Energetics and structures
of grain boundaries in SiC
Marcin Wojdyr,
Izabela Szlufarska, Materials Science and Engineering
In polycrystalline materials the type and structure of
grain boundaries (GBs) are key factors that control
interface-driven processes, such as sintering, GB
diffusion and, in nano-size regime, even plastic
deformation [1].
Although selected special GBs have been already studied
by high resolution transmission electron microscopy (HRTEM)
(e.g., Ref. [2]), determination of atomistic structure
of GBs in SiC is still challenging to accomplish
experimentally. We use atomistic simulations based on
classical interatomic potentials to identify the lowest
energy GB configurations and their properties.
We study GBs in a bicrystal configuration
(Fig 1a). Our simulation box has periodic boundary
conditions in the x and y directions. The
surfaces of the bicrystal are rigid. To make
calculations more efficient, one of the surfaces can be
frozen, but the other must be able to move, to allow
relative translations of the two grains. To find the
atomic structure that has the minimum of energy for a
given misorientation of grains, we develop a global
optimization scheme that combines removing of atoms from
the GB, sampling of configuration space and simulated
annealing.
Several structures generated using our optimization
scheme have been validated against our own ab initio
calculations and against HRTEM images available in the
literature. Such comparison is only possible for
short-period GBs due to the limitations of the ab initio
techniques and due to the limited numbers of
experimental studies on the subject. Having validated
our approach, we use classical simulations to explore
more general structures of GBs in SiC [3].
Fig. 1.
(a) A bicrystal model. GB is formed by two crystalline
grains that are misoriented with respected to one
another (macroscopic degrees of freedom). (b and c) A
symmetric tilt grain boundary <001> Σ17 θ=28.1°. (b) A
fragment of the initial configuration generated for this
boundary, with some of the atoms removed. (c) The same
fragment after relaxation.
[1] I. Szlufarska,
A. Nakano, P. Vashishta, “A crossover in the mechanical
response of nanocrystalline ceramics”, Science 309,
911 (2005)
[2] K. Tanaka and
M. Kohyama, “Atomic and electronic structure analysis of
Σ= 3 incoherent twin boundaries in β-SiC”, J. Electron
Microscopy 51, 265 (2002)
[3] M. Wojdyr, S.
Khalil, Y. Liu, I. Szlufarska “Energetics and structure
of <001> tilt grain boundaries in SiC”, In preparation
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