Oral contributions

abstract: 1.1
Intergranular dislocations relaxation in singular GBs in copper
COUZINIE, J.-P., CNRS / CECM UPR 2801; DECAMPS B., CNRS / LCMTR UPR 209; BOULANGER L., CEA / SRMP; PRIESTER L., CNRS / CECM UPR 2801.
The analysis of the stress relaxation processes in grain boundaries (GBs) is a prerequisite for the understanding of the mechanical properties of polycrytals. Two theoretical models, dicrete product incorporation or core delocalization, have been proposed to explain the absorption of dislocations by grain boundaries (see review paper of Priester [1]). However, these models do not explicitly take into account the stacking-fault energy (SFE) of the material which is expected to play a role in these phenomena. The relaxation processes have been previously studied by High Resolution Transmission Electron Microscopy in semi-conductors displaying a low SFE [2]. They have been followed in details by conventional Transmission Electron Microscopy (TEM) (bright-field and weak-beam) upon in-situ annealing in nickel, metal with a high SFE [3]. Furthemore, the kinetics of the processes have been investigated in stainless steels [4]. But very few studies concern the evolution of intergranular dislocation configurations in low SFE metals. The main objective of the presentation is to analyse the dislocations behaviour upon in-situ TEM annealing in a near sigma 3 GB in copper. Different dislocation configurations resulting from the interactions of lattice dissociated dislocations with the sigma 3 GB are observed and analysed using different TEM techniques and contrast simulation. Their evolution with the temperature involves complex reactions which yield a partial return of the GB towards equilibrium. Results will be discussed and compared to those obtained in nickel.

References :
1. Priester, L., 1996, Interf. Sci., 4 (3-4), 205-19.
2. Thibault Desseaux, J., Putaux, J. L., Bourret, A., Kirchner, H. O. K., 1989, J. Phys., 50 (18), 2525-40.
3. Poulat, S., Décamps, B., Priester, L., 1999, Phil. Mag. A, 79 (11), 2655-80.
4. Swiatnicki, W. A., Grabski, M. W., 1988, Mat. Sci. Eng., 100, 85-92.


abstract: 1.4
Distribution of Solute atoms round a moving dislocation
F.R.N. NABARRO, School of Physics, University of the Witwatersrand, Johannesburg, Private Bag 3, WITS 2050, South Africa.
Cottrell and Jaswon (Proc. Roy. Soc. Lond.  A 199, 104-114 (1949)) gave the detailed theory of the distribution of solute atoms which interact weakly with a slowly-gliding dislocation.  Without giving complete solutions, we show how the analysis can be modified to treat a climbing dislocation.  As already indicated by Takeuchi and Argon, Acta metall. 24, 883-889 (1976), the results are similar for glide and for climb.  Attention is also drawn to the case in which the interaction between the dislocation and a solute atom is large in comparison with thermal energies, and the solute atmosphere approximates a line of excess concentration rather than a concentration dipole.  The stress field of this line of dilatation interacts with solute atoms which produce uniaxial strains.


abstract: 1.6
Dynamic Properties of Dislocations in face-centered cubic Metals: Molecular Dynamics Studies and Discrete Dislocation Simulations
E BITZEK, Universitaet Karlsruhe, Institut fuer Zuverlaessigkeit von Bauteilen und Systemen, Karlsruhe, Germany; D WEYGAND, P GUMBSCH, Universitaet Karlsruhe, Institut fuer Zuverlaessigkeit von Bauteilen und Systemen, Karlsruhe, Germany and Fraunhofer Institut fuer Werkstoffmechanik iwm, Freiburg, Germany.
Atomistic simulations of straight dislocation segments accelerating under an applied shear stress were carried out to study the dynamic properties of edge, screw and mixed dislocations. Using embedded atom potentials for aluminum, nickel and copper, constant temperature molecular dynamics (MD) as well as static simulations were performed. The resulting dislocation trajectories were analyzed with respect to the parameters governing the dynamics of dislocations: the drag coefficient B, the effective mass m as well as the static and dynamic Peierls stress. The results compare well with theoretical results and - where available - with experimental data. Such atomistic simulations can therefore be used to determine the material specific parameters for the dynamics of dislocations.

A similar set of simulations was used to study the interaction of dislocations with localized obstacles like precipitates or voids. Even at room temperature the dynamic simulations show a significantly reduced stress to pass the obstacles compared to the static simulations. This dynamical effect can be attributed to the dislocation inertia. A line tension model for estimating the magnitude of the dynamic effect is presented.

Including inertial effects in discrete dislocation dynamics (DDD) simulations allowed to reproduce the atomistic results of dislocation - obstacle interaction. Provided that the required parameters m, B and the obstacle strength are known, for example from MD simulations or experimental observations, DDD simulations can thus be used to quantify the magnitude of inertial effects for various dislocation - obstacle configurations at the mesoscopic length scale. Such interactions are expected to be important, for example, for low temperature deformation, high strain rate processes or high frequency attenuation.


abstract: 1.9
Activation Energy of a Dislocation Loop in BCC Crystal
KAZUHITO OHSAWA, EIICHI KURAMOTO, Research Institute for Applied Mechanics in Kyushu University, Kasuga-Koen 6-1, Kasuga-shi, Fukuoka 816-8580 Japan.
Dislocation loops are usually observed in heavily irradiated materials. Dislocation loops in BCC crystal especially have a regular hexagon shape and are composed of interstitial atoms located on a $(1,1,1)$ plane. Therefore, they easily move to the $<1,1,1>$ direction. The one-dimensional motion contributes to the diffusion of the interstitial atoms toward sinks e.g., grain boundary and surface etc.. Therefore, the diffusion is very important to the research for void swelling and affects the strength of the irradiated materials. In order to estimate the jumping frequency of the dislocation loop, activation energy, E, is calculated in a simple line tension model in which the dislocation loop is regarded as a flexible string with line tension and self-interactions between the dislocation segments are neglected. In the present model, we assume a sinusoidal function as Peierls potential. The dislocation loop located at a stable Peierls trough is assumed to move to a next trough through a saddle point in phase space between them. Once the activation energy necessary to climb up the saddle point is calculated, the temperature dependence of the jumping frequency is estimated as Cexp(-E/kT). The saddle point configuration is obtained by the variational principle with respect to the dislocation shape. In the process of the energy estimation, we notice a critical length of the dislocation loop to distinguish the nature of the jumping behaviors. According to our analytic solution, if the dislocation loop is longer than the critical length, the loop moves to the next Peierls trough through a conventional double-kink formation process. Then, the activation energy increases moderately with increasing its length and achieves a constant value. On the other hand, if the dislocation loop is shorter than the critical length, the double-kink formation does not occur and the activation energy is proportional to the length of the dislocation loop. The former activation process could be called dislocation like and the latter is point-defect like.


abstract: 1.11
Correlation between creep activation parameters and microscopic dislocation behaviour in g TiAl alloys
COURET ALAIN, JOëL MALAPLATE, DANIEL CAILLARD, CEMES/CNRS, France.
Creep experiments in TiAl alloys provide values of stress exponents ranging between 5 and 8 and activation energies of the order of titanium self-diffusion energy in single-phased gamma TiAl materials. Recovery controlled mechanisms involving dislocations climb have been proposed by several groups. However, the exact correlation between the microscopic dislocation behaviour and theses activation parameters needs still to be elucidated. That is the aim of the present paper with a special attention to stress exponent. Creep experiments have been conducted on two Ti48Al48Cr2Nb2 alloys processed by cast and powder metallurgy at temperatures ranging between 650 and 850°C and stresses varying between 80 and 400 MPa. At 750°C under moderate stress (80MPa to 150 MPa), stress jumps have been performed during primary and secondary stages to measure directly the stress exponent and the activation volume. They have yielded values ranging between 7 and 11 for the stress exponent and between 35 and 45bor3 for the activation volume. Investigations by transmission electron microscopy (TEM) have shown that the deformation is mainly accommodated by ordinary dislocations, of which two types were clearly identified. Type 1 dislocations are moving by glide and it has been shown that they are activated under high local stresses. On the contrary, detailed post mortem analyses supported by high temperature in situ experiments in TEM have demonstrated that type 2 dislocations move by mixed climb. The values of the activation parameters are interpreted in the frame of this mixed climb mechanism in terms of nucleation and propagation of jog pairs. Finally, the driving forces for the activation of this mechanism are discussed.

abstract: 1.14
DISLOCATION BEHAVIOUR IN MATERIALS WITH MIXED COVALENT AND METALLIC BONDING
D. NGUYEN-MANH, UKAEA Fusion, Culham Science Centre, Abingdon OX14 3DB, UK; M CAWKWELL, Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104-6267, USA; M MROVEC, Fraunhofer Institute for Mechanics of Materials, 79108 Freiburg, Germany; R. PORIZEK, V VITEK , Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104-6267, USA; D.G. PETTIFOR , Department of Materials, University of Oxford, Oxford OX1 3PH, UK.

Dislocation core phenomena in materials with structures more complex than close packed fcc, often bring about unexpected deformation modes, strong and unusual dependencies of the flow stress on temperature, strain rate and orientation of the crystal with respect to the loading axes. These deformation aspects are particular accentuated in materials with mixed covalent and metallic bonding since the bond-breaking events associated with dislocation may be inhibited by the angular dependence of covalent bonding, usually arising from d or p electrons. We review recent studies of dislocation behaviour in these materials by developing and using reliable bond-order potentials (BOPs) which is a real-space description of interatomic interactions based on the tight-binding approximation. The distinguishing feature of these BOPs is that both attractive and repulsive contributions to the cohesive energy comprise environmental dependencies (ED). In the former case it is shown here that the screening of the bond integrals, which have been derived analytically within a non-orthogonal tight-binding representation, not only accounts for the discontinuities in the values of ab-initio hopping integrals but also provides a robust and transferable BOPs. The ED that appears in the repulsive part of the binding energy is crucial for the correct evaluation of the elastic moduli, and in particular for fitting the Cauchy pressure. The constructed BOPs were applied in calculations of the core structure of dislocations in bcc transition metals, L10-TiAl and fcc-Ir. In the latter case, for example, we found that the energy of the intrinsic staking fault calculated using BOPs (408 mJ/m$^{2}$) is in excellent agreement with both experiment (420 mJ/m$^{2}$) and ab-initio calculations (365-445 mJ/m$^{2}$). The core structure of the screw and the 60 degree dislocations in iridium have been calculated by using BOPs and flexible Green function boundary conditions that enable updating, in a self-consistent way, of the long range strain field associated with the relaxed dislocation.
This work was funded jointly by the United Kingdom Engineering and Physical Sciences Research Council and by EURATOM.


abstract: 1.15
Dislocations in complex metallic alloys
M. FEUERBACHER, Institut fuer Festkoerperforschung, Forschungszentrum Juelich GmbH, 52425 Juelich, Germany.
Complex metallic alloys (CMAs) represent a class of materials increasingly attracting interest in recent times. These materials possess characteristic structural features substantially deviating from those of simple metals. For example, they possess large lattice constants resulting in a high number of atomic positions per unit cell, and a novel type of local order dominated by icosahedral-symmetric atom coordinations. These properties open up the possibility for novel physical properties.

Representing a new field in materials science, the physical properties of CMAs, in particular the plastic properties of these materials have hardly been investigated to date. However, principle physical questions arise, concerning the mechanisms of plasticity in these materials: What is the structure of defects, particularly of dislocations, if the translational invariant distances are much larger than energetically acceptable Burgers-vector lengths? What are the mechanisms of dislocation movement in such complex structures? Will planar faults be introduced upon dislocation movement? Is the macroscopic plastic behaviour comparable to that of ordinary crystals with simpler structures?

In this contribution we will address these questions, reviewing current work in the field of plasticity of complex intermetallic alloys. We will present experimental results on the investigation of a number of CMAs with up to 1500 atoms per unit cell. It will be demonstrated that completely novel mechanisms occur, involving new types of defect such as the recently discovered metadislocation [H. Klein, M. Feuerbacher, P. Schall and K. Urban, Phys. Rev. Lett. 82, 1999, 3468] and uncommon macroscopic plastic behaviour [M. Feuerbacher, H. Klein and K. Urban, Phil. Mag. Lett. 81, 2001, 639].


abstract: 1.16
A nonlocal-in-stress criterion for dislocation nucleation
RONALD E. MILLER, Carleton University, Ottawa, Ontario, Canada; AMIT ACHARYA, Carnegie Mellon University, Pittsburgh, Pennsylvania, USA.
The generation of new dislocations is an essential aspect of crystal defect physics, especially at small scales, but a fundamental understanding of the mechanical conditions which lead to dislocation nucleation has remained elusive. Here we present a nucleation criterion motivated from continuum thermomechanical considerations related to a recently developed nonequilibrium theory of field dislocation mechanics (Acharya, 2001, 2003, 2004), and demonstrate the criterion's ability to correctly predict dislocation nucleation via direct atomistic simulations. The criterion emerges as a nonlocal one in the stress field and suitable simplification allows it to be phrased in terms of the curl of the stress. We compare and contrast the proposed criterion with other nucleation criteria proposed by Rice (1992) and Li et al. (2002).

Reference:
Miller, R. E. and Acharya, A. (2004) A stress-gradient based criterion for dislocation nucleation in crystals, to appear in J. Mech. Phys. Solids.


abstract: 1.17
On the core structures of dislocations in semiconductors
J. RABIER, J.L. DEMENET, M.F. DENANOT, X. MILHET, Laboratoire de Mécanique et de Physique des Matériaux, UMR CNRS 6617, ENSMA, BP 40109, F-86961 Futuroscope-Chasseneuil Cedex, France.
Perfect dislocations have been found to nucleate at high stresses and low temperature in silicon using deformation under high confining pressures. Those dislocations have been assumed to be in the ``shuffle set'' as compared to the usual dissociated dislocations which are thought to lie in the ``glide set''. These two types of dislocation have been found to control the plasticity of silicon in very different stress conditions and furthermore no conversion from one type to the other type of core structure has been evidenced up to now. Indeed annealing experiments of perfect dislocation microstructures were performed in situ in the TEM in order to promote movement of dislocations or nucleation of new dislocations. It is found that generated ``glide set'' dislocations result from new nucleation events rather than a shuffle glide transformation.

In the same high stress deformation conditions (T$<$150°C, P=5GPa), SiC-4H shows a different type of dislocation microstructures. Indeed perfect and dissociated dislocations appear to be nucleated in the same conditions. This has prompted us to re-examine the nucleation process of shuffle dislocations in compound semiconductors in the light of what has been put forward for silicon [1]. It is proposed that the glide shuffle transition for dislocation nucleation is not only stress dependent but also depends -in compound semiconductors where there is a large difference in mobilities between dislocations having a different chemical nature of the core- on the nature of the leading partial dislocation which would have been nucleated in the ``glide set''.

Reference:
1. M. S. DUESBERY, B. JOOS, Phil. Mag. Lett. 74, 253 (1996).


abstract: 1.21
THE KINEMATICS AND DYNAMICS OF SCREW DISLOCATIONS IN COPPER - A MOLECULAR DYNAMICS STUDY
D. MORDEHAI, School of Physics and Astronomy, The Raymond and Beverly Sackler Faculty of Exact Sciences Tel Aviv University, Tel Aviv 69978, Israel; G. MAKOV, Department of Physics, NRCN, P.O. Box 9001, Be'er Sheva, Israel; I. KELSON, School of Physics and Astronomy, The Raymond and Beverly Sackler Faculty of Exact Sciences Tel Aviv University, Tel Aviv 69978, Israel.
The dynamic properties of dislocations constitute one of the basic building blocks of any theory of plasticity. One of the methodologies to study plasticity is the bottom-up approach, in which rules for dislocations kinematics and dynamics serve mesoscopic simulations. Experiments are not able yet to follow in detail the microscopic dynamic properties of the dislocation, such as dislocation motion or cross-slip, while atomistic simulations may serve as a powerful tool. Using molecular dynamics (MD) methods the dynamic properties of screw dislocations had been studied in detail for Cu, both as a function of the temperature and the applied stress. Upon applying a glide stress on the dislocation a transition from inertial to viscous motion with a stress dependent terminal velocity is observed. The experimentally observed stress dependence of the terminal velocity is reproduced quantitatively by our results [1]. If a narrow dipole of two opposite screw dislocations is introduced into the computational cell then effectively a stress is applied on the cross-slip plane. Then we observed dislocation cross slip and dipole annihilation. Upon applying a narrowing stress on the dislocation the cross-slip rate increased. From these calculations the cross-slip mechanism was identified, and the activation energy and volume were calculated as a function of model parameters, such as dislocation length and dipole width. The MD results allow us to define a set of rules for dislocation kinematics and dynamics. These rules can be used as a basic for a mesoscopic calculation.

References:
1. D. Mordehai et. al. Phys. Rev. B, 67 024112 (2003)
2. http://niva.tau.ac.il/


abstract: 1.22
Seeing Multiple Scales of Relaxation at Dislocation Loops in Electron Diffuse Scattering Patterns
Z. ZHOU, University of Oxford, UK; S L DUDAREV, UKAEA, Culham Science Centre; M L JENKINS, University of Oxford; A P SUTTON, University of Oxford, Helsinki University of Technology; M A KIRK, Argonne National Laboratory.
Recently enhanced capabilities of electron microscopes have made it possible to place a nearly parallel nano-sized electron probe on individual nano-defects. Elastic diffuse electron scattering from single nanometre-sized defects has been successfully measured experimentally. The technique shows promising potential in characterizing nano-defects. Simulations undertaken in the same diffraction conditions are essential for characterizing the dislocation geometry and Burgers vectors. A systematic study was carried out to simulate elastic diffuse scattering from individual nanometer-sized dislocation loops. The displacement fields of dislocations loops, which are necessary for the calculations as input, were obtained from linear isotropic and anisotropic elasticity for both infinitesimal and circular loop models. Atomistic configurations obtained from MD simulations were used to capture the contributions of diffuse scattering intensities from core structures of the defects.

The simulated results show good agreement with other simulations and experiments. It is found that elastic anisotropy makes a significant difference to the diffuse scattering intensity distributions, particularly the orientations of nodal planes, which are important for characterization of loop orientations. By comparing with a database of simulations, it is possible to identify very small defects observed experimentally by electron diffuse scattering, including measurements of their sizes, which has been difficult and inaccurate in TEM weak-beam imaging.


abstract: 1.25
Characterization of Computer Simulated Dislocations
HARTLEY CRAIG S., U.S. Air Force Office of Scientific Research, USA; YURI MISHIN, George Mason University, Fairfax, VA USA.
Simulation of the atomic arrangement around the core of straight dislocations using techniques that employ interatomic potentials provide useful information about the distribution of atomic misfit in the vicinity of the core. This misfit is inevitably spread out over several atomic distances in a manner that depends on the atomic potential and the boundary conditions. Such simulations can, in principle, be employed to modify continuum models of individual dislocations to include features that arise from the discrete nature of the crystal lattice. In order to accomplish this, it is necessary to extract information from the simulations in a form that can be inserted in continuum representations of dislocations. An appropriate procedure for accomplishing this was suggested by Hartley (1). This technique employs the concept of the continuously dislocated continuum introduced by Bilby, Bullough and Smith to calculate components of the Nye tensor in the region surrounding the simulated dislocation. Representing the correspondence between lattice vectors in the perfect crystal (i) and the dislocated crystal (e) as i = Ee, the True Burgers Vector Flux at a point is given by ? = RotE. The distribution of the True Burgers Vector Flux around the dislocation provides a more graphic illustration of the misfit distribution than the more conventional relative displacement maps. The present work illustrates this procedure by calculating components of the Nye tensor in the plane normal to a simulated straight dislocation line from the atomic positions surrounding the dislocation and the corresponding positions in the perfect lattice. Implications of the results on the nature of the dislocation core structure and on the construction of higher order terms in the continuum representation of the dislocation displacement field are discussed.

(1)C. S. Hartley, Linear force and dislocation multipoles in anisotropic elasticity, J. Appl. Phys., 46, pp. 1008-1012 (1975).