Oral contributions

abstract: 3.2
Dislocations in Tetragonal Lysozyme Protein Crystals
HARUHIKO KOIZUMI, MASARU TACHIBANA, MIKI SHIMIZU, KENICHI KOJIMA, Graduate School of Integrated Science, Yokohama City University 22-2 Seto Kanazawa-ku, Yokohama 236-0027 JAPAN.
Protein crystals which were composed of large molecules with high molecular weight have the long lattice constants (several nanometers) of even ordinary protein crystals. Thus the dislocations in protein crystals are very interested. However, as it was quite difficult to grow large high quality protein crystals, nobody could observe dislocations with such long Burgers vectors so far. Recently, however, we have first succeeded to observe dislocations in tetragonal hen egg-white lysozyme single crystals that is one of typical protein crystals using synchrotron X-ray topography. The lysozyme crystals were grown by the concentration gradient method. The maximum size of crystals were $3 \times 3 \times 4\ mm^2$. The high quality crystals were examined by Laue topography with a water filter using synchrotron radiation. As a consequence, straight lines from the core part of the crystal are clear observed. They are grown-in dislocations, since the distribution of line contrasts are similar to those of the dislocations commonly observed in solution-grown crystals. Moreover, according to the invisibility criterion $(g.b = 0)$ of the dislocation images, the Burgers vectors are determined. Thus it is concluded that the predominant dislocations in tetragonal HEW lysozyme crystals are of screw character with $<110>$ Burgers vectors. The magnitude of the Burgers vector to be a $[110]$ direction was $11.1$ nm. In addition, $[001]$ slip dislocations were observed. These dislocation structures were discussed in more details at the conference.


abstract: 3.3
Dislocation Process in Quasicrystals: kink pair formation control or jog pair formation control
SHIN TAKEUCHI, Department of Materials Science and Technology, Tokyo University of Science, JAPAN.
Perfect dislocations can exist in quasicrystals as in crystals, but they accompany not only the usual elastic strain (phonon strain) field but also a phason strain field, which can only be relaxed at high temperatures due to atomic diffusion. As a result, dislocation processes in quasicrystals are only active at high temperatures where atomic diffusion is fast enough. It has been established that at temperatures higher than 0.75Tm, quasicrystals generally undergo thermally activated deformation by a dislocation process. The most characteristic feature is anomalously large work softening at lower temperatures. Recent electron microscopy observations have suggested that dislocations in some quasicrystals move by climb rather than glide at high temperatures. A computer simulation study of dislocation glide in a model quasicrystal has shown that the dislocation glide experiences a high Peierls potential. For a dislocation with a high Peierls potential, glide motion and climb motion of a straight dislocation can be treated almost in parallel in terms of kink-pair formation or jog-pair formation process. By comparing the glide velocity controlled by the kink-pair formation and the climb velocity by the jog-pair formation, we show that when the kink energy is considerably larger than the jog energy, the climb velocity can be higher than the glide velocity. Possible mechanisms of the work softening will also be discussed.


abstract: 3.4
Multi-scale modeling of deformation in polycrystalline thin metal films
ALEXANDER HARTMAIER, MARKUS J. BUEHLER, HUAJIAN GAO, Max Planck Institute for Metals Research, Germany.
The time-dependent irreversible deformation of thin metal films on substrates is investigated on various levels of modeling. The work is focused to the case of polycrystalline films without cap layer, where diffusive processes play an important role. A continuum level model of constrained grain boundary diffusion describes how surface and grain boundary diffusion lead to the relaxation of tractions along the grain boundaries. If the film is perfectly bonded to the substrate, the relaxation of tractions along the grain boundary leads to a crack-like stress concentration close to the film/substrate interface. By virtue of this stress concentration dislocations on glide planes parallel to the thin film are subject to a driving force that would not exist in the case of purely bi-axial loading. This model gives a theoretical basis for experimental observations of such parallel glide processes. To obtain a better understanding of the microscopic phenomena taking place during this creep process, we conducted large scale molecular dynamics (MD) simulations, as well as discrete dislocation dynamics (DDD) studies on intermediate length scales. The results of these numerical simulations reveal that nucleation of glissile dislocations at the tip of the diffusion wedge can be described with a critical stress intensity factor criterion. The validation of this criterion for dislocation nucleation by MD simulations allows us to transfer this information onto the DDD length and time scales. In such DDD simulations film thicknesses from nanometers to microns can be investigated with respect to their deformation behavior. The DDD simulations show that for ultra-thin films the scaling of the flow stress with the inverse film thickness breaks down. Instead such films show a constant flow stress as also observed in experiment. Our studies also reveal under which conditions mechanisms of parallel glide or, respectively, conventional glide mechanisms with threading dislocations are dominant, which gives rise to a deformation map for thin films.


abstract: 3.10
Understanding nanocrystalline deformation mechanisms deduced from molecular dynamics
A G FROSETH, P M DERLET, H VAN SWYGENHOVEN, Paul Scherrer Institut, Villigen, Switzerland.
Atomistic simulations have provided unprecedented insight into the structural and mechanical properties of nanocrystalline materials, highlighting the role of the non-equilibrium grain boundary structure in both inter- and intra-deformation processes. One of the most important results is the capability of the nanosized grain boundary to act as source and sink for dislocations. For materials such as Ni and Cu, the dislocation activity is predominantly governed by extended partial dislocations, whereas for Al the trailing partial is observed soon after the leading or alternatively an anomalous dislocation is emitted. The former results have been used for setting up deformation maps in terms of grain size and flow stress in nanocrystalline metals.

However the extrapolation of the MD results to the experimental regime requires a clear understanding of the temporal and spatial scales of the modelling technique. It will be shown that the different obtained results can be correctly understood in terms of the generalised planar fault (GPF) energy curve for an intrinsic stacking fault, which entails considering both the stable and unstable stacking fault energies, (2) that a value for the critical grain size at which no dislocations are observed cannot be determined uniquely by MD simulations at the present stage of development, and (3) that the existence of extended partial dislocations has to be considered with caution, since their appearance in atomistic simulations might be only an artefact of the high applied stress and the short time scale inherent to such simulations.

Furthermore, it will be shown that the presence of grown-in twins in nanosized grains, which is often the case in experimental samples, introduces alternative deformation mechanisms, such as twin boundary migration. The influence of the grown-in twins on the deformation behaviour is different for nanocrystalline Ni, Cu and Al, and can be explained by the correspondingly different values of the extrema of the GPF energy for the partial dislocation and twin migration slip processes.


abstract: 3.11
In-situ diffraction profile analysis during tensile deformation
ZELJKA BUDROVIC, PETER M. DERLET, HELENA VAN SWYGENHOVEN, Paul Scherrer Institut, Villigen, Switzerland.
Can dislocation activity still be considered as an important deformation mechanism in nanocrystalline metals with a mean grain size below 100nm? Such a question is difficult to answer since direct visualization techniques such as transmission electron microscopy (TEM) fail to give a clear answer. Post-mortem analysis revealed no dislocation debris but also no evidence for a grain boundary sliding mechanism. In-situ deformation in TEM demonstrates dislocation activity to some extent, but these experiments suffer from two drawbacks: (1) the thin film approach leading to artefacts coming from free surfaces and (2) the fact that the propagation of a crack tip is followed which is known to activate different deformation mechanisms.

Molecular dynamics simulations have suggested that dislocations can still be a possible deformation mechanism, even at the very small grain sizes, where now the dislocations are emitted from and absorbed in grain boundaries.

The suggestions from molecular dynamics have inspired us to develop a new in-situ technique, based on well known peak profile analysis methods, for addressing the relationship between microstructure and mechanical properties in nanostructured materials. It is well known that the dislocation network built up during deformation of polycrystalline metals result in an irreversible broadening of the diffraction peaks. Thanks to the high intensity Swiss synchrotron Light Source and the development of a microstrip detector covering a diffraction angle of 60/260, it has been possible to develop an in-situ X-ray technique that allows simultaneous peak profile analysis of several Bragg diffraction peaks during tensile deformation. It is shown that during plastic deformation of electrodeposited nanocrystalline Ni the peak broadening is entirely reversible upon unloading, demonstrating that the deformation process does not build up a residual dislocation network (Budrovic et al, Science, 2004). On the other hand, in ultra fined grained high-pressure-torsion Ni the peak broadening is only partially reversible upon unloading. The behaviour of peak position and peak broadening during tensile deformation, unloading and stress relaxation experiments is discussed in terms of microstructural parameters and those parameters that describe the mechanical behaviour such as flow stress and activation volume. The obtained experimental picture is also discussed in the framework of the deformation mechanisms suggested by molecular dynamics computer simulations.


abstract: 3.15
Dislocation emission from grain boundaries in nano-crystalline Ni
DIANA FARKAS Materials Science and Engineering, Virginia Tech, USA; WILLIAM A. CURTIN, Division of Engineering, Brown University, Providence RI, USA.
We present an atomistic study of the process of dislocation emission from grain boundaries in columnar structures of nano-crystalline Ni. The samples are constructed to include grain boundaries with random tilt misorientations around a [110] axis. The grain boundary planes are also constructed at random and a variety of grain sizes is studied ranging from 4 to 40 nm grain size. After reaching an equilibrium grain boundary structure, tensile deformations of up to 8% were simulated. Dislocations emitted from the grain boundaries were observed increasingly in samples of 6 nm grain size and larger and a quantitative study was performed. . The number of dislocations emitted per unit of grain boundary area present became independent of grain size for grain sizes of around 20nm. The role of grain boundary structure in the emission process will be discussed showing that the presence of intrinsic grain boundary dislocations in the equilibrium grain boundary structure plays a key role in the dislocation emission process.


abstract: 3.17
Plastic Deformation of Al-Cu-Fe quasicrystals embedded in Al$_{2}$Cu at low Temperatures
SHINYA MIYAZAKI, SHINJI. KUMAI, AKIKAZU. SATO, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama, Japan.

Al-Cu-Fe icosahedral quasicrystals are known to be hardly deformable at a lower temperature than 0.8 Tm. The microstructures of the quasicrystals deformed at such temperatures have not been examined in details. Although the quasicrystals are brittle, there is a possibility that they can be deformed if they are covered by other ductile materials. In this study, a few methods are proposed to deform an Al-Cu-Fe quasicrystal. The purpose of this study is to reveal the deformation mechanism at low temperatures and compare the results with those reported for high temperature deformation.

The specimens prepared in this study were composed of Al$_{2}$Cu matrix and Al-Cu-Fe quasicrystal particles. Compression tests of the composite specimens were made at room temperature as well as at 650K (0.6 Tm). Al-Cu-Fe quasicrystal particles embedded in Al$_{2}$Cu could be deformed without cracking At 650K because the matrix Al$_{2}$Cu was sufficiently ductile and protected the quasicrystal particles not to crack. Then, a thin foil specimen for TEM observation could be prepared by usual electrical polishing. TEM observations were made by usual displacement and phase imaging coupled with convergent beam electron diffraction (CBED) method.

In the case of room temperature tests, since Al$_{2}$Cu becomes brittle, the composite specimen was thinned down to a TEM foil specimen before deformation and mounted on a Cu mesh to protect breaking upon thin compression. In this way, plastic deformation could be introduced into Al-Cu-Fe quasicrystals at room temperature.

Dislocations in quasicrystals showed that the deformation was governed by dislocation motion even at this low temperatures. Their motion generally left planner faults along the paths. Properties of the planner faults were examined by visibility criteria.

Many dislocations observed in the bands could be examined individually by use of defocus CBED method which enabled us to analyse the Burgers vectors. The dislocations introduced by deformation at low temperatures will be discussed on the ground of these observations.


abstract: 3.20
A Crystallographic Constitutive Model for Ni3Al (L12) Intermetallics
YOON-SUCK CHOI, UES, Inc., USA; DENNIS M. DIMIDUK, MICHAEL D. UCHIC, Air Force Research Laboratory, USA; TRIPLICANE A. PARTHASARATHY, UES, Inc., USA.
The L12-structured Ni3Al and Ni3(Al, X) intermetallic alloys exhibit a variety of unusual thermo-mechanical flow behaviors. Some key features include an anomalous temperature-dependence of the flow stress and its variation in the micro-strain regime, an anomalous change of work-hardening rate with temperature, a tension-compression asymmetry, and a partially-to-fully reversible flow behavior after a change in temperature. The present study is aimed at building a comprehensive crystallographic constitutive model, which captures the major flow features of Ni3Al and Ni3(Al,X) alloys, and is linked to experimentally observed dislocation events. In this effort, we have critically examined the merits and limitations of existing theoretical models that describe flow behavior, extracted selected formulations and concepts from these models, and have re-formulated these ideas into a combined constitutive model that is based upon verified mechanism-based understanding for this class of alloys. In the combined model, work-hardening is addressed by incorporating two theoretical models proposed by Ezz and Hirsch, and by Caillard, respectively. In Ezz and Hirsch's model the flow stress is partitioned into two (or three) contributions. The first contribution is only temperature dependent, and is thermally reversible upon loading and reloading at different temperatures. The second (remainder) contribution is temperature and strain-rate dependent, and involves rate-controlling work-hardening by forest interactions, which can be described by the multiplication of Frank-Reed sources and their interactions with cross-slip locking and bypass unlocking events. In the present effort we have treated the first term of the Ezz and Hirsch model as being controlled by Caillard's hardening description, in which strain hardening is formulated as a consequence of the exhaustion of mobile dislocations by cross-slip locking from the octahedral plane to the cube cross-slip plane. The combined hardening formulations were incorporated into the strain-rate formulation based on thermally-activated plastic flow proposed by Kocks, Argon and Ashby, which was then implemented into the finite element software ABAQUS via a User MATerial subroutine (UMAT). We have performed parametric studies in order to identify the physical nature of the major input parameters and to evaluate reasonable ranges of the values for these parameters. We will present simulation results for the flow behavior of Ni3(Al, X) crystals over a wide range of temperatures in the anomalous flow regime using this model, and compare the simulation results with experimental data.


abstract: 3.21
Modern Problems of Radiation-Induced Plastic Deformation in Irradiated Structural Materials
ALEXANDER IVANOVICH RYAZANOV, Russian Research Centre ``Kurchatov Institute'', Moscow, Russia.
The radiation resistance of fission and fusion structural materials under neutron irradiation is determined by many physical phenomena. One of the more important from them is a radiation-induced plastic deformation or irradiation creep.

The radiation induced plastic deformation is characterized by the strain rate and it depends on many parameters: initial microstructure, irradiation conditions, accumulation in the matrix under irradiation of defect clusters (dislocation loops, voids and precipitates). The investigation of influence of these parameters on the irradiation creep is very important for understanding of physical mechanisms of this phenomenon. The main aim of the present report is the comparative analyses of physical mechanisms of irradiation creep of structural materials based on modern theoretical models and last experimental results concerning the peculiarities of irradiation creep behavior under neutron irradiation. This report is oriented on the clearing of the effect type of crystal lattice (BCC and FCC), dislocation core microstructure, generation rate of point radiation defects, elemental composition of structural materials, temperature and dose dependencies of irradiation creep under neutron irradiation. The modern theoretical models of irradiation creep of structural materials presented here take into account the some peculiarities of crystal lattice type of materials and defect microstructure evolution including dislocation loop nucleation and growth, climb and glide processes of dislocations under neutron irradiation. The different stages of irradiation creep: transient stage (at low irradiation doses), intermediate and steady state stages (at intermediate and high irradiation doses) are considered here very detail. The presented data in this report include the critical review of last experimental results concerning irradiation creep behavior in different types of structural materials under neutron irradiation. The special part of this report is devoted to the comparison of some theoretical models for the calculations of irradiation creep module with the experimental data obtained under neutron irradiation in fast reactors and ion irradiation. The presented here results allow clarifying the some physical mechanisms of irradiation creep phenomenon in irradiated structural materials.