Poster contributions

abstract: 4.2
Dislocation motion ahead microcrack in thin foil of fcc-metal
P. PANFILOV, Urals State University, Ekaterinburg, Russia.
It is well-known phenomenon that sometimes cracks in crystalline solids could emit dislocations. TEM observations of dislocation structures, which formed ahead microcrack tip in thin foil of fcc-metal, are considered at presented paper. Experiments have been performed on heavy and light metals (iridium and aluminum). Both tension (including in-situ deformation) and bending were used for tests. The cracks, named here as ``microcracks'', had V-shape with an angle near the tip of $5\deg - 10\deg$, while their length varied from few dozens of inter-atomic distances to few micrometers. No zig-zag cracks having ``dragon tooths'' edges (whose growth causes the separation of thin foil!) and dislocation emission around them are discussed in this work. Every atomic scale microcrack in iridium foils emitted one or few 110 dislocations, which move from crack strictly inside the cone-like region having ``strip contrast'' (as it takes place in stacking fault or microtwins). The length of these regions, started from the foil edge, was $\approx 1 \mu m$, and it did not depend on microcrack dimension, whereas ``strip'' contrast became cleaner with microcrack growing up. Simultaneously, the quantity of emitted dislocation increased and twin spots began to resolve on electronograms taken from the region. Estimation has shown that transition of region from ``stacking fault'' to ``microtwin'' state occurred when dozen of dislocations came through and microcrack length grew up few hundreds interatomic distances. The area, where 110 dislocations are created, is not the peak of crack, but it represents itself the upper part of microcrack having size of few dozen interatomic distances. No visible changing of crack tip geometry due to dislocation emission was observed in experiments. The motion of emitted dislocations is crystallographically-sensitive process, so on the cubic plane every micro-crack emits two arrays of dislocations, whereas on 110 such direction can be the sole. Leaving region dislocations begun to move on ``clear'' crystal without any tracks. There are two opportunity for this: (1) dislocations flow to depth of foils if barriers are absent on the way; and (2) array is terminated on power obstacle, such as high dense dislocation nets in iridium. In the first case, dislocation motion leads to thinning of material ahead microcrack, while in the second one, stoppage of dislocations does not mean the end of emission from microcrack. The possible mechanisms of these effects and their relations with fracture mechanisms in fcc-metals are discussed. This work is sponsored by the Rus-sian Foundation for Basic Research (01-03-96438a).


abstract: 4.3
Low-strain fatigue in 316L steel surface grains, a three dimension discrete dislocation dynamics modelling of the early cycles
C. DéPRéS,

CEA/Saclay and GPM2/INPG, France. C. ROBERTSON, CEA/Saclay, France. M. FIVEL, GPM2/INPG, France.
The early stages of the formation of dislocation microstructures in low strain fatigue are analysed, using three-dimensional discrete dislocation dynamics modelling. Simulations under various conditions of loading amplitude and grain size have been performed. Both the dislocation microstructures and associated mechanical behaviour are accurately reproduced in single-slip as well as in double-slip loading conditions. A detailed scheme for the persistent slip band formation is proposed, from the observation of the numerical dislocation arrangements. The surface displacements due to dislocation elimination at the grain surface are determined using a post-processing method, based on an analytical solution adapted to triangular dislocation loops. A complete scheme for the persistent surface slip marking formation and morphology is proposed, in relation with the persistent slip band dislocation arrangements found inside of the grain. Local stress concentrations inside the microstructures and their relations to incipient damage initiation are analysed as well. The details of the stress accumulation mechanisms generating high damage nucleation probability are described.

abstract: 4.5
An indentation method to measure the CRSS of semiconducting materials at elevated temperature
J.-P. RIVIERE, Laboratoire de Physique des Solides et de Cristallogénèse, UMR 8635 CNRS, Meudon, France. L. LARGEAU, G. PATRIARCHE, Laboratoire de Photonique et de Nanostructures, UPR 20 CNRS, Marcoussis, France. E. LE BOURHIS, Université de Poitiers, Laboratoire de Métallurgie Physique, UMR 6630 CNRS, Poitiers, France.
Thermomechanical properties of semiconductors govern the structural quality of heterostructure. Here, we propose an original method to determine the crystal friction in small volume. The experimental requires a conventional microindentation set up except that a trench (3) is to be machined in the holder (2) in such a way that the bottom face of the sample (1) underneath the indenter (4) is not in contact with the holder (2). Single crystals are thinned to $100-300 \mu m$ mechanically and chemically to allow observation of the plastic deformation at the bottom face of the samples. In a first step, the plastic flow in thin samples is modelled by nucleation of dislocations (5) with Burgers vector belonging to several slip planes normal to the indented surface and we assumed that the mobility of the dislocations does not depend strongly on the screw, alpha or beta character. Such assumptions are valid for thin (011) InP indented at $400\deg C$ (Largeau et al. J. Mater Sci. 38, 2004, 943) and punching through the samples can be described by prismatic loops development. Observation of the back side shows in such case a deformed area (6). Assuming the equilibrium of the dislocations punched by the indenter, the increase in size of the back side deformation area when the load F is increased, allows to determine the CRSS:

$$F_{2}-F_{1}=CRSS(P_{2}-P_{1})t$$ (1)

Where P is the perimeter of the back side deformed area and t the thickness of the sample.

In the case of (011) InP we determined a CRSS of 50 MPa at 400°C in good agreement with the literature and taking into account the cooling of the sample by the indenter. Work on (001) and (111) faces of GaAs shows that the previous equation applies to other crystal orientation, once geometrical factors are introduced. Therefore, prospective work using an alumina indenter instead of a diamond one is under way to limit the cooling of the sample by the indenter.


abstract: 4.6
Discrete dislocation dynamics applied to the simulation of crystallographic fatigue crack growth in a FCC polycrystal
GRACIELA M. BERTOLINO, VÉRONIQUE DOQUET, Laboratoire de Mécanique des Solides, CNRS, France. MAXIME SAUZAY, CEA, DEN-DMN-SRMA, France.
High-cycle fatigue is characterized by a large scatter in fatigue lives which reflects the scatter in the mechanical conditions encountered by short cracks in the early stage of their development. Interactions with grain boundaries are responsible for temporary or definitive arrests of the microcracks and often control the endurance limit. The stress distribution in a polycrystal can be very heterogeneous due to the elastic anisotropy of the grains. Moreover, the local texture influences both the ease of crack transfer beyond a grain boundary (G.B.) and the roughness of the crack. The effective crack driving force varies substantially with the local microstructure. An attempt to model this variability is made by coupling finite element computations of the stresses ahead of a tortuous crystallographic microcrack in a FCC polycrystal with simulations of crack growth along slip planes based on discrete dislocations dynamics. Dislocation emission at the crack tip (following Sun, Beltz and Rice criterion) and coplanar alternate dislocation glide (hindered by grain boundaries) under cyclic loading is simulated. Transgranular crack growth rates are deduced from the dislocations flux at the crack tip. Crack growth beyond a grain boundary is assumed to occur when the activation of a dislocation source in the next grain has led to crack initiation there and coalescence with the arrested crack tip. Arrest periods are thus estimated as a function of the resolved shear stress on a potential dislocation source in the next grain. The model predicts a large scatter in growth rates related to the variety of local textures and crack paths. This scatter is predicted to increase as the stress range decreases, consistently with experimental results. The simulations also describe the decrease in the arrest periods at grain boundaries as a crack develops, the influence of the mean grain size on the endurance and the fact that overloads may suppress the endurance limit by allowing arrested cracks to cross the grain boundaries.


abstract: 4.9
Irradiation-induced hardening/softening in $SiO_{2}$ studied with instrumented indentation
SHINSUKE NAKANO, SHUNSUKE MUTO, TETSUO TANABE, Nagoya University, Nagoya, Japan.
It is known that the experimental hardness of $SiO_{2}$ is smaller than the theoretical hardness estimated by the general model incorporating the number of bonds per unit area and the energy gap of covalent materials [1], and one should take into account the rotation and bending of the $SiO_{4}$ framework about their shared oxygen rather than the broken of the bonds.

We carried out a nanoindentation test to precisely track the hardness change of crystalline and vitreous SiO$_{2}$ (c-SiO$_{2}$ and v-SiO$_{2}$, respectively) samples associated with energetic particle irradiation (He$^{+}$ $8.4 \times 10^{14} \approx 3.2 \times 10^{16}$ ions/cm2 at 20keV at room temperature) and explore the mechanism of the plastic deformations in these materials. To extract the mechanical properties of the damaged layer (which is about 450nm from the surface determined by the TRIM code) embedded in the samples, a finite element method (FEM) was applied to reproduce the experimental load-displacement (L-D) curves, using a multiple-layer model.

In c-SiO$_{2}$, the hardness ($H$) increased up to 0.025 dpa (displacement per atom), and then decreased with increasing the irradiation dose. The first increase in $H$ is ascribed to the irradiation-induced hardening. And the softening thereafter is attributed to bond-breaking and tilt of the SiO$_{4}$ units associated with the structural disordering. On the other hand, in v-SiO$_{2}$ $H$ decreased up to 0.3 dpa, and then gradually increased with increasing dpa due to the reported compaction effect [2]. $H$ of c-SiO$_{2}$ and v-SiO$_{2}$ asymptotically approached to the same value at about 1 dpa.

The dislocation structures at the initial stage of irradiation in c-SiO$_{2}$ were observed with TEM and the correlation between $H$ and evolution of dislocation network is discussed.

References:
1. F. Gao et al., Phys.Rev.Lett. 91, 15502 (2003)
2. M. Hasegawa et al., Nucl. Instr. and Meth. in Phys. Res. B, 166-167 (2000) 431-439


abstract: 4.11
Discrete Dislocations Dynamics Investigation on Friction Damping under cyclic Loading

ASTRID WALCKER, DANIEL WEYGAND, Institut fuer Zuverlaessigkeit von Bauteilen und Systemen, Universitaet Karlsruhe (TH), Karlsruhe, Germany; OLIVER KRAFT, Institut fuer Zuverlaessigkeit von Bauteilen und Systemen, Universitaet Karlsruhe (TH) and Institut fuer Materialforschung II, Forschungszentrum Karlsruhe, Karlsruhe, Germany.
The understanding of the small-scale mechanical response of metallic materials to ultra high-frequency cyclic loading conditions has been hardly studied in the past. For dislocations being the carriers of plastic deformation and, therefore, also of fatigue failure, the fundamental question concerns their behaviour under the influence of an oscillating stress. Beside its interesting fundamental aspects, this problem is also highly relevant for reliability issues in MEMS or radio frequency communication devices.

The first step to tackle this problem has consisted in studying the effects of a harmonically oscillating stress on a single dislocation which is represented by an idealised Frank-Read source in a fcc crystal of nickel and aluminum, respectively. To perform this study, a discrete dislocation dynamics method has been extended for dynamic effects.

The central quantity of interest is the frequency dependence of the dissipated mechanical work. This energy loss is calculated from the movement of the dissipative forces exerted on the dislocation, in which the time evolution of the dislocation movement and the forces on the dislocation are delivered by the employed discrete dislocation dynamics code.

The frequency of the cyclic loading has been varied ranging from 1 MHz up to the GHz regime. Additionally, the influence of the friction coefficient (temperature dependent) and the inertia of the dislocation (material dependent: Al versus Ni) on the frequency dependence of the energy dissipation has been studied.

The next step consists in determining the influence of the dislocation-dislocation interaction on the global dissipative response. To this end, simulations analogous to those described above, but with a multi-dislocation configuration, are performed and the results are contrasted to the results gained by simulating the behaviour of the single-dislocation configuration.

Finally, the results are compared both to the classical analytical results of Granato and Lücke and recent simulation results from other groups.


abstract: 4.12
Dynamic Loop Unfaulting in Face-Centered Cubic Al and Cu
ALISON KUBOTA, Lawrence Livermore National Laboratory, USA. MARIA-JOSE CATURLA, University of Alicante, Alicante, Spain. WILHELM WOLFER, Lawrence Livermore National Laboratory, USA.
The dynamic mechanism for loop unfaulting is studied with large-scale classical molecular dynamics simulations in Al and Cu computational cells each with over 4 million particles using the LLNL MCR supercomputer cluster. The process of loop unfaulting may be viewed as one of the most elementary cases of a structural solid state transformation between two stable states of a crystal lattice. These large-scale MD simulations demonstrate their unique capability to uncover and make visible the detailed transformation paths and the dynamics of structural phase transitions. To induce the unfaulting process in the case of small vacancy-type and interstitial-type loops, a short-duration shear stress pulse is applied to one free surface of the computational cell, and the dynamic response of the loop is visualized. From such ``computer experiments,'' we observe for the first time the complex transition paths involved in the unfaulting reaction. Unexpectedly, we find that there exist a great diversity of transition paths with transient dislocations and their reactions; in fact, when repeating some of the simulations at moderate temperatures of 150 K, each displays some differences from the others: no two are ever identical. In spite of this diversity, however, the minimum shear stresses required to induce unfaulting are remarkably similar. This indicates that while a complex energy landscape exists between the two stable loop configurations, the saddle points have similar energies, and the multitude of transition paths should emerge from a general principle. We propose therefore a Ginzburg-Landau functional for all the possible configurations, both stable and transient. This functional depends on a fractional Burgers vector distribution field which may assume finite values on all closed-packed planes which intersect the dislocation loop periphery, and on a generalized stacking fault energy which is a local function of the fractional Burgers vector. The different transformation paths are then variational solutions for the spatial and time-dependent fractional Burgers vector field which minimize this energy functional. This work was performed under the auspices of the U. S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.


abstract: 4.13
Strain rate sensitivity of flow stress under superimposition of ultrasonic oscillatory stress during plastic deformation of RbCl doped with Br or I
TOMIYASU OHGAKU, KATSUKI HASHIMOTO, F. Engineering, Kanazawa University, Kanazawa, Japan.
Flow stress decreases when ultrasonic oscillatory stress is superimposed during plastic deformation of single crystals. Strain rate cycling tests were carried out under superimposition of ultrasonic oscillation during plastic deformation of RbCl single crystals doped with Br- or I-. The relation between the strain rate sensitivity (SRS) of flow stress, which is calculated from the stress change due to strain rate cycling, and stress decrement due to application of oscillation has a stair-like shape and is divided into three regions. In the first region that is a plateau at the small stress decrement, both impurities and forest dislocations act as obstacles to translational motion of dislocations. This is because the amplitude of oscillatory stress is too small to help the mobile dislocation to overcome the obstacles. Then, the SRS keeps constant. This region does not appear for nominally pure RbCl single crystals. In the second region where the SRS decreases with increasing stress decrement, the role of impurities as obstacles is decreasing. Some impurities are overcome by oscillating dislocations and others are not yet. In the third region that is a plateau again, only the forest dislocations act as obstacles and then the SRS keeps constant again. The stress decrement (tau) at the first bending point between the first and second regions is considered to be the effective stress due to impurities. The difference (lambda) of SRS between the first and second plateau regions is also considered to be a part of SRS due to impurities. The tau and lambda were measured in the temperature range from 77K to room temperature. The relations between the tau and activation volume obtained from the lambda reveals the interaction between the dislocation and impurities. The Cottrell-Bilby relation is assumed to be applicable to the case of RbCl crystals doped with Br- or I-. Then, the interaction energies between the dislocation and Br- or I- were determined to be 0.5 or 0.58eV in RbCl, respectively.


abstract: 4.16
Simulation of a tensile test on a thin film on substrate by means of a continuum dislocation-based model
CORNELIA SCHWARZ, RADAN SEDLACEK, EWALD WERNER, Institute for Materials Science and Mechanics of Materials, Department of Mechanical Engineering, Technical University of Munich, Boltzmannstr. 15, 85747 Garching b. Muenchen, Germany.
It is well known, that plastic deformation on the microscale shows a significant size-effect in the sense that plastic response is stronger than expected from the macroscopic point of view. A few benchmark problems have been established for the comparison of simulation and experiment, for instance shearing, torsion, bending and tensile-tests. There have been several approaches to explain this phenonmenon, and a number of models exist which are able to reflect the experimental results.

In a direct comparison, discrete dislocation models seem to be rather promising, but the high computational efforts coming along with them restrict their application to problems of mere academic interest.

Continuum approaches, mostly in the form of strain-gradient theories, therefore deal with only a few variables, but they're purely phenomenologically motivated and in general lack an insight into the microscopic foundations of the effect.

An alternative approach, which differs fundamentally from the common strain-gradient theories, is to be applied in this work to the simulation of a tensile test on a thin film on substrate. It was outlined in [Sedlácek, Kratochvíl, Werner, Phil. Mag. 83 (2003), 3735], and ascribes the emerging size effects to the bowing of dislocations within the constrained workpiece. Rather than a single dislocation, a continuous distribution of dislocations within the rigorous continuum mechanics framework is considered. Our goal is to simulate a strain-controlled tensile test on a thin film on substrate, and to compare the results to those obtained experimentally and to those gained from simulations based on other models.


abstract: 4.17
Simulation of nanoindentation: a discrete dislocation study
H. G. M. KREUZER, R. PIPPAN, Erich Schmid Institute of Materials Science, Leoben, Austria.
Nanoindentation is simulated on the computer by means of a two dimensional discrete dislocation model. In our algorithm the emission of dislocations takes place, whenever the local shear stress at the position of randomly placed dislocation sources in the model material, reaches the critical value of the predifined source strength. During each indentation step, the newly generated dislocation dipoles are brought into the equilibrium arrangement and the arising contact stresses are updated. In order to study the effect of certain microstructural influence factors, such as dislocation obstacles (grain boundaries, thin films etc.) or the given slip geometry on the local plastic material behavior, different calculations were performed. The aim of this work is rather generally to illustrate and discuss the various aspects of plasticity which become apparent especially in the nanometer length scale, and how they can be studied by means of nanoindentation with the background of our simulations.


abstract: 4.18
A study of copper interconnects mechanical behavior in semiconductors
S. LEFEBVRE, MSSMAT ECP, Chatenay Malabry, France and Altis Semiconductor, Corbeil Essonnes, France. T. HOC, MSSMAT ECP, Chatenay Malabry, France. B. DEVINCRE, L. KUBIN, LEM ONERA, Châtillon, France. A. SOUDRY, P. VEKEMAN, Altis Semiconductor, Corbeil Essonnes, France.
Mechanical properties of nanocrystalline metals differ dramatically from their bulk counterparts and have attracted a great deal of interest in recent years, but knowledge of the underlying mechanisms is still incomplete. Miniaturization of micro electronics components, which enables to decrease costs, is a necessity for the semiconductor industry: the thickness of interconnects is now around 150 nm and must soon pass below 100 nm. Unfortunately, at this length scale, mechanical stresses generate a loss of reliability. The aim of this work consists in developing a finite element model of copper behavior which takes size effects into account. We only consider grains larger than 100 nm. In order to properly understand deformation transmission in a polycrystal, we use a 2D simulation of dislocations dynamics. We first wanted to find out what is predominantly responsible, according to grain size, for slip nucleation at the interface: dislocations pile-ups at the boundary or confined geometry and decrease of length scale? In this simulation, we assumed that interfaces act as barriers to dislocation glide and also examined the possibility of dislocation emission at the grain boundary. At the same time, an uniaxial compression test on a bulk copper bicrystal has been performed. The local strain field, the crystal orientation and the stress, obtained respectively by microextensometry, electron beam scattering and X-Ray techniques, have been recorded close to and far from the grain boundary. We noticed a high heterogeneity near the interface and simulated by finite element method the hardening behavior, assuming that the mean free-path is related to the interface distance. The results showed a good agreement between numerical and experimental data.


abstract: 4.20
The Collapse Process and Atomic Scale Mechanism for Stacking Fault Tetrahedron Intersection with Moving Dislocations
YOSHITAKA MATSUKAWA, STEVE J. ZINKLE, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN, U.S.A.
Point defect clusters introduced by neutron irradiation are strong obstacles to dislocation motion. Irradiation hardening in nuclear reactor environments is usually attributed to interaction processes between those point defect clusters and moving dislocations. Stacking fault tetrahedra (SFT) are the dominant vacancy clusters produced by neutron irradiation of many fcc metals. However, the interaction process with dislocations is still unclear due to the complicated crystallographic geometry of SFT. In the present study, the interaction processes were examined by in-situ transmission electron microscope (TEM) straining experiments with quenched gold containing large SFT ($\approx$ 50nm). SFTs were observed to collapse when directly intersected by a moving screw dislocation. Considering that the characteristic property of screw dislocations, cross-slip, was not involved in the process, this collapse process is expected to occur irrespective of dislocation type. The position of the dislocation passage through the SFT was an important factor for the collapse: SFT collapsed when intersected near the base triangular plane, but not when intersected near the apex. Only the base portion divided by intersection with the dislocation annihilated in the collapse process, while the top portion of the original SFT remained behind as a smaller SFT. In order to obtain insights on the mechanism whereby such characteristic collapse process takes place, variations of atom configurations induced by intersection with dislocations were examined using 2-D schematic diagrams illustrating SFT atomic configurations. This simple simulation provided a key to understanding the process. Intersection with a perfect dislocation created an abnormal atom configuration as a step on a stacking fault plane, traditionally called an I-ledge: atoms in adjacent (111) planes overlapped within the I-ledge region. Those overlapping atoms have strong repulsive forces, producing the driving force for SFT collapse. Due to the geometry of an I-ledge, the dislocation chain reaction starting from an I-ledge inevitably collapses only the base portion of the SFT. Also, since the magnitude of the repulsive force depends on the length of the I-ledge, this gives a reasonable explanation for why SFTs tend to collapse only when intersected with dislocations near the base plane.


abstract: 4.21
Some novel aspects of the plasticity of Germanium
CORINNE DUPAS, Ecole Polytechnique Fédérale de Lausanne, Switzerland. TOMAS KRUML, Ecole des Mines de Nancy, France. JEAN-LUC MARTIN, Ecole Polytechnique Fédérale de Lausanne, Switzerland.

The mechanisms underlying the plasticity of DC covalent crystals are assessed. For this purpose, $<123>$ intrinsic Ge single crystals have been tested in compression between 700 and 1000K. The idea was to perform transient tests (stress relaxation and creep tests, dip tests) along the monotonic curve, a technique developed for metallic compounds. The aim is to assess the points which are clearly established and list the questions that still appear to stand out as worth solving.

The main results include two parts: i) dislocation multiplication processes at the onset of deformation ii) the lattice resistance to glide after the lower yield point (LYP). Before the upper yield point, intense dislocation multiplication takes place during the transients. In addition, the shape of the stress relaxation or creep curves is very different from the logarithmic variations observed in metallic crystals. The transient curves are analysed in terms of the Orowan equation and various multiplication laws proposed successively for metals and for covalent crystals. It will be shown that none of these laws satisfactorily describes the transient curves, very likely because they ignore deformation heterogeneity.

Slightly after the LYP, the strain variation of the activation volume allows one to define a critical stress and strain. It likely corresponds to the end of intense dislocation multiplication and the onset of strain hardening. It is thus possible to compare stress-strain curves at that strain, for different deformation conditions. Dip tests have allowed one to separate the two stress components, i.e. the thermal part and the internal stress. The former decreases monotonically over the temperature interval suggesting a single deformation mechanism. The latter corresponds to long range elastic interactions with the substructure of gliding dislocations. Surprisingly, the internal stress is observed to depend strongly on temperature and strain-rate. This can be understood in terms of the influence of these two parameters on the microstructure. These results combined with TEM observations (both static and dynamic) and activation energy measurements, suggest that dislocation velocity is controlled by a lattice resistance to glide combined with point obstacles such as debris or impurities.

abstract: 4.24
Dislocation nucleation at surface step in stressed semiconductor systems
JULIEN GODET, LAURENT PIZZAGALLI, SANDRINE BROCHARD, PIERRE BEAUCHAMP, Laboratoire de Metallurgie Physique de Poitiers, France.

The formation of dislocation at surface defects is a process of particular importance in nanostructured materials submitted to large stress. In these materials, such as nano-grained systems or nanolayers in heteroepitaxy, the Franck-Read mechanism for the dislocation multiplication can not operate. It is then commonly assumed that surfaces or interfaces play a key role in the dislocation formation. In addition, defects like steps, or cleavage ledges, could favor the nucleation, by lowering the activation energy. This last assumption is supported by experimental facts, with dislocation sources located on the cleavage surface and coinciding with cleavage ledges. We investigated the dislocations nucleation from surface steps on silicon submitted to a uniaxial stress. Since the very first stages of dislocation formation are still out of the scope of experimental investigations, atomistic scale simulation are performed. A silicon crystal with a linear step on the surface is built, and an increasing uniaxial stress is applied. A typical system has 80000 atoms and silicon is modeled by various empirical potentials (Stillinger-Weber (SW), Tersoff or EDIP). Several calculations on bulk system strongly strained, performed with the potentials and ab initio methods have allowed to establish that SW is the best one for this kind of study. Then we tested several stress orientations in both traction and compression on a range of temperature from 0K to 1600K. The plastic events always appeared at very large strains, greater than 7.6% in compression and 18.7% in traction. With favorable orientations, perfect 60° dislocations have been nucleated from the surface step in both traction and compression, and more rarely screw dislocations. In all cases, the dislocations glide in the shuffle set planes. Our results also indicates that it is possible to predict the kind of formed dislocation, from the analysis of the Schmid factor and the Peierls stress.

Ab initio calculations are currently in progress on smaller systems, using a DFT-LDA code, SIESTA. Preliminary results with a cell including 200 atoms confirmed what has been obtained with empirical potentials. Large-scale calculations on 500 atoms systems are planned

abstract: 4.26
Molecular Dynamics Simulations of the Dislocation Motion in Precipitation Hardened Materials
C KOHLER , Institute for Materials Testing, Materials Science and Strength of Materials (IMWF), University of Stuttgart, Pfaffenwaldring 32, D-70569 Stuttgart, Germany; P KIZLER, Materials Testing Institute (MPA) University of Stuttgart, Pfaffenwaldring 32, D-70569 Stuttgart, Germany; S SCHMAUDER, Institute for Materials Testing, Materials Science and Strength of Materials (IMWF), University of Stuttgart, Pfaffenwaldring 32, D-70569 Stuttgart, Germany.

Classical molecular dynamics simulations using EAM potentials are performed in order to study the interaction of dislocations with precipitates. The pinning of edge dislocations by gamma' precipitates in Ni-base superalloys and by Cu/Ni precipitates in alpha-Fe is investigated for various forms of the precipitates. The dependence of the critical resolved shear stress on the geometry and the chemical composition of the precipitates is determined.


abstract: 4.27
Cyclic deformation mechanisms in engineering gamma titanium aluminide alloys
M. JOUIAD, A.-L. GLOANEC, LMPM ENSMA, France; M. GRANGE, Snecma Moteurs, France; G. HENAFF, LMPM ENSMA, France.

While many studies have been dedicated during the last years to basic deformation mechanisms in TiAl intermetallic compounds only limited data are available on the cyclic deformation mechanisms, especially in technically relevant alloys. Now the low-cycle fatigue resistance of components can be a design concern and therefore there is a need for a thorough understanding of these mechanisms in relationship with mechanical and microstructural parameters. The present study precisely tackles the issue of the identification of the deformation mechanisms governing the cyclic stress-strain behaviour of a cast Ti-48Al-2Cr-2Nb with a nearly fully lamellar microstructure. Experimental results indicate that, at room temperature, this behaviour and the corresponding deformation mechanisms are strongly dependent on the applied strain range. Thus, at low strain range, where almost no hardening is noticed, deformation is insured by motion of long and straight ordinary dislocations. The moderate hardening observed at intermediate values of the strain range is associated with the formation of a vein-like structure due to progressive tangling of ordinary dislocations. At higher strain range values, twinning, by delaying the formation of this vein-like structure, induces a more pronounced cyclic strain hardening. The influence of parameters such as strain ratio or strain rate on these mechanisms is also discussed. At high temperature (750°C), the material exhibits a stability of the stress amplitude, regardless of the applied strain range. TEM observations of the deformation substructure indicates that twinning is no longer operative at this temperature but that dislocation climb is activated.

abstract: 4.29
Discrete Dislocation Modeling of Nano-Indentation
ANDREAS WIDJAJA, ERIK VAN DER GIESSEN, University of Groningen, The Netherlands.

We analyze a two-dimensional indentation model using a discrete dislocations dynamics framework. We use a planar-symmetric material with random distributions of initial Frank-Read dislocation sources and point obstacles sitting on the slip planes. Finite element method is implemented to calculate image stresses and displacements. Indentation is implemented by incrementally prescribing the displacements according to the indenter shape over the current contact area. The rest of the surface of the crystal is left traction free, and dislocations may leave the crystal through this free surface. The contact area is continuously updated during the calculation. Multiple orientations of slip planes are used. The nano-indentation simulation is performed to study the plastic behavior of the material near the indenter tip caused by dislocation populations. The indentation size effect in term of hardness is detected at shallow indentation depths of a circular indenter.

abstract: 4.32
UNIVERSAL MECHANISMS OF DISLOCATION MOTION AND MULTIPLICATION IN SOLIDS UNDER ULTRASOUND, CREEP, IMPACT AND SHOCK WAVE STRESSES
VALERY P. KISEL , Institute of Solid State Physics, 142 432 Chernogolovka, Moscow district, Russia.
The effect of applied compressive-tensile stresses, S (S=0.6Sy to 95Sy, where Sy is the resolved yield stress) and stress rates, SR (SR = 104 to 106 MPa/s) on dislocation dynamics was investigated in nominally pure NaCl and InSb crystals in the temperature range T= (4 10-4 to 0.65) Tmelt, Tmelt is the melting point. The general damping character of dislocation unpinning, motion and multiplication (crystal work-hardening, WH) under various tests manifests in ultimate mean paths, l$_{ult}$, and the mean number n$_{ult}$ of mobile dislocations. Having covered a certain l$_{ult}$, the n$_{ult}$ dislocations exposed to successive acts of multiplicaftion thus forming slip lines, bands, subgrains, low- and high-angle grain boundaries, point defect clusters. In thin films with the thickness lower than the l$_{ult}$ this dislocation motion may be detected only by the high density of point defect clusters [1] due to the climb of dislocation jogs under extremely high stresses [2]. The next important finding of this work is the fact that the l$_{ult}$, n$_{ult}$ (S, SR, T, impurity state and concentration) dependences under ultrasound, creep, impulse, impact stresses are topologically similar to the appropriate macroscopic quasistatic strain-stress curves, stress changes under relaxation or stepped variations in the stress rate vs flow stress and temperature for the same crystals. It is worth stressing that these WH depen-dences have the same non-monotonous behavior like it has been discovered for the V-shaped frequency dependences of hardening and softening in solids, liquids and biological tissues [3]. This means that the micromechanisms of dislocation dynamics and macroscopic work-hardening are the same for various ranges of S, SR, T, etc. and tests. The third remarkable finding is the scaling of stresses Se at various fixed levels of strain, e = const - the so-called starting stresses for different scales of e-observation: at atomic (amplitude-independent or amplitude-dependent internal friction, deformation photoluminescence, etc.), microscopic (dislocation motion and multiplication) and macroscopic (yield and flow stresses up to fracture) scales of deformation for various crystal classes, different tests and environments. These and structural data, vacancy clusters irrefutably point to the key role of same dislocation mechanisms- cross-slip, climb and Orowan bowing of obstacles - in dislocation unpinning, motion and WH for various crystals, tests and environments.
1.Kiritani M. et al. Mater. Sci. Eng. A, 350 (2003) 1- 250.
2.Kisel, V.P. J. de Phys. (Paris), 46, Suppl. No 12 (1985) C10 (529-532).
3.Kissel, N.S. and Kisel, V.P. Int. Conf. ``Dislocations 2004'', Abstract Book.

abstract: 4.34
Studies of dislocation motion in single crystal and polycrystalline aluminum during uniaxial deformation using photoemission technique
MINGDONG CAI, STEPHEN C. LANGFORD, Physics Department, Washington State University, Pullman, WA 99164-2814, USA; LYLE E. LEVINE, National Institute of Standards and Technology, 100 Bureau Dr., Gaithersburg, MD 20899-8553, USA; J. T. DICKINSON, Physics Department, Washington State University, Pullman, WA 99164-2814, USA.
We report measurements of photostimulated electron (PSE) emission from single crystal Al (99.995%) and high purity polycrystalline Al ($>$ 99.9%) during uniaxial tensile deformation. Photoelectron intensities are sensitive to changes in surface morphology accompanying deformation, including slip line and band formation. In the single crystal material, the PSE intensity increases linearly with strain, with an onset at a strain of 0.08. In the polycrystalline material, the PSE intensity increases parabolically with strain, with an onset at a strain of 0.05. In both materials, time-resolved PSE measurements show step-like increases in intensity consistent with the sequential nucleation and growth of slip bands during tensile deformation. In this sense, we have ?observed? dislocation motion by this technique. Slip bands on the deformed surfaces were subsequently imaged by atomic force microscopy (AFM). Photoelectron measurements can provide reliable, quantitative information on dislocation dynamics.


abstract: 4.35
Dislocation-Solute-Atom Interactions Studied by Amplitude-Dependent Internal Friction
T KOSUGI, Teikyo University of Science & Technology, Japan.
Fundamental process of solid solution strengthening is still under debate. For example, there are conflicting ideas for the breakaway of a dislocation from solute atoms such as Friedel-Fleischer limit due to dislocation-single-atom interaction vs. Mott-Labusch limit due to dislocation-several-atoms interaction.

We have measured the amplitude-dependent internal friction (ADIF) of very pure aluminum (99.9999%) and Al dilute alloys (20-100 ppm) at low temperatures where diffusion of atoms is negligibly small. Two universal behaviors are found in the temperature dependence of the required stress amplitude for a constant ADIF, which is similar to the dependence of flow stress in solid solution.

For relatively low temperatures between 2 - 50 K (or high stress amplitude), the change of the stress with increasing temperature is proportional to - T2/3 above 65% of the stress at 0 K, and then the decrease begin to deviates from the T2/3 dependence. These are well explained by the idea that the fundamental process is due to unpinning of a dislocation from a single solute atom, and the interaction potential is determined successfully from this region [1].

For relatively high temperatures (or lower stress) above 50 K, we found, for the first time, that the stress changes as proportional to T -1 for all dilute Al alloys but for pure Al. This is well explained by the idea that the fundamental process is due to simultaneous unpinning of a dislocation from several solute atoms for dilute alloys [2].

References:
1. T. Kosugi, T. Kino: Mater. Sci. Eng. A164 (1993) 316.
2. T. Kosugi: Mater. Sci. Eng. A309/310 (2001) 203.


abstract: 4.36
Alternative methods to study the specific dislocation structures around a nanoindentation imprint
CHRISTOPHE TROMAS, YVES GAILLARD, JACQUES WOIRGARD, Laboratoire de Métallurgie Physique, Poitiers, France.
In the last ten years, the nanoindentation test has become a commonly used technique to measure mechanical properties form very small volume of materials. Due to the low contact area, very high stresses can be reached locally, sometimes of the order of theoretical elastic limit. Such high stresses can explain dislocation nucleation beneath the indenter, and they may promote some specific dislocation reactions or else unconventional slip systems. For all these reasons, the nanoindentation test can also be used as a nanometre scale mechanical probe to investigate incipient plasticity. However, the dislocations are highly localized around the imprint and confined in a very small volume. Surface observation by atomic force microscopy (AFM) of the slip lines pattern around the indent is a complementary technique of transmission electron microscopy to investigate such dislocation configurations. The slip lines also provide information on the chronology, since they indicate the track followed by dislocations during deformations. These AFM observations have revealed that during nanoindentation in magnesium oxide, unconventional $<011>$(211) slip systems are activated, resulting in the apparition of dislocation helical sources.

To reveal the 3D dislocation structure, we have adapted the etching technique to the scale of the nanoindentation imprint. Nanometre size chemical etch pits, produced at the emergence point of dislocations, are then observed by AFM. Furthermore, thin layers of material can be progressively removed by chemo-mechanical polishing (CMP), thus revealing the dislocation organization in the volume. This AFM-based tomography technique has been used to investigate the dislocation organization around a nanoindentation imprint in magnesium oxide and lithium fluoride single crystals. Specific dislocation interactions, responsible for the deformation behaviour during indentation, have been clearly identified. In particular, the structure resulting from the nucleation of the first dislocations, during the so-called ``pop-in'' phenomenon, has been determined and interpreted. Finally, a simple model has been developed, in the framework of elasticity and based upon the knowledge of! the individual dislocations organization, to describe the dislocations behaviour during the first stage of plastic deformation during a nanoindentation test in MgO.


abstract: 4.37
ECCI observations of Dislocation Structures in Fatigued Austenitic Stainless Steels
YOSHIHISA KANEKO, KEITA FUKUI, SATOSHI HASHIMOTO, Osaka City University, Japan.
In order to investigate factors which govern dislocation structures formed at fatigued materials, surfaces of polycrystalline stainless steels were observed by ``electron channelling contrast imaging (ECCI)'' method. The ECCI enables us to detect dislocations lying close to surface with a scanning electron microscope. Push-pull fatigue tests at constant stress amplitude were carried out in The Fe-19Ni-11Cr stainless steel with an average grain size of 360$\mu$m. The fatigue tests were interrupted at approximately half of fatigue life. In addition to the ECCI observation, the specimens were analyzed by electron back-scattered diffraction (EBSD) method to correlate crystallographic orientation and the dislocation structure of individual grains. Total number of the observed grains was 139. The dislocation structures of the grains could be classified into three types: (i) vein structure, (ii) vein & ladder-like persistent slip band (PSB) structures, and (iii) no feature. For example at the stress amplitude of 140MPa, the grains containing the vein & PSB structures were about 45% of total ones which were located in gage part of the specimen. We investigated both the grain diameter and the Schmid factor dependence of the PSB formation of each grain. All grains whose diameters were more than 900$\mu$m contained the PSB structure, regardless of the Schmid factor. At the grain diameters less than 900$\mu$m, all the above three types were observed. It should be noted that the PSB structures were formed also at the grains where both the grain diameter Schmid factor were almost minimum among the grains. Hence, it seems unlikely that the PSB formation is controlled by the Schmid factor.


abstract: 4.38
Dislocation Patterning around Crack Tips of Fatigued Polycrystalline Copper
YOSHIHISA KANEKO, MASAO ISHIKAWA, SATOSHI HASHIMOTO, Osaka City University, Japan.
Dislocation patterns formed around tips of fatigue cracks were observed using the electron channelling contrast imaging (ECCI) technique in a SEM. It has been difficult to investigate distribution of dislocation structures using a transmission electron microscope (TEM). This is because observable area in the TEM is very limited with compared to the region which is heavy deformed under affection of the crack. On the other hand, relatively large area can be examined by the ECCI technique. Thus, the ECCI technique must be suitable for the dislocation observation near the crack tip.

Fatigue crack growth tests were carried out at single-edge-notched specimens of polycrystalline copper. Surface observation with a conventional optical microscope showed that fatigue crack propagations involved many slip band formations around their tips. After the surfaces were slightly polished, the ECCI observations around the crack tip were conducted. At the regions immediately adjacent to fatigue cracks, dislocation cell structures were solely detected. The dislocation structures of the surrounded areas seem to depend on the mode of slip deformation which had been observed by the optical microscope. The ladder-like dislocation structure was observed at the region where slip bands of single system were operated dominantly. On the other hand, labyrinth structure or cell structures were recognized if the observed areas contained the slip bands of multiple systems.


abstract: 4.39
Ultra-fast elongation of pure aluminum
ICHIRO MUKOUDA, Hiroshima University, Japan. Experiments of ultra-fast elongation were carried out to clarify the mechanism of ultra-fast deformation of metals. Colliding of fast projectile to specimen holder carried out to introduce an ultra-fast elongation of pure aluminum. A projectile was accelerated by compressed gas. Strain rate rose to 1 x 10 4 sec-1. Elongations were done till a part of specimen was fractured. Elongated bulk specimens were observed by electron microscopy. Dislocations and vacancy clusters were observed. Dislocations were formed in isolated configuration. They do not make a group such as cell structure. Vacancy clusters were stacking fault tetrahedra (sft) whose size is 4 nm on average. Thin specimens, which attaches on the fractured edge and whose specimen thickness is less than 100 nm was observed. Defects in thin foil specimens do not contain dislocations and a large number of sft were formed. Stacking fault tetrahedra of vacancy clusters in aluminum forms only in plastically elongated specimens. Sft do not form in aluminum quenched from high temperature and radiation damaged aluminum. The fact that sft form in ultra-fast elongated bulk aluminum and that dislocation do not form cell structure but form isolately means that dislocation multiplication do not occur so fast and sft generation occurs at the beginning of ultra-fast elongation. Following the sft generation, dislocation moves in specimens and stay in isolated state by trapping small vacancy clusters.


abstract: 4.40
Incipient plasticity around nanoindentations in magnesium oxide
YVES GAILLARD, CHRISTOPHE TROMAS, JACQUES WOIRGARD, LMP, Chasseneuil, Vienne, France.
We have developed a new technique based upon controlled chemical etching, atomic force microscopy and chemo-mechanical polishing to study the dislocation structure around nanoindentations. The successive observation in depth of nanoetching patterns performed around the same indentation, allow a 3D reconstruction of the dislocation structure keeping an individual recognition of all the dislocations. This AFM-based tomography technique has been applied to the study of incipient plasticity in magnesium oxide (MgO). Particularly the transition between elastic and elasto-plastic deformation has been underlined. In a number of materials, this transition is characterised by a sudden penetration of the indenter in the material. Depending on the material, this phenomenon, well known as ?pop-in?, is generally associated with nucleation of prismatic or glissiles dislocations or with oxide film breakthrough.

Experiments conducted with classical Berkovich indenter have shown that the dislocation structure just after the pop-in is composed of dislocations half loops lying in classical glide systems of MgO : $<110>\{110\}$. Strong interactions leading to the formation of sessile dislocations have been also emphasized in the vicinity of the contact area. These sessile dislocations prevent the propagation of other dislocations, resulting in a dislocation structure very confined around the residual imprint. The use of spherical indenter with radius of curvature of several micrometers have allow to narrow the plastic deformation to an earlier stage. In this case, the pop-in is broken up into many single events (staircase yielding). Each event appears to correspond to the activation of a glide system. Indeed, the dislocations resulting from the first of these single event are dislocation half loops nucleated in a single $\{110\}$ plane inclined at 45 degrees from the indented surface. The emergence of these dislocations leads to the apparition on the surface of a single step. In particular, no residual imprint is observed. Based on the elastic stress field involved around the spherical indenter before the first event, it has been established that these dislocations result from the propagation of dislocation loops nucleated at the point of maximum resolved shear stress. This point is located under the loading axis at a depth of about half the contact radius.


abstract: 4.42
Calculation of the stored energy of cold-work from discrete dislocation simulations
A. AMINE BENZERGA, Aerospace Engineering, Texas A&M Univ, USA; Y BRECHET, LTPCM, INPG, Saint Martin D'Heres, France; A NEEDLEMAN, Division of Engineering, Brown University, Providence, RI, USA; E VAN DER GIESSEN, Department of Applied Physics, University of Groningen, The Netherlands.
The stored energy of cold work is calculated for crystalline samples where plastic deformation occurs through dislocation glide. Superposition is used to represent the solution of boundary value problems in terms of the infinite fields for discrete dislocations and image fields that enforce boundary conditions. Constitutive rules are used which account for the effects of 3D dislocation dynamics such as dynamic junction formation. Isothermal conditions are assumed and, initially, the samples are taken to be stress-free. At any deformation stage, the stored energy is calculated as the free energy of the sample, after subtracting out the portion associated with the loads, which is equal to the elastic work. Calculations are carried out both under load and after load removal with the line energy contribution accounted for. The extent to which the energy stored in the sample depends on the deformation state is analyzed by considering plane strain tension and bending of crystals in double slip. The effects of crystal orientation and of the amount of supplied mechanical energy are also investigated.

abstract: 4.46
Large-scale atomic-level modelling of strengthening due to localized obstacles
YURI OSETSKY, Oak Ridge National Laboratory, USA.
Strengthening due to localized obstacles to dislocation motion is observed in metals subjected to different conditions such as irradiation by energetic particles, thermal and mechanical treatments. The approach called multiscale materials modelling can predict mechanical property change by decomposing the whole phenomenon into different level scales, each to be studied by the appropriate modelling technique. The important interface lies between the continuum dislocation dynamics models, able to investigate evolution of dislocation networks in the environment of the particular microstructure, and atomistic models, able to investigate atomic-scale details of dislocation interactions with individual obstacles. There is a significant lack of data to build and parameterize mechanisms across this interface. This is mainly because the mutual validation and parameterization of the mechanisms requires simulations by both approaches at overlapping scales, which is technically difficult in the case of atomic-scale modelling. However, increase in computer power and development of new atomistic models allow some particular cases of dislocation-obstacle interactions to be studied. In this paper we present an overview of recent results where dislocation interaction with localized obstacles such as voids, secondary phase precipitates, interstitial loops and stacking fault tetrahedra was studied in some bcc and fcc metals. The overview is focused mainly on structure, temperature and kinetic effects. It is demonstrated that atomistic modelling is able to reveal a number of important effects which are far beyond the continuum approach.


abstract: 4.47
Atomic-scale study of hardening due to copper precipitates in alpha-iron
YURI OSETSKY, Oak Ridge National Laboratory, USA; D. BACON, University of Liverpool, UK.
Formation of secondary phase precipitates is a common phenomenon during irradiation of supersaturated solid solutions and can lead to a significant change in mechanical properties. Theoretical models based on three-dimensional dislocation dynamics (3-DDD) can, in principle, predict hardening effects by simulating dislocation motion and network evolution in the presence of obstacles. Description of the mechanisms of dislocation-precipitate interaction is necessary for such models and is usually based on simple theoretical approaches. We present results of a large-scale atomic-level study of dislocation-precipitate interaction. We have considered initially straight $<111>$ dislocations gliding in alpha-iron containing coherent copper precipitates of size from 0.7 to 6nm over a temperature range from 0 to 600K. The results demonstrate that some features are qualitatively consistent with earlier theoretical conclusions, e.g. the critical resolved shear stress (CRSS) is proportional to L-1 and ln(D), where L and D are precipitate spacing and diameter. Other features, which are intrinsic to the atomic-level nature of the dislocation-precipitate interaction, include strong dependence of the CRSS on temperature, dislocation climb and precipitate phase transformation. It is demonstrated that continuum models based on the constant line tension approach cannot describe either the CRSS or the shape of the dislocation line at the critical stress and its temperature dependence.


abstract: 4.49
Comparison of void strengthening in FCC and BCC metals : large-scale atomic-level modelling.
YURI OSETSKY, Oak Ridge National Laboratory, USA; DAVID J BACON, University of Liverpool, UK.

Strengthening due to voids can be a significant radiation effect in metals. Rationalization of this effect within dislocation dynamics requires knowledge of the dislocation-void interaction mechanism. Treatment of this interaction by continuum dislocation theory can be applied to study qualitative effects at large scales, but fails in cases when atomic structure of the obstacle and dislocation is important. In this paper we report results of large-scale atomic-level modelling of dislocation-void interaction in fcc (copper) and bcc (iron) metals. The main aim of the study is a comparison of the strengthening mechanisms in fcc and bcc lattices, where dissociated and undissociated dislocations are involved. Dislocations with Burgers vectors $1/2<110>$ and $1/2<111>$ have been simulated in copper and iron, respectively. Voids of up to 6nm diameter were studied over the temperature range from 0 to 450K. We demonstrate that atomistic modelling is able to reveal a number of important effects which are far beyond the continuum approach. Some of them are features of dislocation core and crystal structure, others involve dislocation climb, kinetic, temperature and strain rate effects.


abstract: 4.50
Computer Simulation of Dislocation Dynamics in a Solid Solution.
KANIT TAPASA, DAVID J BACON, University of Liverpool, UK; YURI OSETSKY, Oak Ridge National Laboratory, USA.

Glide effects of an edge dislocation in iron containing either carbon or copper atoms in solution have been investigated using an atomic-scale computer simulation method that allows dislocation motion over a long-range on the atomic scale. Models for two different solutes types are considered, namely a substitutional element (copper) and an interstitial one (carbon). The influences of solute at concentrations up to 1at%Cu and 0.5at%C have been treated. The attraction and repulsion of a dislocation due to single solute atoms near the dislocation glide plane have been studied in terms of the interaction energy. The resolved shear stress for glide (Peierls stress) under static conditions (T = 0K) has been investigated, and the dependence of dislocation velocity on stress and temperature under dynamic conditions has been determined and compared with similar modelling for pure iron. The results of the simulations are discussed in relation to the dynamics of dislocation drag and predictions of dislocation-defect interactions obtained from elasticity theory.


abstract: 4.51
Dislocation dynamics described by non-local Hamilton-Jacobi equations.
O. ALVAREZ, Université de Rouen, Lab. Math. R. Salem, France; P. HOCH, CEA / DAM, France; Y. LE BOUAR, LEM, CNRS-ONERA, france; R. MONNEAU, ENPC / CERMICS, France.

We study a simple model of dislocation dynamics involving a non-local Hamilton-Jacobi equation. For this model, we rigorously prove that the mathematical solution exists and is unique. This solution can be approximated and computed numerically. We propose a new scheme based on the level sets method, that we prove to be accurate. We will also present numerical simulations.


abstract: 4.53
Texture and dislocation substructures analysis of a dual phase steel under strain path changes at large deformation
BENOIT GARDEY, SALIMA BOUVIER, VINCENT RICHARD, BRIGITTE BACROIX, LPMTM-CNRS UPR9001, Villetaneuse, France.

During sheet metal forming processes, the material undergoes complex strain paths involving large plastic strains. The combination of several simple loading test sequences is known to be an effective way to investigate the anisotropic plastic behaviour of sheet metals under such real-forming conditions. The aim of the present work is to analyse the evolution of the microstructure of a dual phase steel in term of dislocation organisation under two-stage strain path changes sequences taking into account the evolution of the microstructure in connection with the crystallographic grain orientation in order to achieve a more comprehensive explanation of the anisotropic behaviour under complex strain-path changes for these steels.

The mechanical tests performed are two-stages sequences of simple shear tests with three different strain paths, namely monotonic, Bauschinger and orthogonal loading. Texture measurements are performed that allow focusing the microstructure's observations on ferrite grains that exhibit majority orientations. During a monotonic deformation, well-defined dislocation walls are developed which orientation depends on the grain's orientation. After a Bauschinger strain path change, a partial dissolution of the preformed dislocation sheets is observed whereas after an orthogonal strain path change they remain in most grains while new dislocation microstructures depending on the grains orientation are formed.


abstract: 4.54
An Investigation Of Plastic Zone Deformation Substructures Under The Nanoindentation in an alpha / beta Ti Alloy
GOPAL B VISWANATHAN, EUNHA LEE, The Ohio State University, Columbus, Ohio, USA. SRIKUMAR BANERJEE, BARC, Mumbai, India. DENNIS MAHER, HAMISH L FRASER, The Ohio State University, Columbus, Ohio, USA.

Hardness measurements by indentation is a simple and rapid method for gathering information about the mechanical properties of materials such as elastic modulus, yield stress etc. With its ability to probe a small volume of material, nanoindentation technique is best suited for obtaining the mechanical properties of single crystal alloys from polycrystalline material. In this study this technique has been applied to evaluate the mechanical properties of Ti-Al based alloys containing an alpha/beta two-phase microstructure. Specifically, nanohardness measurements were obtained from individual alpha and beta phases in these alloys. The indentations were placed within the desired grains. The precise individual orientations of the grains were obtained through Orientation Imaging Microscopy (OIM). TEM samples were prepared from the plastic zone beneath the indentation by Focussed Ion Beam (FIB) technique for analyzing the deformation structures. Attempts have been made to correlate the trends that are seen in hardness estimates to various microstructural features such as orientation of individual grains, grain size supported by TEM analysis of the deformation structures. Results indicate that there exist certain relationships between the hardness of the individual grains and their individual orientations. Near the [0001] stress axis the hardness values were the highest and lie in the range 6.3-6.9 GPa. Hardness values decrease as the stress axis deviates from the [0001] orientation. Hardness values were in the medium range i.e. 5.1-5.8 for the grains in the middle of the stereogram and the values were in the lower range i.e. 4.5-5.1, near the [0110] or [1210] orientations. Dislocation analyses indicate that the deformation in individual grain mostly conforms to the Schmid factor type analysis where the slip occurs on those slip systems where CRSS value is high. Additional slip systems are also observed and they are speculated to be present to accommodate the arbitrary shape. An increase in hardness with decreasing depth was observed (especially at depths $<$ 100 nm) in most experiments and it was again depended on the orientations of the individual grains. Based on the TEM dislocation analysis this increase in hardness as been attributed to strain gradient plasticity effects where the geometrically necessary dislocations contribute to higher hardness at smaller depths.


abstract: 4.55
Mechanisms for Creep at Intermediate Temperatures in the Ni-based Superalloy Rene 88DT
GOPAL B VISWANATHAN, PETER M. SAROSI, The Ohio State University, Columbus, Ohio, USA. MICHAEL F. HENRY, GE Global Research Center, Schenectady , New York, USA. DEBORAH WHITTIS, GE Aircraft Engines, Cincinnati, Ohio, USA. MICHAEL J. MILLS, The Ohio State University, Columbus, Ohio, USA.

Creep strength in Ni-base superalloys depends on a number of interrelated microstructural parameters, including the volume fraction, particle size, distribution, and chemical characteristics of the $\gamma$' (Ni3Al type) precipitates that are present in FCC $\gamma$ matrix. Depending on the alloy and deformation conditions, a variety of mechanisms have been reported as operative in the literature. At lower temperatures, deformation seems to be dominated by APB coupled 1/2[110] unit dislocations shearing the $\gamma$ precipitates. At very high temperatures (850-1000°C), climb by-pass and at times looping around the precipitates by 1/2[110] matrix dislocations have been observed. On the other hand, at intermediate temperatures i.e. 650-800°C (depending on the alloy), deformation is extremely planar and complex. The mechanisms responsible for deformation at these temperatures is much less clear and more importantly the effect of microstructure and applied stress is not fully understood.

In this study, the dislocation substructures developed after small-strain (0.5%) creep at 650°C and at stress levels in the range from 115-140 ksi, in Rene 88 DT have been analyzed using conventional and high resolution transmission electron microscopy. Clear differences in creep strength and substructures have been observed for samples heat treated with two different cooling rates from the supersolvus heat treatment temperature. For the more rapidly cooled (400°F/min) and finer microstructure exhibits very low initial creep rates, and deformation microtwinning is the dominant deformation process. For the more slowly cooled (75 °F/min) and coarser microstructure, the creep rates are much faster, and isolated faulting of individual secondary $\gamma$' precipitates is observed. The detailed dislocation analyses leading to these conclusions will be presented. The transition from the isolated faulting mechanism in coarser the $\gamma$' microstructure to microtwinning in the finer $\gamma$ microstructure is attributed to the effect of the volume fraction of the tertiary $\gamma$' precipitates on the stress required to drive 1/2$<110>$ versus 1/3$<112>$ dislocations through the matrix and tertiary particles. A simple model predicting this transition in mechanism will be discussed, as will the possible rate-limiting processes for these two deformation modes.


abstract: 4.56
INHOMOGENEOUS DISLOCATION STRUCTURE IN FATIGUED INCONEL 713LC SUPERALLOY AT ROOM AND ELEVATED TEMPERATURES
MARTIN PETRENEC, KAREL OBRTLÍK, JAROSLAV POLÁK, Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Brno, Czech Republic.

The inhomogeneous distribution of dislocations was studied in specimens of polycrystalline 713LC Inconel superalloy cyclically strained under strain control with constant total strain amplitudes up to failure at temperatures 23 °C, 500 °C, 700 °C and 800 °C. Thin foils were studied in transmission electron microscope and the diffraction patterns and Kikuchi lines were used to determine the grain orientation. Inhomogeneous dislocation microstructure resulting from cycling at all four temperatures was documented. Dislocations were present both in the matrix and in the gamma´ precipitates. Higher dislocation density was in the matrix. Planar dislocation arrangements in the form of bands parallel to the $\{111\}$ planes were observed in specimens cycled at all temperatures. The bands showed up as thin slabs of high dislocation density cutting both the gamma channels and gamma´ particles. The relation of the localized dislocation arrangements and the surface slip bands was discussed.


abstract: 4.57
Strain rate sensitivity of flow stress under superimposition of ultrasonic oscillatory stress during plastic deformation of RbCl doped with Br or I
TOMIYASU OHGAKU, KATSUKI HASHIMOTO, Faculty of Engineering, Kanazawa University, Japan.

Flow stress decreases when ultrasonic oscillatory stress is superimposed during plastic deformation of single crystals. Strain rate cycling tests were carried out under superimposition of ultrasonic oscillation during plastic deformation of RbCl single crystals doped with Br- or I-. The relation between the strain rate sensitivity (SRS) of flow stress, which is calculated from the stress change due to strain rate cycling, and stress decrement due to application of oscillation has a stair-like shape and is divided into three regions. In the first region that is a plateau at the small stress decrement, both impurities and forest dislocations act as obstacles to translational motion of dislocations. This is because the amplitude of oscillatory stress is too small to help the mobile dislocation to overcome the obstacles. Then, the SRS keeps constant. This region does not appear for nominally pure RbCl single crystals. In the second region where the SRS decreases with increasi! ng stress decrement, the role of impurities as obstacles is decreasing. Some impurities are overcome by oscillating dislocations and others are not yet. In the third region that is a plateau again, only the forest dislocations act as obstacles and then the SRS keeps constant again. The stress decrement (tau) at the first bending point between the first and second regions is considered to be the effective stress due to impurities. The difference (lambda) of SRS between the first and second plateau regions is also considered to be a part of SRS due to impurities. The tau and lambda were measured in the temperature range from 77K to room temperature. The relations between the tau and activation volume obtained from the lambda reveals the interaction between the dislocation and impurities. The Cottrell-Bilby relation is assumed to be applicable to the case of RbCl crystals doped with Br- or I-. Then, the interaction energies between the dislocation and Br- or I- were determined to be 0.5 or 0.58eV in RbCl, respectively.


abstract: 4.58
A kind of universality in the growth of long fatigue cracks and the role of dislocations
F. BERGNER, Forschungszentrum Rossendorf, Dresden, Germany.
For a number of thin-sheet wrought aluminium alloys the measured fatigue crack growth rates (constant stress amplitude, constant stress ratio) were observed to fall into a narrow scatterband at a particular value of the cyclic stress intensity factor, DeltaK, but to diverge at increasing values of DeltaK. The members of this group of alloys are characterized by the dominance of strength-controlling precipitates that cannot be sheared by dislocations. The significance of the above mentioned focussing of the fatigue crack growth curves is further enhanced by the following additional findings: Firstly, the growth rates of a number of three plain-carbon steels (yield strength between 330 and 760 MPa) and the magnesium alloy AZ31 strongly deviate if represented over DeltaK but fall into the same scatterband if plotted over DeltaK/G, with G denoting the shear modulus. Secondly, there is another group of thin-sheet wrought aluminium alloys characterized by a dominance of shearable ! strength-controlling precipitates. The crack growth rates of these alloys were observed to be retarded with respect to the focal point for the first group and the amount of retardation turned out to be correlated with the roughness of the crack surface. However, if the retardation is extrapolated to zero roughness, the retardation vanishes, i. e. both groups exhibit the same intrinsic behaviour.

The Paris equation of fatigue crack growth can be reduced to a dimensionless form using a yet unknown modulus, M (a quantity of the dimension of a stress), and a yet unknown length, L. The values of M and L have been estimated from the requirement to correctly reflect the focal point. The resulting characteristic modulus, M 25 GPa, is identified as the shear modulus. The characteristic length, $L \approx 0.16 \mu$m, is proposed to be related to the spacing of dislocation bands appearing in discrete dislocation models [1, 2]. The observations and the proposed interpretation will be outlined in some detail.

References:
1. Riemelmoser FO, et al., Acta Mater. 46 (1998) 1793.
2. Deshpande VS, et al., Acta Mater. 50 (2002) 831.


abstract: 4.59
Dynamics of Drag of Self-interstitial Clusters by an Edge Dislocation in Iron
ZHOUWEN RONG, DAVID BACON, University of Liverpool, Liverpool, UK; YURI OSETSKY, Oak Ridge National Laboratory, USA.
Point defects and their clusters are created and accumulated in metals under fast neutron irradiation, and the ensuing microstructure evolution can lead to change in mechanical properties, such as hardening and reduction in ductility, and plastic instability and strain localization associated with cluster-free channels. The interaction between mobile dislocations and radiation-induced microstructure plays an important role in controlling these effects. In particular, the importance of dislocation interaction with clusters of self interstitial atoms (SIAs) in creation of dislocation decoration, dislocation blocking, yield-stress drop and channel formation is emphasized in the cascade induced source hardening (CISH) model. For example, it is proposed that glissile SIA clusters can move to decorate edge dislocations, thereby lowering their mobility under stress and increasing the flow stress. In this paper, models based on atomic-scale computer simulations are used to investigate the dynamic interaction between an edge dislocation and SIA clusters. The clusters considered are small glissile dislocation loops with perfect Burgers vector parallel to the glide plane of the dislocation: they do not intersect the dislocation. It is demonstrated that glissile SIA clusters can be effectively dragged by a moving dislocation with high velocity. The drag coefficient is estimated as a function of temperature, and loop size and Burgers vector orientation. The data are used to develop a model for loop drag based on the one-dimensional mobility of SIA clusters. The conditions under which the dislocation line breaks away from the decorating SIA clusters have been analyzed. The implications for mechanisms of irradiation effects are discussed.