165 research outputs found
Ginzburg-Landau theory of crystalline anisotropy for bcc-liquid interfaces
The weak anisotropy of the interfacial free-energy is a crucial
parameter influencing dendritic crystal growth morphologies in systems with
atomically rough solid-liquid interfaces. The physical origin and quantitative
prediction of this anisotropy are investigated for body-centered-cubic (bcc)
forming systems using a Ginzburg-Landau theory where the order parameters are
the amplitudes of density waves corresponding to principal reciprocal lattice
vectors. We find that this theory predicts the correct sign,
, and magnitude, , of this anisotropy in good agreement
with the results of MD simulations for Fe. The results show that the
directional dependence of the rate of spatial decay of solid density waves into
the liquid, imposed by the crystal structure, is a main determinant of
anisotropy. This directional dependence is validated by MD computations of
density wave profiles for different reciprocal lattice vectors for
crystal faces. Our results are contrasted with the prediction of the reverse
ordering from an earlier formulation of
Ginzburg-Landau theory [Shih \emph{et al.}, Phys. Rev. A {\bf 35}, 2611
(1987)].Comment: 9 pages, 5 figure
Modeling metallic island coalescence stress via adhesive contact between surfaces
Tensile stress generation associated with island coalescence is almost
universally observed in thin films that grow via the Volmer-Weber mode. The
commonly accepted mechanism for the origin of this tensile stress is a process
driven by the reduction in surface energy at the expense of the strain energy
associated with the deformation of coalescing islands during grain boundary
formation. In the present work, we have performed molecular statics
calculations using an embedded atom interatomic potential to obtain a
functional form of the interfacial energy vs distance between two closely
spaced free surfaces. The sum of interfacial energy plus strain energy provides
a measure of the total system energy as a function of island separation.
Depending on the initial separation between islands, we find that in cases
where coalescence is thermodynamically favored, gap closure can occur either
spontaneously or be kinetically limited due to an energetic barrier. Atomistic
simulations of island coalescence using conjugate gradient energy minimization
calculations agree well with the predicted stress as a function of island size
from our model of spontaneous coalescence. Molecular dynamics simulations of
island coalescence demonstrate that only modest barriers to coalescence can be
overcome at room temperature. A comparison with thermally activated coalescence
results at room temperature reveals that existing coalescence models
significantly overestimate the magnitude of the stress resulting from island
coalescence.Comment: 20 pages, 8 figures, 2 tables, submitted to PR
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