The yield strength anomaly in some L12 compounds has been linked to the thermally assisted cross slip of screw superdislocations. This work continues earlier efforts to understand the yield strength anomaly in L12 alloys using computer simulations of dislocation motion. Dislocations are modelled within isotropic elasticity theory, and simple rules are used to model the cross-slip process in the two dimensional geometry of the simulation. The velocity of a single dislocation in Ni3Al is studied as a function of the applied stress. The observed velocities vary nonlinearly with the applied stress. Further, dislocations are observed to become immobile for small applied loads. At high stresses, the dislocations are observed to advance relatively unhindered by the thermally activated cross slip process. Fluctuations in the velocity of the dislocations are studied, and their autocorrelation function shows an increased correlation time near a threshold stress. This threshold stress is identified with the critical stress proposed in earlier works.

We propose a model to study the plasticity of ice single crystals by numerical simulations. The model includes the long-range character of the interaction among dislocations, as well as the possibility of mutual annihilation of these line defects characterized by its Burgers vector. A multiplication mechanism representing the activation of Frank-Read sources due to dislocation pinning is also introduced in the model.

With our approach we are able to probe the dislocation patterns, which result from the dislocation dynamics. Furthermore, our results exhibit features characteristic of driven dynamic critical phenomena such as scaling behavior, and avalanche dynamics. Some of these results account for the experimental findings reported for ice single crystals under creep deformation, like the power-law distributions of the acoustic emission intensity observed sistematically in experiments.

The goal of this work is to study the interaction of dislocations with local obstacles to glide such as point defects, precipitates and other dislocations. The quasicontinuum method is used as the basis of this study. It is demonstrated that two types of boundary effects are of concern in the calculation of hardening parameters using finite sized simulation cells. A recently developed technique to incorporate periodic boundary conditions in the quasicontinuum method is used to eliminate surface effects which were present in earlier implementations and to simulate a dislocation of infinite extent interacting with an array of obstacles. The second type of boundary effect is due to the boundary conditions on the lateral boundaries. A method based on finite element calculations is proposed for quantifying the effect of lateral boundaries in these simulations. Preliminary results for the validation of the method are presented as well as a simulation of the interaction between a conventional edge dislocation in Al with an array of clusters of Ni atoms.

The mechanical losses due to the bowing of isolated Frank-Read sources under application of periodic loads is studied within a continuum simulation of dislocation dynamics. The dislocations are modelled within isotropic elasticity theory and assumed to be in the overdamped limit. Dislocation radiation effects are neglected. The mechanical losses are studied as a function of bias stress, amplitude of the periodic stress and frequency. The frequencies studied lie between 10 KHz and 1 MHz. Under high stresses applied at low frequencies, a deviation from the expected Lorentzian resonance shape is observed. The physical origins of this deviation are discussed.

The fine structure of dislocations in intermetallic compounds can have a profound influence on their macroscopic mechanical properties. The development of appropriate models of deformation requires consideration of dislocation core structure, possible dissociation or decomposition reactions, overall dislocation morphology and relevant dislocation interactions based on detailed transmission electron microscopy study. This empirical information may be rationalized based on both atomistic and continuum-level dislocation modeling. The specific cases of jogged 1/2<110] ordinary dislocations in γ-TiAl and a<101> superdislocations in NiAl are discussed.

The mechanical loss and shear modulus behaviors of Ni3Al and Ni3(Al, Ta) single crystals have been investigated in the temperature range 100 K – 1300 K. The mechanical loss spectra exhibit two temperature regimes, which are separated by a relaxation peak at nearly 950 K for a frequency of 1 Hz. This relaxation peak has been interpreted by the stress re-orientation of Al-Al elastic dipoles in the (111) octahedral plane [1, 2]. In the low temperature regime, corresponding to the anomaly domain of the flow stress, the mechanical loss of pre-deformed specimens exhibit a strong positive dependence on both the oscillation amplitude and the amount of pre-strain.

Pre-deformations, which were performed either at room temperature or at 100 K, yield a broad maximum in the mechanical loss that extends from nearly 100 K up to 550 K. This maximum is observable for only strain amplitudes larger than 10−4 and entirely vanishes after heating the specimens above 550 K. The increase in mechanical loss has been attributed to the bowing of the superkinks under the action of the applied stress. The gradual and irreversible decrease in damping above 300 K is interpreted in terms of pinning of the screw dislocation segments by a thermally activated process leading to the formation of Kear-Wilsdorf locks.

In the near vicinity of the [021] load orientation, γ-TiAl deforms via ordinary dislocations (Burgers vector b = 1/2<110]). As for deformation by <110] dislocations, the flow stress shows a peak at about 600°C. Results of an extensive microstructural investigation aimed at identifying the origin of this mechanical anomaly are presented. The analysis was conducted on single crystals oriented for single slip. It confirmed that ordinary dislocations tend to align themselves along the screw direction. This preferential line direction becomes gradually accentuated as the deformation temperature is raised up to the peak temperature. This effect is accompanied by a strong tendency towards forming cusps, but there is indication that the immobilisation along the screw direction takes place prior to dislocation pinning. In the vicinity of the peak temperature, screw dislocations gather in the form of bundles. No clear correlation is found between the temperature dependence of the flow stress and that of the density of pinning points. The relationship between these microstructural findings and the occurrence of a flow stress anomaly is discussed.

The flow stress of Al-rich γ-TiAl peaks at a temperature located between about 600°C and 1000°C. We present mechanical data on samples oriented so as to deform on a <011]{111} system in single slip. These data are complemented by TEM observations in samples strained at selected temperatures (RT, 400°C, 600°C and 800°C). The present paper addresses (i) properties of dislocation organisation, (ii) the dissociation mode of <110] dislocations, (iii) locking properties of <110] dislocations and (iv) the formation of faulted dipoles.

Deformation properties of body-centered-cubic transition metals are controlled by the core structure of screw dislocations and their studies involve extensive computer simulations. In this paper we present the recently constructed bond-order potentials (BOP) that are based on the realspace parametrized tight-binding method. In order to examine the applicability of the potentials we have evaluated the energy differences of alternative structures, investigated several transformation paths leading to large distortions and calculated phonon dispersions. Using these potentials we have calculated γ-surfaces that relate to the dislocation core structures and discuss then the importance of directional bonding in studies of dislocations in transition metals.

Recent experiments on deformation of semiconductors show an abrupt change in the variation of the critical resolved shear stress, τy, with temperature, T. This implies a change in the deformation mechanism at a critical temperature Tc. In the cases examined so far in our laboratory and elsewhere, this critical temperature appears to coincide approximately with the brittle-ductile transition temperature, TBDT. In this paper, the deformation experiments performed on the wide bandgap semiconductor, 4H-SiC, over a range of temperatures and strain rates are described together with the characterization of induced dislocations below and above Tc by transmission electron microscopy. Based on these results, and those of Suzuki and coworkers on other compound semiconductors, some understanding of the different mechanisms operating at low and high temperatures in tetrahedrally coordinated materials has been gained, and a new model for their brittle-ductile transition has been proposed.

Using atomistic simulations we investigate the annihilation of screw dislocation dipoles in Cu. In particular we determine the influence of jogs on the annihilation barrier for screw dislocation dipoles. The simulations involve energy minimizations, molecular dynamics, and the Nudged Elastic Band method. We find that jogs on screw dislocations substantially reduce the annihilation barrier, hence leading to an increase in the minimum stable dipole height.

The structure of a dissociated edge dislocation in copper is investigated. Attention is given to the structure of the Shockley partials and the equilibrium size of the fault ribbon. The studies are carried out through Molecular Dynamics simulations. The atomic interactions have been modelled through an Embedded Atom Model (EAM) potential. the implementation of which has been specially designed for this study. Our main results show that the equilibrium distance between partials is very sensitive to the type of boundary conditions imposed on the simulated system.

Recent work has been done on the analysis of elastic stress singularities, such as cracks and dislocations, which propagate at supersonic speeds. Gumbsch and Gao have performed atomistic simulations in which dislocations are created and travel at transonic velocities (speeds which are greater than the material's shear wave speeds but less than the longitudinal wave speed) close to the theoretical value corresponding to the radiation-free state for glide motion. Gao et al. have derived expressions for this radiation-free velocity in both isotropic and anisotropic media. We have performed molecular dynamics simulations showing dislocation nucleation at crystal surface ledges. Dislocations were nucleated at either sub- or transonic velocities, depending upon the ambient temperature, and accelerated to transonic speeds. This paper shows the velocity profiles for the emitted dislocations and compares velocities observed with a theoretical minimum-radiation speed derived by a Strohtype anisotropic elasticity analysis performed by Barnett and Zimmerman. Our findings are particularly exciting considering these simulations were not specifically engineered for the purpose of creating transonic defects, but show agreement with theory nonetheless.

In epitaxial layers, the mechanism of stress relaxation under consideration is in general dislocations. This paper will present experimental evidences for other modes of relaxation. which may occur in some situations especially where a strong size effect or misfit is present. In fact the stress is the driving force for intermixing, twinning, and phase transformation, which are not expected to occur in the bulk. All these experimental results have been sustained by numerical simulations that will also be presented.

It has long been noticed that the effect of Cu solute atoms is important for the microstructural evolution of ferritic pressure vessel steels under neutron irradiation conditions. Despite the low concentration of Cu in steel, Cu precipitates form inside the α-Fe surrounding matrix and by impeding free dislocation motion considerably contribute to the hardening of the material. It has been suggested that Cu-rich clusters and combined Cu solute atoms-defect clusters that may act as initiating structures of further precipitates nucleate during annealing of displacement cascades. In order to assess the importance of the different mechanisms taking place during collision events in the formation and later evolution of these structures, a detailed Molecular Dynamics (MD) analysis of displacement cascades in a Fe-1.3% at. Cu binary alloy has been carried out. Cascade energies ranging from 1 to 20 keV have been simulated at temperatures of 100 and 600 K using the MDCASK code, in which the Ackland-Finnis-Sinclair many-body interatomic potential has been implemented. The behaviour of metastable Cu selfinterstitial atoms (SIAs) in the form of mixed Fe-Cu features is studied as well as their impact on the resulting defect structures. It is observed that above 300 K generated Cu SIAs undergo recombination with no substantial effect on the after-cascade microstructure while at 100 K Cu SIAs remain sessile and exhibit a considerable binding to interstitial and vacancy clusters. Finally, the effect that the production of vacancies via collision cascades may have on the self diffusion of Cu solute atoms is quantitatively addressed by means of determining diffusion coefficients for Cu atoms under different microstructural conditions.

In seeking to understand at a microscopic level the response of dislocations to stress we have undertaken to study as completely as possible the simplest case: a single dislocation in a two dimensional crystal. The intention is that results from this study will be used as input parameters in larger length scale simulations involving many defects. We present atomistic simulations of defect motion in a two-dimensional material consisting of atoms interacting through a modified Lennard-Jones potential. We focus on the regime where the shear stress is smaller than its critical value, where there is a finite energy barrier for the dislocation to hop one lattice spacing. In this regime motion of the dislocation will occur as single hops through thermal activation over the barrier. Accurate knowledge of the barrier height is crucial for obtaining the rates of such processes. We have calculated the energy barrier as a function of two components of the stress tensor in a small system, and have obtained good fits to a functional form with only a few adjustable parameters.

A combination of electric force microscopy (EFM) and non-contact scanning force microscopy (SFM) was used to study micro-indentation-induced dislocation bands in sphaleritic ZnS single crystals. Large local distortions in electrical potential from the dislocation bands were observed in the EFM images. For the first time, the electric charges of resting partial Zn(g) and S(g) dislocations were determined quantitatively. The results compare well with theoretical models.

Typical applications for components and equipment with extreme quality requirements regarding surface roughness, shape accuracy and integrity of the generated surface structure can be found in optical and semiconductor industry. Ultra precision machine tools equipped with sharp, single crystalline diamond provide the necessary machining accuracy. Here the actual cutting process can take place at atomic level, which makes the acquisition of typical cutting process data difficult or impossible. However a detailed characterization and understanding of the process is vital for its effective control as well as for further tool and process development.

Therefore an approach is made that focuses on linking results from atomistic simulations with results and observations from cutting experiments. In this work the potential of molecular dynamics (MD) modeling for studying phenomena related to ultra precision cutting processes will be demonstrated. Observations and first results for machining copper will be presented.

An interatomic potential for aluminum was developed, which is based on empirical tight binding approximations. The model successfully reproduced the shear constants, structure energy differences, and phonon dispersion curves. This transferable potential was applied on static surface relaxations, and shows good agreements with experimental results on the oscillatory damped behavior of the multilayer relaxations and the expansion of the (111) surface.

Materials used in many technical applications contain a broad variety of defects. In both metals and ceramics, pores and inclusions influence significantly the elastic and plastic response. They can limit or broaden the range of applicability of the materials for high performance components and play, therefore, a very significant role in engineering applications.

This study focuses on the onset of dislocation formation and its intensity at pores and hard inclusions during the deformation process. For this purpose a molecular dynamics model of an fcc metal containing pores and/or inclusions was developed. This model material was then sheared at different strains and related development of the dislocation substructures investigated.