abstract: 3.a
Dislocation-interface interactions: dynamic experiments to continuum modeling
IAN ROBERTSON,
ARMAND
BEAUDOIN,
KHALED
AL-FADHALAH,
CHUN-MING
LI,
University of Illinois, U.S.A.
Incorporating the interaction of dislocations with boundaries remains a challenge in the development of predictive large-scale plasticity codes. The focus of this presentation will be on current understanding of the atomic processes governing lattice dislocation interactions with grain and interphase boundaries and on current approaches for incorporating this information, at least phenomenologically, in continuum models. Two examples will be presented to illustrate our methodology. The first considers the prediction of the rolling texture in Cu/Nb nanostructured multilayers, and the second justifies the unique macroscopic stress-strain response of fine-grained polycrystalline Ag containing annealing twins.
abstract: 3.b
A General Methodology to Investigate Size Scales in Crystalline Plasticity
MICHAEL D.
UCHIC,
DENNIS M.
DIMIDUK,
Air Force Research Laboratory, USA;
TRIPLICANE A.
PARTHASARATHY,
ROBERT
WHEELER,
UES, Inc., USA.
Materials scientists have known for many decades that size scales in crystal plasticity-length scales associated with dislocation nucleation, motion, and substructure evolution-greatly influence flow stresses and strain-hardening rates during plastic deformation. Size-scale effects due to internal microstructural features are commonly observed, and are often utilized to improve bulk properties (for example, Hall-Petch hardening). In stark contrast, size-scale effects due to the physical geometry of a sample have been largely overlooked, especially for dimensions smaller than 100 microns. One would expect that as the sample geometry begins to shrink to a few microns, the fundamental response and organization of dislocations should be affected by this change. For example, it is well known that the characteristic size of the dislocation cell substructure formed during plastic deformation of FCC crystals is approximately 1 to 5 microns in diameter-what happens to the flow b!
ehavior when the sample size approaches this length scale?
The relative lack of attention to this issue is likely due to the general deficiency of experimental devices that can fabricate, manipulate, and characterize specimens at this size scale and smaller. Within the past decade or so, the development of instruments tailored for the microelectronics industry has addressed many of these shortcomings. Drawing from this technology base, a new methodology was developed to both fabricate miniature (oriented single-crystal) samples in virtually any inorganic material that have micron-size dimensions, and test the samples in uniaxial compression. The methodology consists of using a Focused Ion Beam microscope (FIB) for sample preparation, together with mechanical testing using a conventional nanoindentation device outfitted with a flat-punch indentation tip.
Using this methodology, the mechanisms for plastic deformation were found to be significantly affected by simply changing sample size at the micron scale. In particular, a dramatic dependence of the flow stress on sample size was observed for the intermetallic alloy Ni3Al, where the sample size ranged from 20 to 0.5 micron in diameter. More subtle effects were observed in pure nickel and Ni-base superalloy for sample sizes ranging from 40 to 5 micron in diameter. Lastly, the current status of the effort to link size-scale effects to the observed deformation substructure is reported.
abstract: 3.c
Small is strong - plasticity in sub-micron metals
EDUARD
ARZT,
GERHARD
DEHM,
T. JOHN
BALK,
HUAJIAN
GAO,
Max Planck Institute for Metals Research, Germany.
The discipline of defect mechanics faces new challenges when applied to micro and nano dimensions. Novel mechanical property combinations become possible when the material is confined - in at least one dimension to less than a micron. For example, thin metal films on substrates exhibit flow stresses that are at least an order of magnitude higher than those of their bulk counterpart; or the fatigue resistance increases dramatically as the thickness falls below the length scale of a persistent slip band. In this presentation an overview will be given of such mechanical effects in small-scale metallic materials. Extensive property measurements, detailed in situ transmission electron microscopy and computer simulation runs have been performed to study the behavior of crystal defects, especially dislocations and vacancies, in metal volumes confined to sub-micron dimensions. Using polycrystalline and epitaxial thin films, we have not only elucidated the strong flow stress effects, but also modelled the thermo-mechanical behavior on the basis of constrained diffusional creep processes. A new and unexpected dislocation mechanism was discovered in ultrathin polycrystalline films, where slip is initiated parallel to the film plane by diffusional processes in the grain boundaries. Newly emerging methods, e.g. in situ straining of thin films in a synchrotron beamline or microdiffraction with sub-micron resolution, will help to advance this field to even smaller dimensions. These findings are of great interest to micro and nano technology, where mechanical effects can, on the one hand, be detrimental to the systems reliability and targeted design of dimensional constraints will create new property combinations, on the other.
abstract: 3.d
Atomic-scale modelling of plastic deformation of nanocrystalline copper
JAKOB
SCHIÖTZ,
KARSTEN W.
JACOBSEN,
Center for Atomic-scale Materials Physics and Department of Physics, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark.
Using large-scale molecular dynamics, we have performed atomic-scale
simulations of nanocrystalline copper with grain sizes from 5 to 50
nm. The simulations show a clear maximum in the flow stress when the
grains are 10-15 nm in diameter. At this grain size, there is a
shift in deformation mechanism, from dislocation-mediated plasticity
at larger grain sizes to grain boundary sliding at smaller. Above the
maximum in hardness, the grain size dependence of the hardness is
consistent with the Hall-Petch relation, stating that the hardness
scales inversely with the square root of the grain size. Below the
maximum in hardness, the large density of grain boundaries prevent
dislocations from contributing significantly to plasticity, but
instead the grain boundaries themselves carry the deformation.
The Hall-Petch relation is normally explained by the creation of
dislocation pileups in the grains. It has not been clear if this
explanation of the Hall-Petch effect is valid for sub-micrometer
grains, but the simulations presented here clearly show the existence
of pileups in the simulation with average grain size of 50 nm. The
dislocation dynamics in the grains is dominated by the grain
boundaries, as almost all dislocation nucleation occurs at the grain
boundaries, which also act as efficient dislocation sinks.
Occationally, dislocations merge and form locks, which break up again
soon after. During the plastic deformation, a large number of
stacking faults and a much lower number of twin boundaries are
created. These do not contribute significantly to the flow stress, as
no work hardening is seen whereas the number of stacking faults
increase with strain.
Reference:
J. Schiötz and K. W. Jacobsen: A Maximum in the Strength of Nanocrystalline Copper, Science 301, 1357 (2003).