Atomistic dislocation models were used to determine the properties of dislocation core fields in Al using an EAM potential. Equilibrium atom configurations were compared with initial configurations displaced according to the Volterra field to determine core displacement fields for edge, screw, and mixed (60° and 30°) geometries. The core field was approximated by a line force defect field lying parallel to the dislocation line direction. Best-fit parameters for the core fields were obtained in terms of the anisotropic elastic solution for a line force defect, from which the line force strengths and the origin of the line forces were determined. The line force stress fields were then used to compute the forces between dislocations for several dislocation configurations. The Volterra field dominates beyond 50b but core field forces modify the equilibrium angle of edge dislocation dipoles and determine the force between otherwise noninteracting edge and screw dislocations at distances out to 50b compared to the Volterra-only forces.

Facets or planar surfaces appear often on crystalline solids, and need to be accurately modeled in studying surface evolution. Here we propose a model in which the radius of curvature of the equilibrium crystal surface is prescribed as a function of crystallographic orientation. In this approach, a facet is represented by the Dirac delta function with the weight of the delta function equal to the width of the facet plane. This model allows sharp corners on solid surfaces, but avoids the non-uniqueness of equilibrium surface profiles that plagues previous facet models. We demonstrate this approach by solving the equilibrium shape and surface energy of triangular crystals.

In a γ/γ' nickel-aluminum alloy, a new phase has been identified along grain boundaries by conventional transmission electron microscopy. This phase is identified to be of the C6Cr23 type structure with lattice parameter 10.48Å, which is approximately three times that of Ni 3Al or nickel. This grain boundary precipitate is found to be coherent with the γ' phase in one grain. The chemical composition of this grain boundary precipitate is determined by energy dispersive x-ray spectroscopy and parallel electron energy loss spectroscopy. The deformation mechanism of this grain boundary precipitate is investigated on post-mortem specimens which have been carefully deformed by nanoindentation after jet-polishing.

Triple junctions are crucial elements in microstructural evolution: for example, their mobility can be rate-limiting if lower than that of grain boundaries. However, very little is known about their atomic-level structure and properties. We studied the atomic structure of multiple-twin triple junctions in silicon, formed by the convergence of two {111} and one {221} symmetric-tilt grain boundaries. Molecular dynamics simulations with the Stillinger-Weber potential and constant-traction border conditions were performed on several triple junction configurations, obtained by different combinations of the three grain boundaries. All the configurations have a positive excess line energy, a measurable volume contraction and display regions of opposite, tensile and compressive, residual stress. Moreover, we tried to elucidate the role of triple junctions as being the seeds of the only microscopic events that can lead to topological changes in the microstructure. Such events, usually dubbed T1 and T2 in mesoscopic models, correspond to grain switching (in the Ashby-Verrall sense) and grain-disappearance events, respectively. We present preliminary results for the atomic-scale modelling of both classes of topological events and discuss the connection between atomistic and mesoscopic modelling of microstructural evolution.

Elementary mechanisms of deformation by fatigue in duplex stainless steels bicrystals are studied by transmission electron microscopy (TEM). An attempt is made to correlate the bicrystal macroscopic behaviour with the interphase interface crystallography.

A possibility to manipulate the microstructure of cold rolled (down to less than 1% of the initial thickness) Fe94Ni4Ti2 and Fe93Ni4 Cr3 foils via inter-phase cycling by nitriding and de-nitriding is investigated. The grains in the as-rolled material had a dominating (001)[110] texture and contained a complicated internal nanostructure due to a dislocation network. The interphase cycling α↔γ'-Fe4N and α↔ε-FexN (x∼3) was investigated as a tool to change or destroy the texture. We observed that the α↔γ'-Fe4N phase transformations weaken but do not erase the texture. A stronger reduction of the texture was obtained in the α↔ε-cycling, where grains with a new orientation appear in XRD scans. During the α↔γ' phase transformation a lamella structure is formed which is coarsening with the number of cycles. It was found that the rates of the α↔γ' and α↔ε interphase transitions were slowing down with the number of cycles. The observation is explained by an increase of kinetic barriers while removing/transforming the rolling-induced defects during the cycling.

The sintering process of ceramics involves grain-boundary migration (GBM) that is accompanied by mass transport across an interface. In this study, electron backscatter diffraction (EBSD) has been used to examine grain-boundary migration in alumina bicrystals with liquid films at the interface. EBSD patterns, taken near the sintered interface, have been used to study the effects of crystallography on GBM and to study the orientation relationships within the migrated regions of the crystal. Results indicate that the direction of migration is not always the same as that predicted by the current theories on GBM. It was also found that there may be small-angle misorientations in the migrated regions.

Microgravity dendritic growth experiments, conducted aboard the space shuttle Columbia (STS-87) in November/December 1997, are analyzed and discussed. In-situ video images now reveal that pivalic acid (PVA) dendrites growing in the diffusion-controlled environment of low-earth orbit exhibit a range of growth behaviors, including steady, transient, and oscillatory states. The observed transient features of the growth process are being studied with the objective of understanding their physical mechanisms. Some transients in the observed growth speed are thought to arise as an intrinsic aspect of the evolving dendritic pattern. Variability in the growth speed observed from a sequence of identical runs at equal supercooling suggests that self-interactions of the dendrite remain important throughout the development of the dendritic pattern. A Greens function analysis of the near-tip diffusion sources contributing to the local field at the tip suggests that strong non-local interactions exist well into the time-dependent side-branch region of real dendrites. Video data obtained at 30 fps allow the first application of discrete Fourier transform methods (Lomb periodograms) to the digitized images of dendritic growths under quiescent microgravity conditions. These observations provide evidence for the appearance of characteristic frequencies in the tip shape and its dynamical behavior. Some of the frequency bands observed coincide closely with the ratio of the dendritic tip growth speed divided by the side branch spacing. Other observed lower frequencies remain as yet unexplained. These data, and their interpretations, are discussed in this paper.

Plastic deformation by dislocation glide is known to be associated with the spontaneous formation of mesoscopic patterns of various types, e.g. cellular dislocation structures during unidirectional deformation and quasi-periodic persistent slip band structures during cyclic deformation. hile it is recognized that dislocation patterning represents a dissipative far-from-equilibrium process, theoretical modelling of those phenomena is complicated by the long-range nature of dislocation interactions inducing collective dislocation behaviour on a mesoscopic scale. n this paper the problem is addressed using a stochastic approach with random uctuations acting on the evolution of the dislocation ensemble. he intensity of the uctuations is determined self-consistently from dynamic dislocation interactions and, hence, re ects correlated dislocation motion. t is shown that those uctuations may induce dislocation patterns by stabilizing non-uniform dislocation distributions. icrostructure-based models are presented for unidirectional and cyclic plastic deformation. n the rst case fractal dislocations distributions corresponding to hierarchically organized dislocation cell structures are obtained, while in the latter case a decomposition into dislocation-rich walls or veins and depleted channels is found, which are associated with the formation of persistent slip bands and matrix structures. he good agreement with experimental observations in single-crystalline f.c.c. metals points at the importance of collective dislocation e ects in the self-organization of those structures.

Mechanism of the slip transmission across the grain boundaries was studied on Σ3 bicrystals of Fe-4at%Si by transmission electron microscopy. In situ straining experiments as well as post mortem observations were performed. Three distinct events were observed in dependence on the angle α between the slip and grain boundary planes, namely transformation of the tertiary slip system in one grain into the secondary slip system in the other grain for α ≍ 90°, cross slip of dislocations of the primary slip system into the {112} plane parallel to the grain boundary for α ≍21°, and an abrupt formation of a sub-grain boundary in one grain for α ≍49°. Reasons for these diverse phenomena will be discussed in terms of interactions between the slip dislocations and the grain boundary dislocations.

We examine the morphological evolution of faceted grain boundaries in gold during annealing. Experiments were performed on <111> oriented gold films composed of two Σ=3 related orientation variants. The boundaries between these variants initially possess a high density of finely spaced (<25 nm) facets on {112} type planes. During annealing a large proportion of these fine-scale corrugations are annihilated, and the facet distribution coarsens significantly. Through in situ transmission electron microscopy (TEM), we directly observe this coarsening process. These results show a more complex behavior than geometric models for facet evolution would suggest and point to the need for an improved understanding of facet-junction properties and the interactions between grain boundary facets and dislocations.

The application of mismatched combinations of heteroepitaxial semiconductors has been quite limited due to the presence of high threading dislocation densities. In recent years, great progress has been made toward solving this problem using compliant substrates and lateral epitaxial overgrowth. We have proposed another approach which we call patterned heteroepitaxial processing (PHP), and which involves post-growth patterning and thermal annealing. In this paper we describe the successful application of the PHP technique to the ZnSe/GaAs (001) material system.

Epitaxial layers of ZnSe on GaAs (001) were grown to thicknesses of 2000 - 6000 Å by photoassisted metalorganic vapor phase epitaxy (MOVPE). Following growth, layers were patterned by photolithography and then annealed at elevated temperature under flowing hydrogen. Threading dislocation densities were determined using a bromine/methanol etch followed by microscopic evaluation of the resulting etch pit densities.

We found that as-grown layers contained more than 107 cm-2 threading dislocations. The complete removal of threading dislocations was accomplished by patterning to 70 μm by 70 μm square regions followed by thermal annealing for 30 minutes at temperatures greater than 500°C. Neither post-growth annealing alone nor post-growth patterning alone had a significant effect. By studying the annealing temperature dependence, we have determined that the dislocation removal by PHP is thermally activated. It appears that the maximum dimension for patterned regions in PHP is determined by the annealing temperature rather than an effective range for image forces.

These results show that PHP can be used to completely remove threading dislocations from lattice-relaxed heteroepitaxial layers. In principle this approach should be generally applicable to mismatched heteroepitaxial materials.

The interactions between lattice dislocations and grain boundaries were studied in nickel bicrystals. Three types of grain boundaries, according to their energy, were investigated : singular σ3 {111}, vicinal near σ11 {311} and general near σ11 {332} grain boundaries. The experiments were performed by transmission electron microscopy using a set of techniques : conventional, weak beam, in situ and high resolution transmission electron microscopy. Dislocation transmission from one crystal to the other was only observed for σ3 {111} GB. It consists in a decomposition within the grain boundary of the trapped lattice dislocation followed by the emission of one partial in the neighbouring crystal. A high resolved shear stress is required to promote the emission process. Most often, the absorbed lattice dislocations or extrinsic grain boundary dislocations react with the intrinsic dislocation network giving rise to complex configurations. The evolutions with time and upon thermal treatment of these configurations were followed by in situ transmission electron microscopy. The evolution processes, which differ with the type of grain boundaries, were analyzed by comparison with the existing models for extrinsic grain boundary dislocation accommodation. They were tentatively interpretated on the basis of the grain boundary atomic structures and defects obtained by high resolution transmission electron microscopy studies.

The dynamics of dislocations at yield can be understood within the framew ork of the depinning transition of elastic manifolds in random media. Close to the threshold stress for their long-range motion, the geometry and dynamics of dislocations are characterized by a set of critical exponents. We consider a single flexible dislocation gliding through a random stress field, taking in to account long-range self stresses, and estimate the critical stress where depinning takes place. Simulations of a discretized lattice model confirm the analytical estimate and yield numerical values of the critical exponents which are in agreement with theoretical predictions for an elastic string mo ving on a plane.

The microstructure determines for the most part mechanical properties of castings. This is the case especially with age-hardening aluminum alloys. To predict the phase transformation in cast parts during and after solidification a sophisticated approach to a coupled modeling of various simulation phenomena is presented. This approach couples a diffusion equation solver which considers the online estimation of thermodynamic equilibria with a macroscopic model for temperature calculation. The coupled micro-model predicts the dendrite solidification of ternary alloys. To simulate the phase transformation in temperature zones below solidification a homogenization model is added. Permanent mould casting experiments with ternary aluminum alloys are used to validate these models.

Semisolid alloys exhibit a shear rate history dependent flow behavior. The increasing interest on numerical modeling of thixo-casting processes makes it quite important to understand the flow behavior of semisolids. Its apparent viscosity is the key parameter for the numerical models. In this paper a two phase approach is used to investigate the rheology of semisolid alloys. It is assumed that both, liquid and solid phase, can be regarded as inter-penetrating continua with its own viscosity. Thus, each phase is thought to behave as a Newtonian fluid. The simulation results show that the rheology of the two-phase flow is determined by the interaction between the solid and the liquid, i.e. the momentum exchange between the two phases. Non-Newtonian flow behavior of the solid-liquid mixture is predicted although both phases are considered as Newtonian fluids.

Polysynthetically-twinned titanium aluminide (PST-TiAl), a fully lamellar γTiAl + α2-Ti3Al dual-phase alloy, is under evaluation for applications in rotary components in aircraft and automobile industries due to its high specific strength, and a high strength-retention capability at elevated-temperatures. However, the low ductility at room- to mid-high temperatures of the material hinders its application. Additions of certain tertiary elements to the binary TiAl system appear to improve the ductility at room- to mid-high temperatures, thus a balance among strength, ductility, and fracture toughness can be expected. In this article, segregation of tertiary elements to the lamellar interfaces is investigated. Single crystals of a TiAl with 0.6% atomic percentage tertiary additions are grown by an optical float-zone method. Segregation to the lamellar interfaces and the microstructure of the interfaces are investigated. Structures of the lamellar interfaces are characterized, and microchemistry and distribution habits of these elements along the γ+α2 lamellar boundaries as well as the γ-γ lamellar and domain boundaries are analyzed.

The paper presents an electron microscope study of phase transformation and recrystallization in an intermetallic α2(Ti3Al) + γ(TiAl) titanium aluminide alloy, after long-term creep. The mechanisms are closely related to the atomic structure of the α2/γ phase boundaries and are probably driven by a non-equilibrium of the phase composition leading to the dissolution of the α2 phase. The α2 /γ transformation is accompanied by the formation of precipitates, because the γ(TiAl)phase has a significantly lower solubility for interstitial impurities than the α2(Ti3Al) phase.

The inuence of mesoscopic strain-rate uctuations on the n ucleation of localized deformation bands (PLC bands) in dynamically strain ageing materials is discussed. It is shown that uctuations ma y lead to a macroscopic localization of plastic ow even if linear stability analysis (LSA) of the macroscopic constitutiv e equations predicts stable deformation beha vior. The resulting quantitative corrections to linear stability analysis are evaluated, and implications for the reliabilit y of LSA results are discussed.

The shape-memory effect relies on deformation in the martensitic state by motion of intervariant boundaries in response to an applied stress. A mechanism for this process in terms of interfacial defect generation, motion and interaction is proposed. This mechanism is conservative and is expected to operate with modest applied stresses and without thermal activation. Experimental observations of intervariant boundaries are discussed and atomistic simulations of the mechanism in such boundaries in hexagonal metals are presented.