We present a new Screened Bond-Order Potential (SBOP) for molybdenum in which the environmental dependence of two-center tight-binding bond integrals has been implemented via a recently developed analytic expression. These bond integrals reproduce very well the numerical ab-intio values of screened LMTO bond integrals. In particular, they display the large discontinuity in ddπ between the first and second nearest neighbor of the bcc lattice whereas they do not show any discontinuity in ddσ. This dependence can be traced directly to the angular character of the analytic screening function and is shown to be critical for the behavior of the second nearest neighbor force constants. The new BOP eliminates the problem of the very soft T2 phonon mode at the N point that is found in most two-center tight-binding models. Preliminary study of the core structure of 1/2<111> screw dislocations performed using SBOP indicates that the core is narrower and less asymmetric than structures found in previous studies, in agreement with recent ab-initio calculations.

Advanced martensitic steels are leading candidate materials for fusion reactor structural components due to their resistance to void swelling, and good balance of physical and mechanical properties. However, irradiations at temperatures below about 400°C result in increases in the cleavage-to-microvoid coalescence transition temperature, as well as reductions in the upper-shelf tearing toughness. This paper reports on the transition regime fracture toughness properties of two unirradiated 7-9Cr martensitic alloys, F82H and T91. Effective fracture toughness-temperature curves, Ke(T), were measured in the transition regime using relatively small pre-cracked specimens. Three sets of data obtained on different specimen sizes and geometries for the two steels are analyzed. All the data can be placed on a single so-called master-curve, when shifts due to size/geometry and material are taken into account. The results from mechanical tests, finite element method (FEM) simulations of crack tip fields and confocal microscopy-fracture reconstruction observations are presented. The link between the different mechanisms taking place at various length scales, resulting in the complex process of cleavage fracture, is discussed.

Multiple layers of an Interstitial Free (IF) steel microstructure are polished and analyzed using EBSP and Orientation Imaging Microscopy (OIM) technique. This enables to build a 3D finite element mesh which represents the actual grains arrangement. Then a 30 % reduction plane strain compression is applied on this mesh in order to capture the heterogeneous deformation of the aggregate. After deformation, many grains show large orientation spreads, and thus do not follow a Taylor type assumption. This computation also shows that the stored energy distribution depends on the chosen definition.

Classical plasticity theories generally assume that the stress at a point is a function of strain at that point only. However, when gradients in strain become significant, this localization assumption is no longer valid. These conventional models fail to display a ‘size effect’. This effect is seen experimentally when the scale of the phenomenon of interest is on the order of several microns. Under these conditions, strain gradients are of a significant magnitude as compared to the overall strain and must be considered for models to accurately capture observed phenomena.

The mechanics community has been actively involved in the development of strain gradient theories for many years. Recently, interest in this area has been rekindled and several new approaches have appeared in the literature. Two different approaches are currently being evaluated. One approach considers strain gradients as internal variables that do not introduce work conjugate higher order stresses. Another approach considers the strain gradients as internal degrees of freedom that requires work conjugate higher order stresses. Experiments are being performed to determine which approach models material behavior accurately with the least amount of complexity. A key difference between the two models considered here is the nature of the assumed boundary conditions at material interfaces. Therefore, we are investigating the deformation behavior of aluminum/sapphire interfaces loaded under simple shear. Samples are fabricated using ultra-high vacuum diffusion bonding. To determine the lattice rotations near the boundary, we are examining the samples with both electron backscatter diffraction methods (EBSD) in the scanning electron microscope and with a variety of diffraction techniques in the transmission electron microscope. The experimentally found boundary conditions shall be subsequently used to determine whether the simpler internal variable model is adequately descriptive or if the greater complexity associated with the internal degree of freedom approach is warranted.

Fixed and induced point dipoles have been implemented in the Ewald and Particle-Mesh Ewald (PME) formalisms. During molecular dynamics (MD) the induced dipoles can be propagated along with the atomic positions either by interation to self-consistency at each time step, or by a Car-Parrinello (CP) technique using an extended Lagrangian formalism. The use of PME for electrostatics of fixed charges and induced dipoles together with a CP treatment of dipole propagation in MD simulations leads to a cost overhead of only 33% above that of MD simulations using standard PME with fixed charges, allowing the study of polarizability in largemacromolecular systems.

We present FACET: a two dimensional simulator to model polycrystalline thin film growth, which links atomic scale processes to macroscopic phenomena. The model is based on the concept of describing the crystal surface in terms of preferred facets. Line segments were used to depict the profile of the grain and grain boundaries. Multiple nuclei are semi-randomly distributed along the textured or non-textured surface, and crystallographycally appropriate facets are created in the nucleation simulation. We use a Kinetic Lattice Monte Carlo (KLMC) method to calculate the inter-facet diffusion rates and use a continuum approach to grow the facets, hence the multiple grains. The software is Windows(95/98/2000/NT) based and has an integrated Graphical User Interface, within which a user can input deposition conditions and experimental and simulation data, visualize the nucleation and growth of the grains, and obtain the final grain structure and texture.

We present a simulation technique that allows the calculation of a phase coexistence curve from a single nonequilibrium molecular dynamics (MD) simulation. The approach is based on the simultaneous simulation of two coexisting phases, each in its own computational cell, and the integration of the relevant Clausius-Clapeyron equation starting from a known coexistence point. As an illustration of the effectiveness of our approach we apply the method to explore the melting curve in the Lennard-Jones phase diagram.

Mechanical elastic and inelastic properties of thin coatings are often studied by means of the curvature measurement technique in combination with the Stoney formula. It is then implicitly assumed that the elastic substrate is initially flat which implies that the curvatures theoretically remain constant over the substrate. If the substrate has a slight initial curvature, a different situation arises. In this case, the curvatures will vary over the substrate. It has recently been shown that in spite of this, the stresses that appear in the thin coating will remain constant over the surface of the substrate. Therefore, measured curvatures can generally not be used to extract layer stresses without a proper compensation for the initial curvature. A general method for the extraction of mechanical properties from curvature measurements on non-planar substrates is outlined. The method is valid for linear as well as non-linear shell theories. The compensation needed to evaluate a coating on a circular substrate with spherical initial curvature is studied for all relevant parameters. The results are particularly discussed in relation to curvature measurements on silicon wafers.

A recent approach which combines first-principles and experimental data to produce highly accurate and reliable interatomic potentials is tested for the case of bcc and fcc Fe. The Embedded-Atom-like potential accurately reproduces the basic equilibrium properties of bcc Fe, including elastic constants, phonon properties, and vacancy formation energies, as well as the correct relative stability of structures with coordination numbers ranging from 12 to 4. This potential was used in a simulation study of lattice trapping effects during the cleavage fracture of bcc Fe. A strong directional anisotropy for crack propagation was observed due to lattice trapping effects. The strongest trapping effects were observed for cleavage along the {110} planes and it was found that lattice trapping strongly favors cleavage along the {100} planes.

A hybrid quantum mechanical/molecular dynamics simulation scheme is developed by embedding a quantum mechanical system described by the real-space density-functional theory in a classical system of atoms interacting via an empirical interatomic potential. A novel scaled position method for handshake atoms coupling the quantum and the classical systems is introduced. Hybrid simulation run for oxidation of Si (100) surface is performed to demonstrate seamless coupling of the quantum and the classical systems.

Plastic deformation in bcc metals at low temperatures and high-strain rates is controlled by the motion of a/2<111> screw dislocations, and understanding the fundamental atomistic processes of this motion is essential to develop predictive multiscale models of crystal plasticity. The multiscale modeling approach presented here for bcc Ta is based on information passing, where results of simulations at the atomic scale are used in simulations of plastic deformation at mesoscopic length scales via dislocation dynamics (DD). The relevant core properties of a/2<111> screw dislocations in Ta have been obtained using quantum-based interatomic potentials derived from model generalized pseudopotential theory and an ab-initio data base together with an accurate Green's-function simulation method that implements flexible boundary conditions. In particular, the stress-dependent activation enthalpy for the lowest-energy kink-pair mechanism has been calculated and fitted to a revealing analytic form. This is the critical quantity determining dislocation mobility in the DD simulations, and the present activation enthalpy is found to be in good agreement with the previous empirical form used to explain the temperature dependence of the yield stress.

A two-dimensional micromechanical model based on the finite element method is presented to model two-phase ferritic/pearlitic steels, by aid of generalised plane strain elements. A periodic representative cell containing 100 ferrite grains, and the desired fraction pearlite is used. By applying periodic boundary conditions, loading by an average stress or strain state is possible.

Uniaxial tensile tests were performed on specimens containing the ferrite and pearlite microstructures, and on two-phase materials containing 25% and 58% pearlite respectively. The stress-strain data of the pearlite material is used to fit a laminar dependent Taylor relation to represent the pearlite workhardening. Thereafter, laminar spacings in the two-phase materials are measured, and the total stress-strain response of the materials is modelled. Comparisons between generated data and experiments show good agreement up to a strain of 2%.

We calculate the electronic structure of AlΣ9 tilt grain boundary with substitutional Na impurity atoms by first principles pseudopotential method. Results show that by Na segregation Al grain boundary expands and the valence charge density decreases significantly along the boundary. There is no stronger bond than metallic bond in the boundary even with Na impurity. We therefore conclude that the mechanism of Na-promoted Al grain boundary embrittlement should be one kind of ‘decohesion model’.

The discontinuous yielding of a model material, which contains an edge dislocation moving in the atmosphere of solute atoms, is studied by Kinetic Monte Carlo (KMC) simulations. The stress-strain curves for a constant strain rate were obtained at different temperatures. The dislocation moves discontinuously, producing three types of serrated yielding behavior at intermediate temperatures for different imposed strain rates. Positive dependence of flow stress on temperature and negative strain rate sensitivity were observed in the regime of discontinuous motion. The present model, though highly simplified and not taking into account the collective behaviors of dislocations in real materials, does exhibit some of the basic features observed in experiments.

The effect of crystal orientation on the discontinuous deformation and coarse slip bands in aluminum single crystals has been investigated by finite element simulation. The numerical analysis is based on a rate-dependent crystal plasticity model recently formulated by us with modifications made here that explicitly accounts for the three strain hardening stages and the effect of the interaction of dislocations and alloying elements on setting the hardening rules on the individual slip systems of the aluminum alloy crystal. The dependence of the magnitude of the serrations of tensile stress-strain curves on orientation and its evolution over the three strain hardening stages of Al-0.3%Mg single crystals observed experimentally has been successfully explained in this investigation.

To understand how dislocations form ordered structures during the deformation of metals, we perform computer simulation studies of the dynamics and patterning of screw dislocations in two dimensions. The simulation is carried out using an idealized atomistic model with anti-plane displacements only; we show that this system is an analog of the two-dimensional XY rotor model. Simulation studies show that under a constant applied shear strain rate, the flow of dislocations spontaneously coalesces to form narrow dislocation-rich channels separated by wide dislocation-free regions, so that the applied strain is localized into slip bands. We argue that this pattern formation represents a phase separation into low/high defect density phases associated with the XY model, and conjecture that thermodynamic forces drive strain localization.

A new model for the discretisation of dislocation lines is presented, which is optimised for mesoscale simulations of dislocation dynamics. By comparison with the existing edge-screw model, the present one provides a better description of the stress field close to the dislocation lines. It simplifies the modelling of dislocation reactions and accelerates computations by allowing to make use of larger time steps. An application to attractive junctions and forest hardening is briefly sketched.

Dislocations decorated by both clusters of self-interstitial atoms (SIAs) and small dislocation loops, are one of the microstructure features which can play an important role in post-irradiated deformation processes. The interactions between dislocations and clusters are important and are usually treated within the framework of isotropic elasticity theory. However, it is still not clear whether or not these interactions, especially for small clusters at short distances, can be treated accurately by elasticity theory. Comparative studies by atomistic simulation and elasticity theory can clarify this. Here we present a simple example of such a study where interactions between a glissile SIA cluster and an edge dislocation are studied in bcc-Fe and fcc-Cu using both techniques. In Fe we have studied the interaction of a dislocation with Burgers vector b= 1/2<111> lying along <112> direction with a SIA cluster with the same b situated at different distances below the extra half-plane. In Cu, the dislocation and cluster had b = 1/2<110> and the dislocation line was along the <112> direction. Interactions with clusters of diameter about 1nm were simulated. Elastic calculations were made within the isotropic theory with parameters estimated from atomistic simulation. The results obtained by both techniques are discussed and some preliminary conclusions for different cases are drawn.

We propose a new method to simulate the interface diffusion in the solid-state bonding-process. This method is more significant under the condition of low pressure and low temperature. It is available for the bonding of two bodies which consist of the atoms of the same kind and have slight surface roughness. In a conventional method, the elastic deformation during the bonding process is considered. The interface diffusion is enhanced and the bonding time decreases when the external pressure changes at appropriate frequency. In order to clarify the enhancement effect of changing the pressure, we examine three cases, i.e., the 1st pressure type is constant pressure, the 2nd type is zero pressure, and the 3rd type is the on-off pressure. Our results suggest the on-off pressure decreases the required time for the perfect bonding if we choose on appropriate frequency.

Repeated stress-relaxation experiments are used to characterize the deformation parameters in 3 types of single crystals (Cu, Ni3Al and Ge) in which different dislocation mechanisms are known to operate. Mobile dislocation exhaustion rates and the amplitude of the yield point at reloading after stress-relaxation are measured. These two parameters are discussed in terms of the work-hardening coefficient in monotonic tests.