**Phase-field-crystal study of grain boundary premelting and shearing in
bcc iron**

A Adland and A Karma and R Spatschek and D Buta and M Asta, PHYSICAL REVIEW B, 87, 024110 (2013).

DOI: 10.1103/PhysRevB.87.024110

We use the phase-field-crystal (PFC) method to investigate the
equilibrium premelting and nonequilibrium shearing behaviors of **001**
symmetric tilt grain boundaries (GBs) at high homologous temperature
over the complete range of misorientation 0 <. theta < 90 degrees in
classical models of bcc Fe. We characterize the dependence of the
premelted layer width W as a function of temperature and misorientation.
In addition, we compute the thermodynamic disjoining potential whose
derivative with respect to W represents the structural force between
crystal-melt interfaces due to the spatial overlap of density waves. The
disjoining potential is also computed by molecular dynamics (MD)
simulations, for quantitative comparison with PFC simulations, and
coarse-grained amplitude equations (AE) derived from PFC that provide
additional analytical insights. We find that, for GBs over an
intermediate range of misorientation (theta(min) < theta < theta(max)),
W diverges as the melting temperature is approached from below,
corresponding to a purely repulsive disjoining potential, while for GBs
outside this range (theta < theta(min) or theta(max) < theta < 90
degrees) W remains finite at the melting point. In the latter case, W
corresponds to a shallow attractive minimum of the disjoining potential.
The misorientation range where W diverges predicted by PFC simulations
is much smaller than the range predicted by MD simulations when the
small dimensionless parameter epsilon of the PFC model is matched to
liquid structure factor properties. However, it agrees well with MD
simulations with a lower epsilon value chosen to match the ratio of bulk
modulus and solid-liquid interfacial free energy, consistent with the
amplitude-equation prediction that theta(min) and 90 degrees -
theta(max) scale as similar to epsilon(1/2). The incorporation of
thermal fluctuations in PFC is found to have a negligible effect on this
range. In response to a shear stress parallel to the GB plane, GBs in
PFC simulations exhibit coupled motion normal to this plane or sliding.
Furthermore, the coupling factor exhibits a discontinuous change as a
function of. that reflects a transition between two coupling modes.
Sliding is only observed over a range of misorientation that is a
strongly increasing function of temperature for T/T-M >= 0.8 and matches
roughly the range where W diverges at the melting point. The coupling
factor for the two coupling modes is in excellent quantitative agreement
with previous theoretical predictions **Cahn, Mishin, and Suzuki, Acta
Mater. 54, 4953 (2006)**. DOI: 10.1103/PhysRevB.87.024110

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