Nanocrystalline materials have demonstrated very interesting changes in physical, chemical and mechanical properties at severely diminished length scales. This article focuses on the topic of nanocrystalline ceramic composites specifically designed for applications requiring improved fracture toughness. The difficulty in producing fully consolidated ceramic composites that retain a nanocrystalline structure is the main hurdle for thorough investigations in this area. This obstacle has been overcome in the current investigation through the use of a fast, comparably lower temperature, sintering technique e.g., Spark Plasma Sintering. Alumina based nanocomposites incorporating carbon nanotubes and additionally incorporating nanocrystalline niobium have yielded fracture toughness values that have exceeded that for pure nanocrystalline alumina by more that 300%. This introduces the question of whether this improvement is merely additive or evidence of a synergistic toughening mechanism involving ductile phase and fiber toughening.

In the present study, the focus is on the analysis of the effect of the mold dimensions on the temperature distribution of a sintered body during plasma activated sintering. The temperature distribution of a cylindrical mold with various dimensions was measured using K-type thermocouples. The temperature homogeneity of the mold was studied based on the direction and dimensions of the mold. A temperature gradient existed in the radial direction of the mold during the plasma activated sintering. Also, the magnitude of the temperature gradient was increased with increasing sintering temperature. In the longitudinal direction, however, there was no temperature gradient. The temperature gradient of the mold in the radial direction decreased with increasing inner diameter of the mold and its length.

The role of slip transfer processes across the heterophase interfaces in two-phase TiAl intermetallics has been studied. Polysynthetically twinned crystals of TiAl (PST-TiAl) have been used as model systems for individual grains in technologically relevant polycrystalline lamellar TiAl alloys. Compressive plastic loads have been applied for orientations of the lamellar interfaces parallel and perpendicular to the loading directions to produce hard mode slip activity. Transmission electron microscopy has been used to determine the active deformation modes in the constituent phases and to study the slip transfer across heterophase interfaces. The results are discussed with respect to the mechanical behavior of PST-TiAl and lamellar TiAl alloys, which is of relevance to in-service performance and metallurgical processing operations.

The strength of structural materials strongly depends on the structure and properties of grain boundaries. Interfaces usually act as barriers to dislocation motion and therefore strengthen materials with decreasing grain size, quantitatively described by the well-known Hall-Petch relation. However, interfaces in nanocrystalline materials are often covered with impurities or second phases, which may influence the mechanical properties. With nanoindentation testing it is now possible to probe the strength of interfaces like grain boundaries directly on a nanometer scale. Therefore this method was used to investigate the properties in the vicinity of grain boundaries in polycrystalline materials with conventional grain size and in ultrafine-grained metals prepared by equal channel angular pressing (ECAP), where no impurities are introduced during processing. Measurements on an austenitic steel clearly show a decreasing hardness close to the interface opposite to the general expected behavior of strengthening. In this case segregation effects strongly influence the mechanical properties near the boundaries. The nanoindentation investigations on ultrafine-grained Al and Cu show a strong strain rate sensitivity. Inelastic effects are also found between unloading-loading segments during indentations.

Previous studies in our laboratory have shown that individual nanoparticle chain aggregates (NCA) exhibit remarkable mechanical behavior when under strain inside the transmission electron microscope. NCA made of various materials (e.g. carbon, metal oxides, metals, etc.) were strained by as much as 100% when tension was applied to them. After breaking, the NCA rapidly contracted to form more compact structures. In this study, molecular dynamics (MD) computer simulations are employed to investigate, at the atomic scale, the behavior of short nanoparticle chains under strain and to obtain quantitative information of the forces involved in chain straining and fracturing. The interaction potential used is that of copper obtained with the embedded atom method (EAM). Although the methodology is generally applicable, copper was selected as a test material because reliable interatomic potentials are available. Seven single- crystal nanoparticles, each 2.452 nm in diameter, are placed in contact in two chain configurations, linear and kinked. The structures are initially relaxed adiabatically with MD steps for 225 ps, at a starting temperature of 300 K. The bonding energy between any two nanoparticles in contact ranges from about 20 eV to 30 eV at 0 K. The two relaxed chain configurations are strained along their longest dimension, to the breaking point, at strain rates spanning from 0.3 m/s to 10 m/s. We identify mechanisms of stress accommodation that lead to plastic deformation and eventually fracture for both chain configurations, linear and kinked, and we construct the corresponding stress-strain curves. The two chain configurations exhibit different mechanical behavior. Applications of our experimental and simulation studies on NCA are to the behavior of nanocomposite materials, including carbon black reinforced rubber, sampling of aggregates by high speed impactors and the formation of flexible coatings of nanoparticles.

Layered structures of Cu and Ta thin films on silicon are well established for many technological applications in microelectronics. Electronic circuits used for flexible displays or wearable electronics are becoming increasingly popular. For such applications, the Cu/Ta system must be transferred to flexible substrates, incorporating a design rule for several percent of total strain. We have investigated the deformation behaviour of different Cu/Ta and Ta/Cu/Ta thin film systems on a flexible polyimide substrate subjected to total strains of more than 5%. A novel synchrotron X-ray diffraction technique allowed us to characterize the evolution of mechanical stress in very thin metallic films during isothermal tensile tests. We found that samples with a Cu film thickness below 300 nm showed a sudden stress decrease at a total strain of about 2.5%. This stress drop was attributed to fracture of the entire film system, initiated by cracks in the Ta layers.

Atomistic simulations have provided unprecedented insight into the structural and mechanical properties of nanocrystalline materials, highlighting the role of the non-equilibrium grain boundary structure in both inter- and intra-grains deformation processes. One of the most important results is the capability of the nanosized grain boundary to act as a source and sink for dislocations. However the extrapolation of this knowledge to the experimental regime requires a clear understanding of the temporal and spatial scales of the modelling technique and a detailed structural characterisation of the simulated samples. In this contribution some of the synergies that can be developed between atomistic simulations and experiments for this research field are briefly discussed by means of some typical examples.

The critical conditions for dislocation nucleation from the surface steps of various geometries are analyzed based on the Peierls-Nabarro dislocation model. By modeling a surface step as part of a three- dimensional crack surface, the half space problem is transferred into an equivalent three dimensional crack problem in an infinite medium. The profiles of embryonic dislocations, corresponding to the relative displacements between the two adjacent atomic layers along slip planes, are then rigorously solved through the variational boundary integral method. The critical conditions for dislocation nucleation are determined by solving the stress dependent activation energies required to activate embryonic dislocations from their stable to unstable saddle point configurations. For a given slip plane, the effects of step geometry such as the step height and inclined angle on dislocation nucleation are analyzed in detail. The results show that the atomic scale steps may reduce the critical stress required for dislocation nucleation from the surface by several factors. Compared to previous analyses of this type of problem based on continuum elastic dislocation theory, the presented analysis eliminates the uncertain core cutoff parameter by allowing for the existence of an extended dislocation core as the embryonic dislocation evolves. Because of the serious limitation of direct atomic simulation for this type of problem, the presented methodology of incorporating atomic information into continuum approach appears to be particularly noteworthy for providing insights of energetics of the atomic processes involved in dislocation nucleation.

Two types of glass plates coated with uniformly aligned polyurethane films were produced by repeating the cycles of the alternative exposures of the glass plates to 1,4-phenylene diisocyanate (PDI) and 2-butyne-1,4-diol (BDO) and to PDI and terephthalic acid bis-(2-hydroxy ethyl) ester (TBE) for 500 times at elevated temperatures. The glass plates coated with uniformly aligned poly-(PDI-BDO)500 (500 represents the number of the cycle) produced monolayers of closely packed 2D arrays of silicalite-1 crystals with the average size of 370 × 200 × 500 nm on the glass plates upon immersion of the glass plates into a dense gel consisting of tetraethylorthosilicate (TEOS), tetrapropylammonium hydroxide (TPA+OH-), and water (mole ratio = 7:1.5:330) followed by the hydrothermal reaction at 180°C for 2 h. The silicalite-1 crystals were aligned with the c-axes perpendicular to the substrate plane. Upon switching the polymer from poly-(PDI-BDO)500 to poly-(PDI-TBE)500 the orientations of the silicalite-1 crystals in the 2D arrays changed from c to a axes perpendicular to the substrate. This report therefore demonstrates that the uniformly aligned polyurethane films serve as the templates for the growth of closely packed multi-crystal arrays of silicalite-1 in uniform orientations and the nature of the polyurethane film affects the resulting orientations of the crystals. We propose that the supramolecularly organized organic-inorganic composites consisting of the hydrolyzed organic products and the seed crystals responsible for the above phenomena.

A novel synchrotron-based X-ray diffraction technique is presented by which it is possible to characterize the evolution of stresses in polycrystalline metallic films thinner than 100 nm. The film under investigation is deposited on a flexible polyimide substrate which is subjected to a uniaxial tensile test. The evolutions of longitudinal and transverse elastic strains in the film are monitored simultaneously by means of a high-resolution area detector. The strain resolution is better than 10−4. The samples are typically subjected to 6 % total strain and subsequently unloaded. First experiments carried out on Au films in the thickness range between 20 nm and 1000 nm show the usefulness and the power of this new technique.

Atomistic simulations are carried out to study the effect of atomic sliding capability at the interface between a plastically deforming film and a stiff substrate. Molecular statics modeling is utilized to corroborate the overall film response and the nano-scale defect mechanisms. A free-sliding interface is shown to be able to cause “reflection” of oncoming dislocations and enhance film plasticity. A rigidly bonded interface, on the other hand, is seen to resist approaching dislocations. Partial sliding results in a transitional behavior between the two extremes, as revealed in our parametric analysis. The sliding capability of interface atoms is also seen to dictate the overall film response.

An experimental evaluation method of interface strength between ductile submicron-dots and a hard substrate is developed. The validity is examined with copper (Cu) cylindrical dots of submicron scale on a silicondioxide (SiO2) substrate. A hard-layer of tungsten (W) is employed to restrain the deformation and concentrate the stress near the free-edge of Cu/SiO2. A diamond tip is dragged horizontally along the SiO2 surface and the load is applied to the side edge of the W layer at a constant displacement rate using a modified atomic force microscope. Both the lateral and the vertical loads and displacements are continuously monitored during the test. After the tip hits the W layer, the lateral load, Fl, increases almost proportionally with the lateral displacement, σl. The Cu dot with the W layer then is clearly separated from the SiO2 along the interface. The restraint by the W layer works well so that there are little damages in both the delaminated W/Cu dot and the substrate. The delamination lateral load, FlC, is successfully evaluated.

The low ductility of nanostructured materials is attributed to the deficit of dislocation activity in the nanometer range. Recent scientific interest in nanostructured materials stems from reports of alterative combinations of mechanical properties, although a low ductility is typically reported. One promising approach based on the concept of multiple length scales is illustrated by a “bimodal” microstructure, i.e. containing a mixture of nanostructured and coarse grains. The present work reports a numerical study of the tensile deformation and fracture of a nanostructured Al alloy with a bimodal microstructure. In the theoretical framework used in the present study, the elastic-plastic behavior and deformation processes are approximated by Ramberg-Osgood formula and finite element method, respectively. The numerical results are found to be in a good agreement with the experimental behavior.

This work focuses on the development of a classical molecular dynamics (MD) framework that can be used to analyze the quasi-static strength, elastic-constants, and dynamic strength of nanostructured Fe2O3+Al reactive metal powder (NRMP) mixtures. An interatomic potential is developed for the Fe2O3+Al NRMP system, accounting for the behavior of Al, Fe, Fe-Al intermetallics, Fe2O3, and Al2O3. This potential can be regarded as a generalization and combination of several existing potentials for the individual components in the system. The potential incorporates electronegativity equalization to account for pressure-induced phase transformation. In addition, the parameter set of the potential is capable of predicting the elastic properties of components over a range of 0 - 900 K.

A fact that has been neglected in most theories of brittle fracture is that the elasticity of a solid clearly depends on its state of deformation. Metals will weaken, or soften, and polymers stiffen as the strain approaches the state of materials failure. It is only for infinitesimal deformation that the elastic moduli can be considered constant and the elasticity of the solid linear. We show by large-scale atomistic simulations that hyperelasticity, the elasticity of large strains, can play a governing role in the dynamics of fracture and that linear theory is incapable of capturing all phenomena. We introduce a new concept of a characteristic length scale ξ for the energy flux near the crack tip and demonstrate that the local hyperelastic wave speed governs the crack speed when the hyperelastic zone approaches this energy length scale. The new length scale ξ, heretofore missing in the existing theories of dynamic fracture, helps to form a comprehensive picture of crack dynamics, explaining super-Rayleigh and supersonic fracture. We further address the stability of cracks, and show agreement of the Yoffe criterion with the dynamics of cracks in harmonic systems. We find that softening hyperelastic effects lead to a decrease in critical instability speed, and stiffening hyperelastic effects leads to an increase in critical speed. The main conclusion is that hyperelasticity plays a critical role in forming a complete picture of dynamic fracture.

To model size dependent plastic deformation at micron and submicron length scales the theory of mechanism-based strain gradient plasticity (MSG) was developed. The MSG approach incorporates the concept of geometrically necessary dislocations into continuum plastic constitutive laws via Taylor hardening relation. This concept is extended here to develop a mechanism-based strain gradient theory for crystal plasticity (MSG-CP) based on the notions of dislocation density tensor and resolved density force corresponding to the Peach-Koehler force in dislocation theory. An effective density of geometrically necessary dislocations is defined on the basis of resolved density force for specific slip systems and is incorporated into the plastic constitutive laws via Taylor relation.

The principal goal of the present paper is to demonstrate an application of modern software engineering tools for modeling virtual reality and molecular dynamics of novel nanocomposites. The main technical components of presented system are 1) software and nanoengineering tools for modeling of virtual reality, molecular dynamic and 3D video images of novel diamonds- containing nanocomposites and 2) Problem tracking system to be used during modeling of virtual reality. For a realistic simulation of the stability behavior of the reinforced material, the nonlinear intramolecular inter-actions between neighboring atoms have to be taken into account. A comparison shows the buckling sensitivity of different geometries. In order to reduce computational costs, it is necessary to develop suited homogenization techniques, so that shell elements can be applied.

This paper reports experimental work using an electric field close to a substrate to induce morphological patterns in a thin polymer film. The film was spin-coated onto a glass wafer, and subsequently heated to above its glass transition temperature to allow viscous flow. An electric field was applied by two parallel electrodes spacing 10 mm apart. The initially flat polymer/air interface lost stability and formed islands. The self-assembled islands exhibited a narrow size distribution and demonstrated spatial ordering. We attribute the pattern formation to the minimization of combined interface energy and electrostatic energy.

Elevated temperature tensile tests of different microstructures arising from different heat treatments of the Fe-based metallic glass Vitroperm (Fe73.5Cu1Nb3Si15.5B7) are presented. An anneal at 600°C for 1h yields a single phase μ-Fe microstructure with equiaxed, randomly oriented 15 nm grains, which is an ideal candidate for study of material properties at diminishing length scale. This microstructure has good stability during tensile testing at 600°C, showing a strain rate exponent correlating to grain boundary sliding (m=0.5), but little ductility, and strengths to 1250 MPa. The brittle behavior could be attributed to the lack of dislocation activity at such length scales. At temperatures up to 725°C, grain growth occurs, leading to elongations as large as 65% at flow stresses of 250 MPa. Precipitation of a second Nb-rich phase accompanies the grain growth. This investigation is supported by NSF, Division of Materials Research, grant NSF-DMR-0240144.

A new simple method – unconfined twist was employed to prepare ultrafine grained (UFG) Fe wire. A coarse grained (CG) Fe wire with a diameter of 0.85 mm was fixed at one end, and twisted at the other end. After maximum twist before fracture, in the cross-sectional plane, concentrically deformed layers with a width of several micrometers formed surrounding the center axis of the wire. The near-surface deformed layers consist of lamella grains with a width in submicrometer range. In the longitudinal plane, deformed bands (with a width of several micrometers) formed uniformly, which were composed of lamella crystallites (with a width in submicrometer range). The tensile yield strength and ultimate strength of the twisted Fe wire are increased by about 150 % and 100 % compared with the values of its CG counterpart.