Plastic deformation due to thermal stresses has been investigated for different metallic films deposited on Si or α-alumina substrates. We conducted post-mortem TEM and SEM investigations of samples that underwent thermal cycles in order to capture the microstructural changes imposed by thermal stresses. The ultimate goal is to determine the dominant plasticity mechanisms responsible for such changes. In-situ thermal cycles performed inside the TEM allowed direct and real-time observations of dislocation behaviour under stress. It is shown that dislocation density drops in Al/Si, Au/Si and in Cu/α-alumina thin film systems. Except in the case of pseudo-epitaxial Cu on sapphire, the interaction of dislocations with the interfaces (passivation, oxide, adhesion layer) is attractive and leads to the disappearance of interfacial dislocations. In this light, the generalized observation of high tensile stresses that arise in metallic films at the end of cooling is explained in terms of insufficient dislocation sources instead of classic strain hardening. Diffusional processes can substitute for a lack of dislocation, but the low relaxation strain rate that would be excpected should lead to high stresses during the cooling stages of thermal cycles.

An analysis of the fluctuations of a {100} FCC interface has been used to locate the surface roughening transition in kinetic Monte Carlo simulations. These simulations were conducted using a spin 1/2 Ising model treatment of the solid-liquid interface. A temperature gradient was imposed on the simulation cell providing a restoring force to the interface. Fluctuations in the number of solid atoms in the system were measured using time correlations. The fluctuation dissipation theorem was employed to relate the characteristic times retrieved from the time correlation to the kinetic coefficients of the system. A dramatic change was found in these characteristic times, which locates the surface roughening transition.

Island coalescence during the early stages of polycrystalline film formation is thought to lead to the large values of tensile stress observed in experiments. Continuum models have been developed which are in semi-quantitative agreement with in-situ experimental observations of stress evolution during coalescence. However, the size of coalescing particles is in the nanometer range, so that atomistic treatments might be expected to more accurately reveal the mechanisms leading to coalescence stresses. We have performed molecular dynamics (MD) computational experiments in which small metallic clusters both in free space and on weakly bound substrates are allowed to coalesce. These atomistic simulations complement the finite element modeling (FEM) and analytical work on the tensile strains developed during grain boundary formation by coalescence. Additionally, the atomistic simulations implicitly capture the effects of anisotropy and special orientations. The MD simulations also provide atomic-level detail of the boundary structure that is absent in continuum models. We have focused on simulations of Ag as a high mobility material at and above room temperature and have begun studying Ni as a material that can behave either as a high or low mobility material depending on the deposition temperature(i.e. Ni has a higher activation energy for adatom motion than Ag.). We find that for small islands, the initial boundary formation occurs in times on the order of nanoseconds. For nanometer-sized clusters in free space and on traction-free substrates, the cluster can undergo large rotations prior to boundary formation. The simulated boundary heights fit well with both a FEM model and an analytical model. Work is ongoing to further quantify the results of the simulations and relate the atomistic forces and positions to the stress state. We are pursuing methods for including the effects of traction at the interface, both through continuum and discrete methods.

The 3D x-ray crystal microscope is an emerging tool for the study of mesoscale structure in polycrystalline materials. With this nondestructive device, local crystalline orientation, phase, elastic and plastic strain tensors can be measured with submicron spatial resolution in three dimensions. A key step in analyzing the Laue patterns from the 3D microscope is indexing the reflections, which determines the orientation of the sub-grain. With current algorithms, the angles between pairs, triplets and quadruplets of reflections are compared to theoretical angles to make guesses as to the reflection indices. The ability to index a pattern can however be compromised by both elastic and plastic deformation of a grain; elastic deformation changes the angles between reflections and plastic deformation increases the uncertainty in the centroid of each reflection. Here we report on the use of an indexing algorithm that simultaneously fits all peaks from a subgrain. This algorithm is more robust than previous methods and allows for indexing of deformed or strained grains. Some applications to studies of mesoscale materials properties are described.

A new experimental apparatus has been developed for performing compression deformation experiments on high-purity Mo single crystals. These experiments provide data that can validate 3-D dislocation dynamics (DD) simulations. The experiments are performed under conditions that allow unconstrained deformation; thus, a relatively uniform state of axial stress is maintained during deformation. In the following sections, we describe the new experimental apparatus and our results from experiments performed at ambient temperature at a strain rate of s–1. Validation criteria based on the Mo experiments may include comparing the stress-strain response using 3-D strain information, the predicted slip-system yield, and work-hardening behavior.

The substructure inhomogeneity of real textured metal materials was studied by use of recently developed methods of X-ray diffractometry and data treatment. Main regularities of substructure inhomogeneity were revealed for the first time by the analysis of generalized pole figures. The structure of deformed metal includes an extremely wide spectrum of substructure conditions. The crystallographic orientation of grains is the most effective criterion for systematization of substructure inhomogeneities in textured materials. The dispersity of grains and the distortion of their crystalline lattice are minimal in texture maxima and increase up to highest values by passing to texture minima. The distribution of lattice elastic deformation in rolled metals most often shows a cross-wise pattern, consisting in alternation of quadrants with predominance of elastic extension or elastic compression.

We discuss dislocation multiplication and long range interactions in the framework of threedimensional (3-D) statistical dislocation dynamics. It is found that the long range interactions render the conservation equation for the ensemble-averaged dislocation density nonlinear and nonlocal. It is also shown that the conservation equation for the dislocation density must be supplemented by a gauge constraint in order to account for dislocation multiplication.

Thermal stress evolution in polycrystalline thin films is analyzed using discrete dislocation plasticity. Stress develops in the film during cooling from a stress-free configuration due to the difference in thermal expansion coefficient between the film and its substrate. A plane strain formulation with only edge dislocations is used and each grain of the polycrystal has a specified set of slip systems. The film–substrate interface and the grain boundaries are impenetrable for the dislocations. Results are presented for two film thicknesses, with higher hardening seen for the thinner films.

The pile up patterns arising in nanoindentation are shown to be indicative of the sample crystal symmetry. To explain and interpret these patterns, complementary molecular dynamics simulations and experiments have been performed to determine the atomistic mechanisms of the nanoindentation process in single crystal Fe{110}. The simulations show that dislocation loops start from the tip and end on the crystal surface propagating outwards along the four in-plane <111> directions. These loops carry material away from the indenter and form bumps on the surface along these directions separated from the piled-up material around the indenter hole. Atoms also move in the two out-of-plane <111> directions causing propagation of subsurface defects and pile-up around the hole. This finding is confirmed by scanning force microscopy mapping of the imprint, the piling-up pattern proving a suitable indicator of the surface crystallography. Experimental force-depth curves over the depth range of a few nanometers do not appear smooth and show distinct pop-ins. On the sub-nanometer scale these pop-ins are also visible in the simulation curves and occur as a result of the initiation of the dislocation loops from the tip.

The aim of this study is to specify the Güneisen coefficient variation versus volume γ(V). First for isotropic solid, and in the anharmonic approximation we recall the various classical laws on γ(V). On the other hand, the shock wave physics have shown the experimental evidence of the linear relationship between shock velocity (U) versus jump of particle velocity (u) with some further consequences on γ(V) [cf. MRS Symp. Proc. vol 731 paper W-8.1 (Spring 2002)]. In this paper, we point out the failure of the γ(V) representation in the anharmonic approximation to reproduce the shock wave data U(u). Taking now into account the anharmonic effects in solids, we propound a new analysis of the γ(V) law which is well suited for the shock wave physics of solids.

Motivated by recent theoretical and experimental progress, large-scale atomistic simulations are performed to study plastic deformation in sub-micron thin films. The studies reveal that stresses are relaxed by material transport from the surface into the grain boundary. This leads to the formation of a novel defect identified as diffusion wedge. Eventually, a crack-like stress field develops because the tractions along the grain boundary relax, but the adhesion of the film to the substrate prohibits strain relaxation close to the interface. This causes nucleation of unexpected parallel glide dislocations at the grain boundary-substrate interface, for which no driving force exists in the overall biaxial stress field. The observation of parallel glide dislocations in molecular dynamics studies closes the theory-experiment-simulation linkage. In this study, we also compare the nucleation of dislocations from a diffusion wedge with nucleation from a crack. Further, we present preliminary results of modeling constrained diffusional creep using discrete dislocation dynamics simulations.

Si and InSb were subject to depth sensing multi-cycling nanoindentation. The load-depthcurves exhibited hysteresis loops which are explained in terms of pressure induced phase transformations. In order to study the impact of crystal distortions on phase transformation, the specimens were subject to boron implantation (ion energy 180 keV) of different implantation doses (1014 to 1017 ions/cm2) and indented without annealing. In InSb, the hysteresis loops disappeared after implantation of 1016 ions/cm2, and for Si with its stronger bonds, a dose of 3*1016/cm2 is required for the same effect. Indentation cycling with constant maximum load results in a sudden disappearance of the hysteresis loop after a small gradual loop area reduction during the first initial cycles.

A dislocation core model is developed in terms of a singular decomposition of the elastic field surrounding the defect in a power series of 1/rn. The decomposition is a Laurent expansion beginning with the term corresponding to the Volterra dislocation and continuing with a series of dipoles and multipoles. The analysis is performed for an edge dislocation in an fcc lattice. The field surrounding the dislocation is derived by means of atomistic simulations. The coefficients of the series expansion are determined from the elastic field using path independent integrals. When loaded by a shear stress smaller than the Peierls stress, the core distorts. The distortion up to the instability (Peierls stress) is monitored based on the variation of these coefficients. The stacking fault separating the two partials is characterized, by using a similar procedure, as a source of elastic field.

Most existing continuum kinematic metrics used for correlating experimental data sets from finite deformation tests are traceable to a two reference state configuration analysis proposed primarily by Lee[1]; namely, a deformed elastic-plastic back to an idealized zero stress plastic state. These analyses admit that material discontinuities would exist in arriving at a virtual state of zero residual stress, however, the elastic-plastic deformation process is considered a mathematically continuous spatial function process. Hence, atomic length scale metrics are not analytically introduced because of the continuity requirements of the embedded deformation function space. This is not a valid physical description given that the microscopic dislocation mechanism for the non-recoverable kinematics of crystal material deformations creates discontinuities between atomic planes of grains in a material sample. In addition, the mathematical structure of current continuum kinematic metrics is deterministic. This is also not physically realistic in describing materials' responses that are dependent on intrinsic metallic grain characteristics. This follows since an ensemble of material specimens have a randomness for the physical numbers per unit material volume per unit species volume for both dislocation density and grain lattice structure density function characteristics. These random functions are best described as classical probability density functions in a spatial, time, and species variable domain. A stochastic description for dislocation density in solids has been developed, and explicitly introduced in a finite deformational functional[2]. This finite deformational functional will be extended to include an explicit dependence on a stochastic grain lattice structure density function. Finally, analyses of this dislocation-grain deformational functional provide stochastic kinematic metrics for void or microcrack-type opening deformations at grain boundaries that contain length scale measures based on the physical attributes of both dislocations and grains.

Dissociation of perfect 1/2<110> single dislocations into two 1/6<112> Shockley partial dislocations in ZrN was observed by transmission electron microscopy (TEM). The 1/2<110> single dislocations have a super-jog character and are not coplanar with the dissociated Shockley partials. This sessile arrangement of dislocations may be responsible for the brittleness of ZrN. The wide separation of the partial dislocations bounding stacking faults indicates that the stacking-faults energy (SFE) is low in ZrN. The low SFE can be explained on the basis of a high vacancy concentration, which was confirmed by the appearance of diffuse intensity maxima in electron diffraction patterns due to short-range ordering (SRO) of N vacancies. In-situ heating experiments in the TEM revealed that the diffuse intensity maxima disappear during heating and reappear on cooling. This indicates that N (or N vacancy) diffusion scrambles the SRO arrangement of N vacancies during heating. The width of the stacking faults in ZrN increases with temperature, indicating that the SFE decreases as the vacancy concentration increases.

Mechanisms of misfit strain relaxation in epitaxially grown Bi4-xLaxTi3O12 (BLT) thin films deposited on SrTiO3 (STO) and LaAlO3 (LAO) substrates have been investigated by means of transmission electron microscopy (TEM). The misfit strain of 20 nm thick BLT films grown on STO substrate was relaxed by forming misfit dislocations at the interface. However, cracks were observed in 100 nm thick BLT films grown on the same STO. It was confirmed that cracks were formed because of high misfit strain accumulated with increasing the thickness of BLT, that was not sufficiently relaxed by misfit dislocations. In the case of the BLT film grown on LAO substrate, the magnitude of lattice misfit between BLT and LAO was very small (~1/10) in comparison with the case of the BLT grown on STO. The relatively small misfit strain formed in layered structure of the BLT films on LAO, therefore, was easily relaxed by distorting the film, rather than forming misfit dislocations or cracks, resulting in misorientation regions in the BLT film.

Aerosol deposition method (ADM) for shock-consolidation of fine ceramics powder to form dense and hard layers is reported. Submicron ceramic particles were accelerated by gas flow in the nozzle up to velocity of several hundred m/s. During interaction with substrate, these particles formed thick (10 ∼ 100 μm), dense, uniform and hard ceramics layers. Experiments were fulfilled at room temperature. The results of fabrications, microstructure and mechanical properties of oxides (Al2O3; (Ni,Zn)Fe2O3; Pb(Zr0.52,Ti0.48)O3 and non-oxide (AlN; MgB2) materials are presented. Every layer has polycrystalline structure with nano-meter order scale.

The structural evolution of polytetrafluoroethylene (PTFE) crystalline polymer latex films is studied at hundreds nanometer length scale by atomic force microscopy and Brillouin light scattering. In a controlled sintering process the transition is observed from the original particle distribution towards a ‘fibrillar’ structure of crystalline regions embedded in a disordered matrix. This transition is accompanied by a cross-over from localized acoustic excitations to propagating acoustic phonons, related to mesoscopic elastic properties. After sintering, a ‘mark’ of the original particulate structure persists, suggesting that filming of crystalline polymers may be analogous to sintering of ceramic powders. Films of crystalline polymers can thus be exploited as model systems to study the elasto-optical properties of granular and disordered media.