1,877 research outputs found
The vacancy - edge dislocation interaction in fcc metals: a comparison between atomic simulations and elasticity theory
The interaction between vacancies and edge dislocations in face centered
cubic metals (Al, Au, Cu, Ni) is studied at different length scales. Using
empirical potentials and static relaxation, atomic simulations give us a
precise description of this interaction, mostly in the case when the separation
distance between both defects is small. At larger distances, elasticity theory
can be used to predict this interaction. From the comparison between both
approaches we obtain the minimal separation distance where elasticity applies
and we estimate the degree of refinement required in the calculation. In this
purpose, isotropic and anisotropic elasticity is used assuming a perfect or a
dissociated edge dislocation and considering the size effect as well as the
inhomogeneity interaction
Monte Carlo Study of the Precipitation Kinetics of Al3zr in Al-Zr
Zr precipitates in Al to form the phase Al3Zr. For a low supersaturation in
Zr of the fcc solid solution, it has been observed that during the
precipitation first steps the Al3Zr precipitates have the metastable L12
structure and that they transform themselves to the stable DO23 structure for
long enough annealing time. The aim of this study is to model the kinetics of
precipitation during this nucleation stage. We use FP-LMTO (Full-Potential
Linear-Mu n-Tin-Orbitals) calculations to fit a generalized Ising model
describing thermodynamics of the Al-Zr system. As we are interested in the
nucleation stage, the structures considered to obtain the interactions of the
Ising model are lying on a perfect fcc lattice having the lattice parameter of
Al. This allows us to stabilize the L12 structure with respect to the DO23. In
order to be able to take into account the influence of local environment on
kinetics, interactions for the tetrahedron of first nearest-neighbors are
considered, and for the pair of second nearest neighbours so as to stabilize
the L12 structure. We then generalize our description of the configurational
energy of the binary Al-Zr to the one of the ternary Al-Zr-Vacancy system by
including interactions with vacancies. Saddle point energies for the migration
of the vacancy are fitted using experimental di usion coe cients. This model is
then employed in a kinetic Monte Carlo simulation which considers the di usion
through the jumps of a vacancy. Thus we are able to study the Al3Zr kinetics of
nucleation.Comment: Proceeding of the Third International Alloy Conference, Lisbon 2002.
Published in P.E.A. Turchi, A. Gonis, K. Rajan and A. Meike (Eds.), Complex
Inorganic Solids - Structural, Stability, and Magnetic Properties of Alloys,
(Springer Verlag, New York, 2005), pp. 215-24
Classical nucleation theory in ordering alloys precipitating with L12 structure
By means of low-temperature expansions (LTEs), the nucleation free energy and
the precipitate interface free energy are expressed as functions of the
solubility limit for alloys which lead to the precipitation of a stoichiometric
L12 compound such as Al-Sc or Al-Zr alloys. Classical nucleation theory is then
used to obtain a simple expression of the nucleation rate whose validity is
demonstrated by a comparison with atomic simulations. LTEs also explain why
simple mean-field approximation like the Bragg-Williams approximation fails to
predict correct nucleation rates in such an ordering alloy
Precipitation kinetics of Al3Zr and Al3Sc in aluminum alloys modeled with cluster dynamics
Precipitation kinetics of Al3Zr and Al3Sc in aluminum supersaturated solid
solutions is studied using cluster dynamics, a mesoscopic modeling technique
which describes the various stages of homogeneous precipitation by a single set
of rate equations. The only parameters needed are the interface free energy and
the diffusion coefficient which are deduced from an atomic model previously
developed to study the same alloys. A comparison with kinetic Monte Carlo
simulations based on the vacancy diffusion mechanism shows that cluster
dynamics correctly predicts the precipitation kinetics provided a size
dependent interface free energy is used. It also manages to reproduce
reasonably well existing experimental data.Comment: Acta Mater. (2005), in pres
Hydrogen and vacancy clustering in zirconium
The effect of solute hydrogen on the stability of vacancy clusters in
hexagonal closed packed zirconium is investigated with an ab initio approach,
including contributions of H vibrations. Atomistic simulations within the
density functional theory evidence a strong binding of H to small vacancy
clusters. The hydrogen effect on large vacancy loops is modeled through its
interaction with the stacking faults. A thermodynamic modeling of H segregation
on the various faults, relying on ab initio binding energies, shows that these
faults are enriched in H, leading to a decrease of the stacking fault energies.
This is consistent with the trapping of H by vacancy loops observed
experimentally. The stronger trapping, and thus the stronger stabilization, is
obtained for vacancy loops lying in the basal planes, i.e. the loops
responsible for the breakaway growth observed under high irradiation dose.Comment: submitte
Dislocation dynamics simulations with climb: kinetics of dislocation loop coarsening controlled by bulk diffusion
Dislocation climb mobilities, assuming vacancy bulk diffusion, are derived
and implemented in dislocation dynamics simulations to study the coarsening of
vacancy prismatic loops in fcc metals. When loops cannot glide, the comparison
of the simulations with a coarsening model based on the line tension
approximation shows a good agreement. Dislocation dynamics simulations with
both glide and climb are then performed. Allowing for glide of the loops along
their prismatic cylinders leads to faster coarsening kinetics, as direct
coalescence of the loops is now possible
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