4,362 research outputs found
Large scale ab-initio simulations of dislocations
We present a novel methodology to compute relaxed dislocations core configurations, and their energies in crystalline metallic materials using large-scale ab-intio simulations. The approach is based on MacroDFT, a coarse-grained density functional theory method that accurately computes the electronic structure with sub-linear scaling resulting in a tremendous reduction in cost. Due to its implementation in real-space, MacroDFT has the ability to harness petascale resources to study materials and alloys through accurate ab-initio calculations. Thus, the proposed methodology can be used to investigate dislocation cores and other defects where long range elastic effects play an important role, such as in dislocation cores, grain boundaries and near precipitates in crystalline materials. We demonstrate the method by computing the relaxed dislocation cores in prismatic dislocation loops and dislocation segments in magnesium (Mg). We also study the interaction energy with a line of Aluminum (Al) solutes. Our simulations elucidate the essential coupling between the quantum mechanical aspects of the dislocation core and the long range elastic fields that they generate. In particular, our quantum mechanical simulations are able to describe the logarithmic divergence of the energy in the far field as is known from classical elastic theory. In order to reach such scaling, the number of atoms in the simulation cell has to be exceedingly large, and cannot be achieved with the state-of-the-art density functional theory implementations
Generalized stacking fault energetics and dislocation properties: compact vs. spread unit dislocation structures in TiAl and CuAu
We present a general scheme for analyzing the structure and mobility of
dislocations based on solutions of the Peierls-Nabarro model with a two
component displacement field and restoring forces determined from the ab-initio
generalized stacking fault energetics (ie., the so-called -surface).
The approach is used to investigate dislocations in L1 TiAl and CuAu;
predicted differences in the unit dislocation properties are explicitly related
with features of the -surface geometry. A unified description of
compact, spread and split dislocation cores is provided with an important
characteristic "dissociation path" revealed by this highly tractable scheme.Comment: 7 two columns pages, 2 eps figures. Phys. Rev. B. accepted November
199
Hydrogen-enhanced local plasticity in aluminum: an ab initio study
Dislocation core properties of Al with and without H impurities are studied
using the Peierls-Nabarro model with parameters determined by ab initio
calculations. We find that H not only facilitates dislocation emission from the
crack tip but also enhances dislocation mobility dramatically, leading to
macroscopically softening and thinning of the material ahead of the crack tip.
We observe strong binding between H and dislocation cores, with the binding
energy depending on dislocation character. This dependence can directly affect
the mechanical properties of Al by inhibiting dislocation cross-slip and
developing slip planarity.Comment: 4 pages, 3 figure
A nonplanar Peierls-Nabarro model and its applications to dislocation cross-slip
A novel semidiscrete Peierls-Nabarro model is introduced which can be used to
study dislocation spreading at more than one slip planes, such as dislocation
cross-slip and junctions. The strength of the model, when combined with ab
initio calculations for the energetics, is that it produces essentiallyan
atomistic simulation for dislocation core properties without suffering from the
uncertainties associated with empirical potentials. Therefore, this method is
particularly useful in providing insight into alloy design when empirical
potentials are not available or not reliable for such multi-element systems. As
an example, we study dislocation cross-slip and constriction process in two
contrasting fcc metals, Al and Ag. We find that the screw dislocation in Al can
cross-slip spontaneously in contrast with that in Ag, where the screw
dislocation splits into two partials, which cannot cross-slip without first
being constricted. The response of the dislocation to an external stress is
examined in detail. The dislocation constriction energy and the critical stress
for cross-slip are determined, and from the latter, we estimate the cross-slip
energy barrier for straight screw dislocations.Comment: Submitted for the Proceedings of Multiscale Modelling of Materials
(London, 2002
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