyclic straining cycles, preexisted defects will be driven out of the crystal progressively and can eventually generate a very clean crystal without any observable defects. We termed this as fatigue healing. Unlike the mechanical annealing phenomena reported previously, the main advantage of fatigue healing is that it can clean the crystal with initial high density of defects without changing the geometry of the samples obviously. In addition, the samples treated with fatigue healing show much higher yield stress. Our findings are expected to find applications in industries that need clean crystals.

Specimen size and grain boundaries are both known to influence mechanical properties. The combined effect of specific grain boundaries with micron-scale sample dimensions can be studied by compression testing of pillars with defined grain structure.
In this study, single-, bi- and polycrystalline pillars down to 500 nm are investigated by in-situ SEM and AFM compression experiments and electron backscattered diffraction at different stages of the deformation. General experimental constraints in pillar compression testing are studied by different conditions of aspect ratio, taper and frictional contact between tip and pillar. Preferential conditions for a reliable evaluation of strain hardening are found to be a taperless pillar shape, high lateral compliance of the system and cyclic loading for reduced lateral constraints between sample and indenter.
A novel setup for in-situ AFM imaging during the compression of micron-sized pillars is introduced to investigate the deformation at the pillar surface. Surface steps of sub-nm size are detected and the activity of individual slip planes is monitored over the deformation process, allowing for unprecedented characterization during pillar compression. Specimen with large grain size or single-crystalline pillars deform by delocalized dislocation slip. The dislocation activity at individual slip planes is observed to saturate at step sizes of 40 nm for micron-sized pillars. Bi-crystals with a high-angle grain boundary impose an additional constraint on the deformation, which leads to the activation of harder slip systems, residual orientation gradients and dislocation interaction, explaining the increased strain hardening behavior. For polycrystalline samples with decreasing grain size, a transition from dislocation slip to grain boundary mediated processes is observed by in-situ AFM, while grain boundary sliding dominates the behavior for nanocrystalline pillars.
The influence of initial microstructure on the deformation behavior will be discussed in light of the current theories on size effects to gain a better understanding towards materials with high strength and ductility.

In this study the length scale dependence of the operative mechanisms of time-dependent plastic deformation in high purity (99.99%) Au was studied using compression tests performed on micro-pillars and micro-spheres of less than 6 mu;m size. Micro-pillars of various initial diameters ranging from 1 to 5 µm were fabricated using focused ion beam milling from a polished poly-crystalline Au sample. Constant loading rate uniaxial micro-compression and constant compressive load creep tests were performed on the pillars at room temperature. The flow stress of the micro-pillars was highly dependent upon the pillar diameter and increased with decreasing pillar diameter. Constant load creep tests, of 1800 to 3600 second duration, indicated that the deformation process is highly non-uniform and displayed numerous strain jumps with the jump frequency increasing when the pillar diameter is small. Electron backscatter diffraction indexing of the crystal orientation allowed us to relate the strain jump frequency to the resolved shear stress applied to the active slip planes within the deforming micro-pillar.
A different, and fairly new, approach to analysing the time-dependent deformation mechanism was undertaken by performing constant loading rate and constant load deformation tests at room temperature on singe-crystalline Au micro-spheres of 2 to 6 mu;m diameter. The micro-spheres were fabricated using e-beam lithography, sputter-deposition, followed by in-vacuum annealing at 1000°C. Based upon our findings from these tests we describe several features that are related to the apparently stochastic mechanisms of time-dependent plastic deformation of fcc materials, such as Au, at small length scales.

Cylindrical pillars in the diameters of 250 nm to 4 µm are produced by focused ion beam (FIB) from <100>-orientated NaCl, KCl, LiF and MgO single crystals. Uniaxial microcompression technique is used to study the mechanical behaviors of those pillars. A prominent size-related plasticity is observed, showing material flow strengths increase with decreasing pillar diameters. The compressive size effects for NaCl, KCl, LiF and MgO pillars, as evaluated by the log-log slope of normalized strength vs. pillar diameter, are -0.64± 0.02, -0.72± 0.02, -0.68± 0.02 and -0.8± 0.03, respectively. These ionic crystals exhibit very similar size-dependent effect to face centered cubic (fcc) metals. The size dependence correlated with Peierls stresses and critical temperatures is applied to compare ionic crystals with metals. In addition, LiF and MgO pillars exhibit higher normalized stresses than NaCl and KCl pillars, which might be attributed to relatively lower ion beam induced defect densities in LiF and MgO. The microcompression results also show that this size-related plasticity in NaCl is independent of the temperature from 300 K to 500 K and the loading rate from 1 to 40 nm s-1.

We have investigated the deformation of single crystal Au nanowires using in-situ tensile testing in the TEM and the SEM. Both the evolution of the defect morphology and the stress-strain behavior of the 20 nm to 300 nm diameter nanowires have been studied. Two different deformation mechanisms are inferred from the defect morphologies: (1) surface nucleation of partial dislocations leading to the storage of stacking faults and layer-by-layer growth of nanotwins, and (2) surface nucleation of full dislocations resulting in interactions and dislocation tangles, as observed in bulk specimens. It will be shown that surface facets, stress state, and initial defects are more important in determining the deformation mode in nanoscale samples than the actual sample size. A quantitative nucleation rate model will be presented which can be used to predict the active defects, the nature of defect storage, and the flow stresses.

It has been reported that the time-dependent plastic deformation, often referred to as creep, can be more active at the small scale and can occur even at room temperature in some materials (e.g., nanocrystalline materials and amorphous alloys). Analyzing the creep in the small-scale can be valuable not only for solving scientific curiosity but also for obtaining practical engineering information about the lifetime or durability of advanced small-scale structures or devices. In this work, we suggest two novel ways to estimate the nano-scale creep. Through the novel methods, we could successfully estimate the time-dependent plasticity of nanomaterials and discuss it in terms of stress exponent and activation volume. *This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2010-0025526).

Hydrogen is considered to be an alternative energy source significantly reducing the amount of pollutants, such as greenhouse gas. A cost-effective infrastructure is required to transfer hydrogen from the production site to the point of use. Possible use of the existing natural gas (or crucial oil) pipeline has been considered for hydrogen transmission. However, hydrogen may lead to a serious degradation of mechanical properties in the pipeline steels. To gain a better understanding of the hydrogen effect, here the small-scale mechanical behavior of hydrogen-charged API X70 pipeline steel was systematically investigated through nanoindentation experiments. Somewhat interestingly, the influence of hydrogen on the nanohardness was found to be seriously affected by indenter sharpness; i.e., hydrogen enhanced the hardness during Berkovich indentation, whereas hydrogen-induced softening was observed during (sharper) cube-corner indentation. These contradictory results are discussed in terms of the changes in applied stress and plastic zone size with different indenter angle. *This research was supported by the Korea Research Council of Fundamental Science and Technology (KRCF) through National Agenda Project

Abstract
Development of high-energy density dielectrics with low temperature coefficients of capacitance that are integratable with underlying electronics are needed for extreme environment, defense and automotive applications. The presentation will emphasize sintering of Ca(Zr0.985Ti0.015)O3 ceramics from fine powder derived from mixed oxide method. Highly dense and fine grain Ca(Zr0.985Ti0.015)O3 ceramics were obtained using 2-step sintering profiles. The dielectric properties, including dielectric constant, dielectric loss, electrical breakdown and temperature stability, will be reported and correlated to data from microstructural analysis.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Lightweight porous metallic materials are generally created through specialized processing techniques. Their unique structure gives these materials interesting properties which allow them to be used in diverse structural and insulation applications. In particular, highly porous Al structures (Al foams) have been used in aircraft components and sound insulation; however due to the difficulty in processing and random nature of the foams, they are not well understood and thus they have not yet been utilized to their full potential. The objective of this project was to determine whether a relationship exists between the relative density (porous density/bulk density) and the mechanical properties of porous Al structures. To this end, a combination of computer simulations and experiments was pursued to better understand possible relationships. A Finite Element Method (FEM)-based software, COMSOL Multiphysics 4.3, was used to model the structure and to simulate the mechanical behavior of porous Al structures under compressive loads ranging from 1-100 MPa. From these simulated structures, the maximum von Mises stress, volumetric strain, total strain energy, displacement and strain energy density were calculated. These simulation results were compared against data from compression experiments performed using the Instron Universal Testing Machine on porous Al specimens created via a computer-numerically-controlled mill. CES EduPack software, a materials design program, was also used to estimate the mechanical properties of porous Al and open cell foams for values not available experimentally, and for comparison purposes. This program allowed for accurate prediction of the mechanical properties for a given percent density foam, and also provided a baseline for the solid Al samples tested. The main results from experiments were that the Young&’s modulus (E) values for porous Al samples (55.8% relative density) were 15.8-16.6 GPa depending on pore diameter, which is in good agreement with the CES EduPack predictions; while the compressive strength (σc) values were 155-185 MPa, higher than those predicted by CES EduPack. The key finding from FEM simulations using 3D models (55.8% relative density) was the onset of yielding at 13.5-14 MPa, which correlates well with CES EduPack data. Overall results indicated that a combination of experiments and FEM simulations can be used to calculate structure-property relationships and to predict yielding and failure, which may help in the pursuit of simulation-based design of metallic foams. In the future, more robust modeling and simulation techniques will be explored, as well as closed cell Al foams and different porous geometries. This study will help to improve the current methods of characterizing porous materials and enhancing knowledge about their properties for alternative energy, use while promoting their design through integrated approaches.

Processing of materials during casting and how the solidification affects the resultant
microstructure and ultimately its mechanical performance is of great interest to Los Alamos
National Laboratory and the materials science community in general. This presentation will
focus on using synchrotron X-ray radiography and tomography for the real-time in-situ
examination of Al-Cu eutectics and Al-In monotectics during melting and solidification as well
as using laboratory based micro and nano scale X-ray tomography for post-mortem analysis. Xray
computed tomography on multiple length scales provides an opportunity to explore the
morphological results to the microstructure of materials as a result of solidification conditions.
The real-time examination of materials during melt and solidification at 32-ID-C at the
Advanced Photon Source (~2.7 mu;m voxel size) using an RF generator and graphite rod coupled
to the sample foils and rods which allowed for directional solidification for both the thin sections
(200 micron thick) for radiography and thin, 1-mm posts for tomography were demonstrated.
High speed tomography with a camera frame rate of 2.8 ms and one rotation per second
produced one complete CT in 0.5 seconds. Post-mortem examination of the materials using both
micro- and nano-scale laboratory based X-ray tomography instruments was also performed. With
micro tomography, high-resolution radiography was obtained of the solidified foils, then they
were cut down to posts for micro CT, and finally again into 60-mu;m diameter posts for nano
tomography. Co-registration of the micro scale and nano scale X-ray images was possible. Our
laboratory based nano tomography instrument has a 65 mu;m field-of-view, and a 50 nm voxel
size, giving a 3D spatial resolution of approximately 150 nm. Using micro-scale radiography and
tomography allows us to see differences in the bulk structure such as coarsened dendrites in
hypoeutectic Al-Cu, but when X-ray CT is applied on the nano-scale, a 2-mu;m interdendritic
structure of lamella can be visualized and the resultant structure due to solidification rate can be
compared. With decreasing solidification rate, the eutectic takes on a coarser structure.
Interestingly, post-mortem images of Al-In eutectics show structures that are very similar in
geometry, but have increasing Feret Shape with increasing size.

The goal of this study was to evaluate and predict the mechanical properties of sandwich structures, consisting of two 6061 Al sheets and a polyurethane foam core, as a function of three different foam densities (62 kg/m3, 160 kg/m3, and 320 kg/m3). The sandwich structure dimensions were 12 mm x 25 mm x 390 mm, and load levels ranged from 2000-3200 N as previous independent experiments. Computer simulations and experiments were carried out in this project in order to create an efficient procedure for future design considerations. A Finite Element Method (FEM) program was used to predict the critical buckling load of the sandwich structure (individual components and entire system) when subjecting it to uniaxial compression loads. The results were compared with previous independent experimental findings. From the FEM simulations, other mechanical properties were analyzed including the maximum von Mises stress, displacement, and volumetric strain among others. An experimental three-point bending test was also conducted using a Universal Instron Machine to measure the mechanical properties of the sandwich structure. Predicted results showed that the critical buckling load increases as foam density increases, implying that the density and strength of the foam in the sandwich structure have a direct relationship, and affect the overall properties of the entire foam-sheet system. A design-based software was also used to estimate the mechanical properties of other similar sandwich structures. In brief, simulations and experimentation combined can be a powerful tool for the study, prediction and design of new or enhanced materials. The original motivation for this project was to evaluate the possible use of this sandwich structure in a Small Formula One Car for the Society of Automotive Engineers (SAE) at UC Merced. Future work will be pursued on the feasibility of these sandwich structures as vehicle impact absorbers in particular. This project has also implications in areas such as allowing future automobiles to be lighter and safer, which in return would increase fuel efficiency.

Single crystal superalloys are used in the blades of advanced gas turbines, which operate in the creep range where they have to withstand mechanical loading at elevated temperatures. Creep is characterized by the evolution of strain with time, which limits service life of high temperature components. Most importantly, creep rates strongly depend on stress and temperature. It is well known that creep is governed by dislocation glide and climb processes which have been observed in the TEM and which are incorporated in micromechanical models. In the present study we use discrete dislocation dynamics (DDD) simulation to study the evolution of dislocations in a typical #61543;/#61543;&’ microstructure of a single crystal superalloy under different loading conditions. A hybrid glide-climb mobility model is used to conduct the interaction of dislocations with #61543;&’ particles. We focus on the early stages of creep, where dislocation plasticity is confined to narrow #61543; channels. In a first order approximation we assume that our system has no misfit and that the #61543;/#61543;&’ microstructure is stable. Using the implemented dislocation glide-climb mobility it appears that at elevated temperatures, i.e. when climb rates are high, the creep strains accumulated even for resolved shear stress which are smaller than the critical stresses required for dislocations to enter into the #61543;-channels. Simulated creep microstructure consists of long bent dislocations which form complex networks. At higher stresses, however, dislocations can penetrate the #61543; channels and deposit 60° mixed or screw dislocations at the #61543;/#61543;&’ interfaces. These observations are in a good agreement with previous experimental findings. In the present study an effort is also made to study the influence of crystallographic loading direction on creep.

One dimensional metal boride nanostructures have attracted tremendous attention due to their outstanding chemical inertness, high temperature stability, excellent mechanical properties, and low thermal expansion coefficient. Zirconium diboride (ZrB2) is a highly covalent refractory ceramic material with a hexagonal crystal structure. ZrB2 is an Ultra High Temperature Ceramic (UHTC) with a melting point of 3246 °C. This along with its relatively low density of ~6.09 g/cm3 and good high temperature strength makes it a candidate for high temperature aerospace applications such as hypersonic flight or rocket propulsion systems. In addition, ZrB2 can be used in nuclear area, such as fuel rod cladding, structural and flow mixing grids, instrumentation tubes, and guide thimbles. In this abstract, we report the facile synthesis, structural, and mechanical characterization of ZrB2 nanorods. Single crystalline ZrB2 nanorods were synthesized for the first time via a facile route at a relatively low temperature of 800 °C. The X-ray diffraction (XRD) analysis revealed that the as-synthesized nanorods have hexagonal phase of ZrB2 with lattice constants of a= 3.17, b= 3.17 and c = 3.53 Å, and space group of P6/mmm. The high-resolution transmission electron microscopy (HRTEM) characterization showed that individual ZrB2 nanorods are single crystals with the growth direction along the [001] orientation. Nanoindentation tests were performed directly on individual nanorods to probe their mechanical properties. The elastic modulus of the ZrB2 nanorods was measured to be 110.5 GPa, and the hardness 11.6 GPa. Such structural and mechanical information provides design guidelines for developing ZrB2 nanostructure-based nanodevices and nanocomposites, and lays a constitutive foundation for modeling the nanostructures of ZrB2 and other boride nanostructures.

Utilizing architectural features as key elements in defining multi-dimensional material design space promises to enable independent manipulation of the currently coupled physical attributes and to develop materials with unprecedented capabilities. Creation of extremely strong yet ultra-light materials can be achieved by capitalizing on the hi-enot;rnot;anot;not;rnot;chical design of nanostructured metallic lattices which promise superb thernot;mo-mechanical pronot;not;not;pernot;not;ties at exnot;trenot;menot;ly low mass densities (lighter than aerogels), making these solid foams ideal for many scientific and technot;nonot;lonot;ginot;cal applications. Yet, the dominant denot;fornot;mation mechanisms in such “meta-materials”, where individual constituent size (nanometers to microns) is companot;rable to the characteristic length scale of the material, are essentially unnot;known. To harness the lucrative properties of 3-dimensional hierarchical structures, it is critical to assess mechanical properties of constituent materials at each relevant scale while capturing the overnot;all structural complexity.
We present the fabrication methodology and mechanical properties of nano-trusses, or 3-dimensional hollow metallic and ceramic lattices, whose solid constituent dimensions are on the order of tens of nanometers, tube diameters are below 1 micron, and unit cell sizes are ~ 2 microns. Compression experiments are performed in an in-situ mechanical deformation instrument, SEMentor. Focus is on the interplay between the internal critical microstructural length scale of materials and their external limitations in revealing the physical mechanot;nisms governing the mechanical deformation, where competing material- and structure-induced size effects drive overall properties. As an example of coupled effects of microstructural and extrinsic nano-dimensions, we report a FIB-less nano-fabrication technique for nanocrystalline platinum nanopillars and present experimental and computational results on the tensile, fracture, and compressive response of these structures.

Changing the size of a structure from the dimension we are familiar with to extremely small scales changes the order of governing physical phenomena to determine mechanical behavior. By specific micro-architecture one can take advantage of the occurring mechanical size effects. Such structures can reach a level of strength up to several times higher than analogous macroscopic structures and may in relation exceed the mechanical properties of bulk material.
Using 3D laser lithography microstructured metamaterials with new combinations of strength and low density have been fabricated. Thereby several structures of high strength and stiffness were transferred to the micrometer length scale. Polymeric templates were applied to produce several metallic and ceramic structures by means of coating techniques such as electroplating and atomic layer deposition. Stability problems such as buckling of shells and columns as well as the mechanical mechanisms predominating the material behavior were analyzed.

A vast variety of techniques are readily available to design and fabricate uniform, structural components via rapid prototyping manufacture. Still, no means are currently accessible for the additive layer processing of structurally efficient fibre reinforced polymer composites. At present, difficulties arise prevalently from the impracticality of using continuous fibre strands within a rapid prototyping framework, and the poor mechanical performance generally observed with short fibre filled materials.
One possible approach to overcome the limitation of short fibres is to control staggered periodicity of the fibres, as observed in biological fibrous systems, such as collagen. This approach would also facilitate their incorporation into the rapid prototyping environment, providing the fibre architecture can be controlled and manipulated through an external field.
In this study, the application of an ultrasonic field effect technique is employed to control short fibre orientation. While acoustic levitation techniques are most commonly studied within the biological and medical frameworks, a number of authors have also reported the successful casting of ultrasonically arranged particles inside various matrix media. Further, additional control over reinforcement distributions, potentially leading to increased structural efficiency and growing flexibility in the processing of future composite materials may be achieved. Finally, engineered reinforcement across all three spatial dimensions is thought to decrease the susceptibility of current composites to impact damage, wear, longitudinal micro-buckling, delamination, and fatigue.
There are currently four distinct approaches available for the generation of acoustic radiation forces: mode switching, focused ultrasonic beams, linear arrays of transducers facing a reflective surface, and the emission of counter-propagating travelling waves. In the present context, counter-propagating devices are considered the most adequate as potential changes in the system's resonant frequency - due to the presence of large numbers of fibrous entities - can be avoided. Further, nodal positions are not fixed but by altering the phase relationship, particles can easily be moved along the `x-y plane' in an arbitrary fashion to a predetermined location. Piezoelectric transducers are chosen as the acoustic driver, due to their wide band of operating frequencies, high electroacoustic efficiency, and economic cost.
This technique has the potential to mimic many aspects of nature&’s fibre architecture and to develop the next generation of bio-inspired advanced fibre reinforced composite structures. Already, theoretical models predict the instantaneous formation of aligned reinforcement patterns that are dependent primarily on the external ultrasonic field applied. These predictions are validated in experiments, and composite samples can be made and tested.

A special case of size-effects is observed in miniaturized specimens where the microstructural length scale i.e. average grain size (dg) and a structural length scale, typically thickness (t), are similar i.e. there are just a few grains across the limiting specimen dimension. In such cases, we report strongly coupled t-dg effects on plastic deformation characteristics. While previous studies have reported weak effect of grain boundaries on plastic deformation in such cases, we show that for dg ~11µm - 45µm in annealed Cu foils under uniaxial tension, the magnitude of thickness effects on strength and ductility is strongly dependent on grain size. From a strength stand-point, thickness effect is more prominent in specimens with smaller dg where microstructural constraint is high. In all cases tested in this study, reducing specimen thickness was found to reduce foil strength. However, the overall yield/flow stress did not follow a unique relation with either t or dg, and instead, depends on t/dg ratio. A phenomenological model is presented to account for the coupled t-dg effects on yield strength. From a ductility stand-point, we show that both, thickness and grain size control strain to failure. Reducing thickness, for constant dg, tends to reduce the strain to failure due to pre-mature strain localization as a result of surface roughening. Similarly, for a constant t, reducing dg also reduced ductility in this study. Post-mortem investigations of the fracture surface revealed the existence of both intergranular and intragranular fracture modes. Large grains mostly appear to fail via grain thinning in miniaturized specimens. In contrast, smaller grains (or specimens with t > dg) showed considerable evidence of intergranular failure and the presence of voids at the fracture surface, even in specimens with t = dg.

(*M.R. and Y.M. contributed equally to this work.)
We present a self-cleaning gecko-inspired micro-fibrillar adhesive with adhesion and contact self-cleaning performance comparable to the gecko's toes. In order to ensure reliable adhesion between their molting cycles, geckos have evolved remarkable mechanisms to keep their toes clean. Until now, the strongest synthetic gecko-inspired adhesives had not matched their natural counterparts in this key characteristic. We present a synthetic gecko-inspired adhesive that can rapidly regain adhesion lost due to contamination through a process of contact self-cleaning. Furthermore, the presented adhesive exhibits attachment strength nearly equal to the gecko&’s toes.
Using an established fabrication process combining soft mold casting and dipping, three types of mushroom-shaped elastomeric adhesives were created. After contaminating the adhesives with dirt particles of varying sizes self-cleaning experiments revealed that the ratio of fiber dimensions and size of the dirt particles plays a key role in contact self-cleaning. Under ideal conditions, we observed the gecko-inspired adhesives exhibit attachment strength of 140 kPa and adhesion recovery up to 100%.
Finally, we developed a theoretical study describing contact self-cleaning through the rolling and sliding of the contaminant dirt particles. The model suggests there are optimal loading conditions that enable contact self-cleaning.

Bulk single crystal silicon is brittle at room temperature, but it behaves as a ductile material when its temperature exceeds 540C, a phenomenon known as brittle to ductile transition (BDT) temperature. It has been postulated that BDT is size dependent. Several experiments have been reported where either temperature was varied for a given specimen size, or size was varied at room temperature. The results were inconclusive and often contradictory. Here we carried out bending tests on single crystal silicon samples with varying geometry (10 micro meters to 700 nm) and temperatures (room temperature to 400C). Tests were carried out in situ in scanning electron microscope. Our results show that BDT is indeed size dependent. It decreases with size, e.g., it is 293C for 720 nm sample. We propose a mechanism based model to explain our observations. The mechanism is as follows. Ductility is mediated by dislocation dynamics. They initiate from the surface and the bulk. Activation energy for initiation from the surface is lower compared to that from the bulk. Increasing temperature helps overcoming the barrier for both, and silicon can be plastically deformed with stress lower that brittle fracture stress. But, with decreasing size, the surface to volume ratio increases. Thus, there is increasing fraction of surface dislocations that contribute to plasticity. Due to their lower activation barrier for initiation, they can be generated at lower temperature. Hence BDT decreases with size.

Titanium layers can be used to promote adhesion between polymer substrates for flexible electronics and the Cu or Au conducting lines. Good adhesion of conducting lines in flexible electronics is critical in improving circuit performance and increasing circuit lifetime. In this study, the thickness and the response at high temperature of Ti adhesion layers on polyimide were examined. The coatings exhibit a stark transition in mechanical properties in both cases, as evidenced by uniaxial straining of blanket coatings. As the layer thickness of Ti coatings increases from 10 nm to 50 nm, the cracking of the coatings transitions from very straight, parallel cracks to very jagged ones. Cross-sectional TEM shows the presence of an amorphous interlayer, of constant thickness, between the Ti and PI, in addition to a thin surface oxide layer, also of constant thickness. A very similar transition in crack morphology is observed with the as-deposited samples having jagged cracks and films strained at 350C had very straight cracks. In this case a sharp transition in coating adhesion was observed, the adhesion of the as-deposited Ti being around three times greater than that of the others. Cross-sectional TEM revealed that while all samples had an interlayer, nanocrystallites were observed in the interlayers of the 350C tested films. This result points to the thermodynamic instability of the interlayer Ti forms with PI and the role that this interlayer plays in the adhesion-boosting properties of Ti. Taken as a whole, this work serves to highlight the important role that a Ti-PI interlayer seems to play in the mechanical properties of Ti coatings, in particular their adhesion.

In an effort to develop high performance steel, predicting the mechanical property can contribute to considerable savings of cost and time. Reliable prediction of the strength of an advanced steel calls for a clear picture of the correlation between microstructure and mechanical properties of the material. In the present study, attempts were made to predict the strengths of advanced steels by performing nanoindentation experiments on each phase in the microstructures. Consideration of the indentation size effect and application of a simple rule-of-mixture led us to a reasonably successful estimation of strengths of the steels from phase hardness. Using this approach, influences of Nb, V and Mo on the mechanical behavior of the steels were systematically explored in terms of various strength mechanisms.

Pyramidal microindentation tests were performed at various loading rates from 1 to 2000 mN/sec on annealed samples of pure Copper, 70/30 brass, and 5052 Aluminum to study the effect of indentation strain rate on the indentation depth dependence, from h = 500 nm to 10 mu;m, of the average indentation stress. The average indentation stress displayed a clear strain rate sensitivity which was indentation depth dependent. The model of Nix and Gao was applied to calculate the density of statistically stored dislocations (SSDs) and geometrically necessary dislocations (GNDs). The SSD density exhibited a decreasing trend with increasing indentation strain rate while the GND density displayed only the characteristic decrease with increasing h however was essentially independent of indentation strain rate and test material. This is, to our knowledge, the first data reported that confirm the fundamental premise upon which the Nix and Gao theory is based; namely, the GND density is solely dependent upon the indentation geometry and not the deformation characteristics of the indented material.
The measure indentation strain rate was predicted by applying an Arrhenius-type relationship to predict the strain rate in terms of the total dislocation density and the Gibb&’s free energy ΔG0 of the microstructural obstacles that limit the rate of dislocation glide. This description also allowed us to calculate the indentation depth dependence of the normalized apparent activation volume of the deformation process. We observed that ΔG0 decreased, while activation volume increased, with increasing h. Both parameters were dependent upon the type of materials that was being indented.

We develop a quantitative analysis of how the plastic deformation in a metallic glass is more uniform if its Poisson ratio \nu is higher. The plasticity of metallic glasses under ambient conditions is mediated by shear localized in thin bands, and can be characterized by experiments on the bending of thin plates. We extend the analysis by Conner et al. [J Appl Phys 94 (2003) 904-911] of bands in bent plates to include the micromechanics of individual shear bands. Expressions are derived for the shear-band spacing and the offset on each band. Both these quantities are predicted to decrease as \nu is increased. The predictions are tested against measurements on metallic glasses with a wide range of \nu. Good agreement is found, supporting the new model for the shear-band spacing, and pointing the way towards more diffuse deformation, and consequently improved plasticity and toughness, of metallic glasses as \nu increases toward the limiting value of 0.5.

Nanocrystalline metals have an unusual propensity to undergo mechanically-induced abnormal grain growth resulting in a sharply bimodal grain size distribution. Yet our ability to detect bimodal grain distributions is largely confined to brute force cross-sectional metallography and laborious transmission electron microscopy. The present study presents a new method for rapid detection of unusually large grains embedded in a sea of much finer grains. Traditional x-ray diffraction based grain size measurement techniques such as Scherrer, Williamson-Hall, or Warren-Averbach rely on peak breadth and shape to extract information regarding the average crystallite size. However, these line broadening techniques are not well suited to identify a very small fraction of abnormally large grains. The present method utilizes statistically anomalous intensity spikes in the Bragg peak to identify regions where abnormally large grains are contributing to diffraction. This needle-in-a-haystack technique is demonstrated on a nanocrystalline Ni-Fe alloy which has undergone fatigue-induced abnormal grain growth. In this demonstration, the technique readily identifies a few large grains that occupy less than 0.00001% of the interrogation volume.
Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy&’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Nanocrystalline thin films can exhibit novel, exceptional properties due to their special microstructure. Sputter-deposited and electrodeposited Ni(W) films exhibit outstanding mechanical properties and thermal stability up to temperatures exceeding 400 °C [1,2], above which the nanocrystalline microstructure gets destroyed due to grain growth. This notably good thermal stability, as well as the enormous mechanical stability, can be linked to a highly planar-faulted microstructure.

Alloying Ni with W by co-sputtering on Si substrates leads to a highly planar-faulted micro-structure, i.e. with high densities of {111} twin and stacking faults. The films exhibit a sharp {111} fiber texture with the faults oriented perpendicular to the growth direction of the nanocrystalline, columnar grains. Due to the strongly aligned planar faults, diffraction pat-terns consist of so-called affected reflections, i.e. shifted and broadened by planar faults, and discrete, unaffected spots.

Whereas the positions of the unaffected reflections can well be used to calculate mechanical, residual stresses, the affected reflections reflect the planar fault probability via their positions and widths. Selected area electron diffraction data are usually not statistically representative and, therefore, X-ray diffraction (XRD) data were used, recording intensity distributions along affected streaks in the reciprocal space. We will demonstrate a new technique which is able to extract twin densities from measured intensities in reciprocal space by a dedicated application of the program DIFFaXplus [3].

Room temperature XRD investigations at the synchrotron ANKA (Karlsruhe, Germany) demonstrated that low W-content Ni(W) films exhibit a fcc-like stacked structure, whereas high W-content Ni(W) films show a (partially) hcp-like stacked structure.

The thermal stability of Ni(W) thin films with 9-21 at.% W was analyzed in-situ by tracing the planar fault density at the synchrotron ESRF (Grenoble, France), heating the films to a maximal temperature of T_max = 800 K. These XRD investigations demonstrate that the fcc-like stacked Ni(W) are less thermally stable than the (partially) hcp-like stacked Ni(W) films. The new method of twin density investigation enhances the understanding of the microstructure-property relationship: the underlying mechanisms of thermal stability can be identified and will help to tailor specific microstructures for thermally stable, nanocrystalline materials.


[1] Detor, A. J. & Schuh, C. A. (2007). J. Mater. Res. 22, 3233.

[2] Welzel, U., Kuemmel, J., Bischoff, E., Kurz, S. & Mittemeijer, E. J. (2011). J. Mater. Res. 26, 2558.

[3] Leoni, M., Gualtieri, A. F. & Roveri, N. (2004). J. Appl. Crystallogr. 37, 166.

Cobalt nanoparticles are important catalysts for Fischer-Tropsch synthesis, which converts carbon monoxide and hydrogen into hydrocarbons and water. This reaction can be used to make liquid fuels from abundant carbon sources such as coal or biomass. Traditionally, catalyst particles are prepared by wet-impregnation methods, which result in a mixture of sizes on the nano-scale. Using a colloidal synthesis, monodisperse cobalt particles ranging from 4nm to 15nm were prepared. We investigated the interaction between reactant gases and cobalt nanoparticles of varying sizes by in situ soft x-ray absorption spectroscopy (Advanced Light Source, Berkeley, CA) performed in a novel gas cell. This gas cell was designed to operate in 1 atm of gas and up to 350C.
By following the chemical state of the surface oxygen and the cobalt particles during the reaction, we discovered that carbon monoxide dissociation, the first step in the reaction mechanism, depends strongly on catalyst particle size. Smaller particles (4 nm) were found to be less effective at dissociating CO than larger particles (15 nm). This finding could provide an explanation for the observation in the literature that small nanoparticles are not as effective at converting CO to alkanes. In catalysis, smaller particles are often favored because of the high surface-area to volume ratio, but our study suggests that in the case of cobalt, as size is reduced from 15 nm to 4 nm, catalytic properties are adversely affected. Smaller is not necessarily better.

Multilayers with a different scale of periodicity offer the chance to tune, to optimize and to understand the phase formation in intermetallic systems. Annealing experiments with low heating rate allow investigating the stepwise formation of phase sequences with thermodynamical and different kinetical contributions. In contradiction to that high heating rates may offer the demonstrative self propagating reactions in relevant systems such as Ruthenium Aluminide. Scaling the periodicity in this case allows not only for single step formation of RuAl, but also for the precise control of the reaction velocities and amount of heat which offer unique potential applications, e.g. in micro joining. Another way of controlled local reaction at high heating rate is interference patterning with defined lateral periodicity using a short pulse Laser in the nano sec regime. The only local formation of intermetallic phases in such multilayers leads to well defined lateral hard-soft pattern with long range order. Careful characterization of microstructural morphologies in the nano and the atomic scale help to understand the control parameters of these effects.

X-ray tomography as been commercially available since the 1970's; only since the 1990's with a rapid growth in computer power and x-ray source development has it become commonplace in the laboratory. Unfortunately during most of this time, x-ray tomography has been limited to generating 'pretty', albeit useful pictures of materials. These 3D images give scientists a qualitative understanding of the sample morphology, distribution of voids or inclusions, or damage features. More recently, scientists have begun to take this data one step further and added the ability to quantify these features, void sizes, shapes and their distributions. Coupling this with an in-situ cell such as tension, compression, tearing, or heating, now it is possible to quantifiably measure how these features are changing as a result of response of the sample to this dynamic change. Now it becomes possible to measure these changes and to compare the performance of the material as a result of age, previous stress, or other environmentally induced effects.
MPM modeling, used for modeling large deformations, becomes even more robust when modeling from a real data set. The initial dataset of uncompressed data was binned down to be fed into the MPM model using CartaBlanca coded to simulate the foam material under uniaxial compression without any adjustment to material parameters as a first try. The main goal to date has been to demonstrate that the code is capable of handling large deformation and complex irregular geometry of the material that contains voids of various shapes.
In this talk we will discuss our recent work with polymer foams using a compression/tension load cell within a micro computed tomography instrument and our use of a nano computed tomography instrument to examine fine structure. We will demonstrate how when coupling in-situ dynamic and a quantification package, five dimensional information can be collected. Three dimensions are the 3D image, a fourth dimension will be time or compressive load, with the fifth dimension corresponding to the change in statistics (ex. voids colored by equivalent diameter). Using this information, we can compare formulation variations as a result of some previously applied stress such as age degradation, compression set or radiation damage. We can compare performance in-situ and even measure the Poisson ratio. A comparison of the 3D images of the in situ compressed foam, and the MPM modeled foam shows good agreement in the morphological changes to the foam structure. Feeding the initial state images and their statistics into an advanced numerical simulation code that is based on MPM adds a level of rigorousness to the modeling process that until now was never possible.

During the last decades, many different sensing concepts have been developed ranging from proof-of-concept studies towards commercially available products. (Nano)granular metals have also found their ways into this field, however, often accompanied by some drawbacks such as high operating temperatures, complicated formation and / or clearance procedures. At the same time but in a different scientific field, focused electron beam induced deposition (FEBID) from the gas phase attracted more and more attention due to its capability to deposit functional material in a direct write manner on even non-flat surfaces with spatial nanometer resolution. Although the functionalities span over a broad range from electrically insulating, conducting, magnetic, or catalytic, a major problem remained: chemical impurities, mostly carbon, which suppressed the intended functionality. However, in some cases this “drawback” can be used as enormous advantage which is content of this contribution. We demonstrate how FEBID can be used to fabricate a composite system of Pt nano-particles (2-3 nm in diameter) which are homogeneously embedded in an insulating carbon matrix. The fine control during deposition allow for special geometries which enable a new sensing concept for polar analytes in gaseous and liquid environments based on variable tunneling probabilities due to changing intergrain-coupling strengths. The advantages of FEBID based sensors are the fast preparation without post-treatment steps by means of high temperatures or special environments, fast response times and reversible characteristics without any reformation or clearance procedures. These unique properties are based on the small metal grain sizes and the insulating matrix in between which will be explained in detail together with an outlook how this simple concept can be used for new applications.

In the low-symmetry martensite phase, magnetic shape-memory alloys exhibit strong magneto-structural coupling. This coupling, when combined with a low twinning stress, enables large magnetic-field-induced deformation. Inversely, deformation by stress-induced twinning changes the magnetization. The ability to control magnetization at the nanoscale by patterning (100) surfaces of Ni-Mn-Ga single crystals with arrays of nanoindentations provides a means to store magnetic information. We show, using magnetic force microscopy, that nanoscale indentation-induced surface corrugations modify the magnetic stray field. At low indentation load and large indentation spacing, each indentation appeared with a separate magnetic contrast signature. With increasing load and decreasing spacing, the stray fields of neighboring indentations coupled. An external magnetic field and a mechanical stress changed the orientation of the magnetization 90° via twinning. Further, the magnetization, and therefore, the contrast from the magnetic stray field, switched 180° by reversing the applied magnetic field parallel to the crystallographic c direction. With local control of the c direction, corrugation-induced magnetic patterns on Ni-Mn-Ga provide a mechanism for four-state memory devices.

The functionality of shape memory alloys is enabled by the reversible phase transformation between austenite and martensite phases under external thermal, mechanical, or magnetic stimuli. In this talk, we will present our recent findings of the size effects in thermoelastic shape memory alloy small wires with a bamboo grain structure. Shrinking the wire diameter causes a shift in the energetics and thermodynamic driving force for the phase transformations. As a result, the critical temperatures or stresses required for inducing the transformations, as well as the superelastic hysteresis and morphological evolutions in a thermal or mechanical cycle, become dependent on sample size. Size reduction coupled with grain structure design also leads to drastically improved fatigue properties and rapid thermal energy exchange. We will discuss our experimental measurements of these properties in small shape memory wires, as well as the understanding gained from our mesoscale simulations on such size effects.

The influence of film thickness on the B2-B19 martensitic transformation properties of nanoscale Ti51Ni38Cu11 thin films with thicknesses ranging from 750 to 50 nm is reported. For these films an unexpected behavior of the phase transformation temperatures was observed: Af and Os initially decrease with decreasing film thickness but increase sharply again for thicknesses <100 nm. The phase transformation temperatures and thermal hysteresis width range from 58 to 35 °C (Af) and 14 to ~0 K, respectively. Furthermore, it is shown that with decreasing film thickness a change in the tetragonality of the B19 martensite phase occurs. This leads to fulfilling the lambda;2-criterion, causing a vanishing hysteresis for a film thickness of 75 nm. [1]
Furthermore, the effect of the film thickness on the transforming matrix composition was investigated by X-ray photoelectron spectroscopy depth-profiling. With decreasing film thickness the chemical composition of the transforming matrix phase changes significantly due to the formation of oxide top layers and interfacial layers, e.g. for a film thickness of only 50 nm the Ti content is reduced by ~ 9 at. % whereas the Ni content is increased by ~ 4 at. %. These results are of importance for design and application of nanoscale Ti-Ni-Cu shape memory alloy actuators.
Reference:
[1] D. König et al., Acta Mater., 60, (2012), 306-313

The structural, thermal, magnetic, and mechanical properties of Ni-Mn-Ga magnetic shape-memory alloys depend strongly on composition. Sputter deposition of films from alloy targets shifts the composition where the composition change depends on sputter parameters. Films produced via physical vapor deposition from alloy targets have sound film-substrate adhesion and film uniformity, however, accurate control of composition is difficult. Co-sputter deposition from multiple targets allows the flexibility of varying deposition rates and film compositions. We developed a robust procedure with three targets (Nickel, Nickel-Gallium, and Manganese) to deposit Ni-Mn-Ga films with defined composition, structural and magnetic properties. The sputter power was controlled independently and systematically for each target. For a film with target composition 50 at.-% Ni 30 at.-% Mn 20 at.-% Ga, Energy-Dispersive X-ray Spectroscopy yielded the composition 50.5 at.-% Ni 29.2 at.-% Mn 20.3 at.-% Ga. XRD and TEM revealed the 14M modulated martensite structure , a <242> fiber texture, an average grain size of about 100 nm, and the occurrence of nano-twinning within the grains. The martensitic start and finish temperatures, measured with multi-beam optical sensor wafer curvature deflectometry, were 160 °C and 133 °C respectively, indicating stress-induced martensite formation at high temperature. Co-deposition using three targets provides a method to control the composition of Ni-Mn-Ga films and to adjust film properties such as martensite structure and transformation temperature.