**Molecular dynamics simulation of migration behavior of FCC-BCC atomic
terrace-step phase boundary in iron-based alloy**

ZZ Wei and X Ma and CB Ke and XP Zhang, ACTA PHYSICA SINICA, 69, 136102 (2020).

DOI: 10.7498/aps.69.20191903

The martensitic transformation between the high-temperature face-
centered cubic (FCC) phase and the low-temperature body-centered cubic
(BCC) phase in iron-based alloys has been studied for years, which plays
a critical role in controlling microstructures and hence properties of
the alloys. Generally, the BCC structure martensitic phase forms from
the FCC parent phase, involving a collective motions of atoms over a
distance less than the interatomic distance in the vicinity of the
interphase boundary. Thus the structure of interphase boundary
separating the FCC and BCC phases is the key characteristics to
quantitatively understanding the mechanism and kinetics of martensitic
transformation. Due to the difficulty in observing the atomic motions
taking place at a velocity as high as the speed of sound, the
experimental investigation on the migration of FCC/BCC interphase
boundary during the transformation is as yet limited. Noteworthily,
molecular dynamics (MD) simulation has been applied to studying the
martensitic transformation, in particular for investigating the mobility
of the FCC/BCC interphase boundary in iron. However, in most of the MD
studies the atomistically planar interfaces of *111*(FCC) // *110*(BCC)
are considered as the initial configuration of the interphase boundary
between FCC and BCC phases, which is in contradiction to the high-
resolution TEM observations. In fact, the FCC/BCC interphase boundary,
which is known as the macroscopic habit plane, is a semi-coherent
interface consisting of several steps and terrace planes on an atomic
scale. In the present work, the atomic configuration of a terrace-step
FCC/BCC interphase boundary of iron is built in terms of the topological
model. The MD simulation is conducted to clarify the mechanism of
interphase boundary migration in the FCC-to-BCC transformation. The
results show that the FCC/BCC boundary migrates along its normal at the
expense of FCC phase as a result of the lateral motions of the
transformation dislocations. Meanwhile, the interphase boundary
maintains the stable terrace-step structure during the transformation.
Further examinations reveal that the transformation dislocations move
steadily at a velocity as high as (2.8 +/- 0.2) x 10(3) m/s, affecting
the migration of the interphase boundary with a constant velocity of
about (4.4 +/- 0.3) x 10(2) m/s. The effective migration velocity of
FCC/BCC interface exhibits dynamic properties consistent with the
characteristic features commonly observed in a displacive martensitic
transformation. Additionally, the motion of transformation dislocations
gives rise to the macroscopic shape strain composed of a shear component
Gamma(yz) = 0.349 parallel to the boundary and a dilatation Gamma(zz) =
0.053 normal to the boundary in the MD simulation, which is close to the
crystallographic calculations by the topological model.

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