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Figure 1.
The above pictures were obtained from a dislocation dynamics simulation
of plastic flow in a Mo model system using the newly developed parallel
ParaDiS code. The simulation was done on the
ASC White and Thunder
supercomputers. The simulation has reached more than a couple of percent
in strain. It has been instrumental in the discovery by the ParaDiS team
of a new type of
dislocation microstructure: the dislocation multi-junction. The picture
on the right shows a multi-junction formed by 4 dislocation lines with
different Burgers vectors (represented by red, orange, yellow, blue
respectively). Further studies show that these multi-junctions are unlike
the conventional junctions formed by two dislocation line interactions,
and they present much larger resistance to plastic flow. The
multi-junctions have recently been confirmed by TEM observations in
lab experiments by Hsiung (LLNL). Massive dislocation dynamics simulations also show
that the multi-junctions can cause ultra strain hardening for deformation
in certain crystallographic directions in BCC metals.
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Dislocation Dynamics for Single Crystal Plasticity of BCC Metals
Meijie Tang
Methods: Dislocation Dynamics
Collaborators: V. Bulatov, G. Hommes, M. Hiratani, A. Arsenlis, M. Rhee, (LLNL); W. Cai (Stanford), M. Fivel (INP-Grenoble) and G. Xu (UC-Riverside)
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The single crystal plastic behavior of bcc metals (Tantalum as a prototype) has
been simulated using our dislocation dynamics (DD) code. One of the key input
to the DD simulation is the dislocation mobilities. For bcc metals, the screw
dislocations are known to control the plastic deformation process. They move by
a thermally activated kink pair mechanism and the activation enthalpy dominates
the mobility of the screw dislocations. The activation enthalpy has been
empirically fitted to experimental data for Ta using the Kocks-Argon-Ashby
model. It has also been calculated by atomistic calculations using MGPT
potentials, in particular at high pressures. The kink pair mechanism affects
the yield properties as well as the strain hardening properties. The strain
hardening is simulated by the dislocation junction mechanism. An averaged
junction strength parameter has been obtained from experiments. The strain
hardening behavior of bcc metals with kink pair mechanism is found to be
different from that of fcc metals. It is shown that the forest hardening in bcc
is made of two contributions, a free-length effect that logarithmically depends
on the length of the mobile screw segments and a line tension effect linearly
proportional to the dislocation obstalce density. Both terms depend on strain
rate and temperature strongly. One main thrust of the DD simulations for bcc
metals is to link with single dislocation properties calculated at the
atomistic level such as the Peierls stress and activation enthalpy, in
particular for the study of plastic deformation at high pressures and other
extreme conditions. In addition, the DD simulation code has been coupled with
a finite element method to account for boundary conditions more rigorously. It
allows one to deal with surface effects and inhomogeneous loadings. The application
of the coupled code is mainly for thin films or other structures at smaller
length scales.
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SELECTED PUBLICATIONS
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- M. Tang, L. P. Kubin, G. R. Canova,
"Dislocation mobility and the mechanical response of bcc (Ta) single crystals: a mesoscopic approach",
Acta Mater. 46, 3221 (1998).
- M. Tang, B. Devincre, L. P. Kubin,
"Simulation and modeling of forest hardening in BCC crystals at low temperatures",
Modeling Simul. Mater. Sci. Engi. 7, 893 (1999).
- M. Tang, M. Fivel, L. P. Kubin,
"From forest hardening to strain hardening in body centered cubic single crystals: simulation and modeling",
Materials Science and Engineering A 309-310, 256 (2001).
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Metals & Alloys
| Condensed Matter Physics
| Physics & Adv. Tech.
| LLNL
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Maintained by Robert E. Rudd -- Last updated on 2 May 2005.
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