476 5 Computer Simulation of Grain Boundary Motion
The distinction between high-angle and low-angle GBs was captured in the
simulations, as evident from the GB migration activation parameters. The
GB migration activation enthalpy for the two true low-angle twist GBs, Σ41
and Σ85, was about 0.12eV. As confirmed in the simulations, the low-angle
twist GBs consist of discrete screw dislocation networks that remain stable
throughout the studied temperature range. Hence, the migration mechanism
of the low-angle twist GBs must be linked to a movement of the screw dislo-
cation network itself. By contrast, the high-angle twist GBs can move in the
low temperature regime by collective shuffle mechanisms [522, 537] which are
usually in-plane rearrangements. At elevated temperatures, an out-of-plane
component of the atomic displacement appears which might be the cause for
a change of the GB migration mechanism as reflected by the drop of the GB
mobility above 900 K.
Using the OCDF as the DF concept for the studied twist GBs yields identi-
cal results for the GB mobility [524], the activation parameters of GBM [524]
as well as for the found GBM mechanism [524].
5.4 Motion of Tilt Boundaries
While the application of an elastic strain on the simulation box worked suc-
cessfully to drive a twist boundary, it failed for tilt boundaries [524]. This is
due to the fact that the application of any stress or strain state necessary to
yield an elastic DF to a tilt boundary whose structure is composed of edge
dislocations invariably caused GB sliding in addition to GB migration [524].
This eventually will stall GB migration and, therefore, renders MD simula-
tions of GB migration unsuccessful [524]. These deficiencies can be overcome
by the use of an OCDF.
All simulations of the migration of Σ5, 53.13
◦
[001] tilt boundaries were
obtained by MD simulations utilizing the Doyama EAM potential and by ap-
plying 3D periodic boundary conditions.
The studied GB system consisted of 100 (210) planes, where initially 60
(210) planes belonged to the lower grain and 40 (210) planes to the upper
grain. Eight (002) planes were stacked in x-direction and the overall number
of atoms was 4800. The relaxed GB energy was 0.899 J/m
2
. The interatomic
distance of (210) planes was 0.2236068 a
o
.
Finite temperature simulations were performed where only the h
33
box
variable was allowed to adjust to the present stress state according to the
Rahman-Parrinello scheme. Under this constraint for the simulation box it
was found that indeed the Σ5 (210) tilt GB can move continuously in the
presence of an OCDF. Two examples of GB position vs. time profiles are
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