Atomistic simulations have become a powerful tool in materials research due to the extremely fine spatial and temporal resolution provided by such techniques. In order to understand the fundamental principles which govern material behavior at the atomic scale and directly connect to experimental works, it is necessary to quantify the microstructure of materials simulated with atomistics. Specifically, quantitative tools for identifying crystallites, their crystallographic orientation, and overall sample texture do not currently exist. Here, we develop a post-processing algorithm capable of characterizing such features, while also documenting their evolution during a simulation. In addition, the data is presented in a way that parallels the visualization methods used in traditional experimental techniques. The utility of this algorithm is illustrated by analyzing several types of simulation cells which are commonly found in the atomistic modeling literature, but could also be applied to a variety of other atomistic studies which require precise identification and tracking of microstructure.

Tracking Microstructure of Crystalline Materials: A Post-Processing Algorithm for Atomistic Simulations

Molecular dynamics simulation of polycrystal silver nanowires under tensile deformation

Phase equilibrium lines (denoted ph-eq lines) of face-centered-cubic (fcc) and
body-centered-cubic (bcc) phases, as well as saddle-point equilibrium lines (denoted sp-eq
lines) in Al and Ca are studied by first-principles total-energy calculations. For a
non-vibrating crystal of Al we determine the transition pressure
p
 t
 = 2.62 Mbar from fcc to bcc phase. The
sp-eq line lies between the two ph-eq lines, merges with the bcc-eq line at
V = 61 au3/atom (p = 1.64 Mbar) and with
the fcc-eq line at V = 42.4 au3/atom
(p = 5.50 Mbar), gives the Gibbs free energy barrier
ΔG = 0.64 mRy/atom at p
 t
 .
The bcc phase is unstable below 1.64 Mbar, while the fcc phase is unstable above 5.50
Mbar. In a non-vibrating crystal of Ca two sp-eq lines (denoted sp1-eq line and sp2-eq
line, respectively) are found corresponding to two phase transitions: one is from fcc to
bcc at p
 t1 = 89.6 kbar, the other is from bcc
to fcc at p
 t2 = 787 kbar. The sp1-eq line
merges with the bcc-eq line at V = 231 au3/atom
(p = 50 kbar) and with the fcc-eq line at
V = 183 au3/atom (p = 174 kbar), gives a
barrier of Δ
 G
 1 = 0.62 mRy/atom at
p
 t1. The sp2-eq line merges with the bcc-eq
line at V = 90 au3/atom (p = 981 kbar) and
with the fcc-eq line at V = 110 au3/atom
(p = 624 kbar), gives a barrier of
Δ
 G
 2 = 1.1 mRy/atom at
p
 t2. The bcc phase is stable in the range
from 50 kbar to 981 kbar but unstable outside this range, while the fcc phase is unstable
in the range from 174 to 624 kbar but stable outside this range. This work confirms all
the features of the sp-eq line described in our recent work [S.L. Qiu, P.M. Marcus, J.
Phys.: Condens. Matter 24, 225501 (2012)] and finds two additional features:
(1) there are two sp-eq lines corresponding to the two phase
transitions between fcc and bcc phases in Ca; (2) fcc phase of Ca
is unstable between the two merge points on the fcc-eq line but stable beyond them, while
bcc phase of Ca is stable between the two merge points on the bcc-eq line but unstable
beyond them.

Saddle-point equilibrium lines between fcc and bcc phases in Al and Ca from first principles

Although the catalytic properties of Au-Pt alloys have received widespread attention, there are a few of relevant studies on their microscopic deformation process. Considering that Au-Pt alloys are noble metals, and the costs of experiments are extremely high. Direct research experiments are extremely expensive. So the molecular dynamics method which can directly observe the evolution process of microstructure and analyze the mechanical behavior of each deformation stage of the material was adopted to explore the underlined relationship between the temperature and the compression deformation mechanism. The results show that during the process of compression deformation, lots of atoms in the crystal are converted into boundary atoms, the dislocation multiplication and the grain fragmentation occur. In different temperature intervals, along with the compression deformation, low temperature, medium temperature and high temperature recovery appear. Due to the influence of the dual mechanism of deformation and recovery, the deformation mechanism is different at different temperatures, at low temperature stage is mainly the dislocation multiplication and the grain fragmentation, at middle temperature stage is the combination of the opposite sign dislocation, and at high temperature stage is mainly the merger of the grain. 

Study on the Microscopic Compression Deformation Mechanism of Nanocrystalline Single-phase Gold Platinum Alloy

Atomistic simulations have become a powerful tool in materials research due to the extremely fine spatial and temporal resolution provided by such techniques. To understand the fundamental principles that govern material behavior at the atomic scale and directly connect to experimental works, it is necessary to quantify the microstructure of materials simulated with atomistics. Specifically, quantitative tools for identifying crystallites, their crystallographic orientation, and overall sample texture do not currently exist. Here, we develop a post-processing algorithm capable of characterizing such features, while also documenting their evolution during a simulation. In addition, the data is presented in a way that parallels the visualization methods used in traditional experimental techniques. The utility of this algorithm is illustrated by analyzing several types of simulation cells that are commonly found in the atomistic modeling literature but could also be applied to a variety of other atomistic studies that require precise identification and tracking of microstructure.

By molecular dynamics simulations employing an embedded atom model potential, we investigate the fcc-to-bcc phase transition in single crystal Al, caused by uniform compression. Results show that the fcc structure is unstable when the pressure is over 250 GPa, in reasonable agreement with the calculated value through the density functional theory. The morphology evolution of the structural transition and the corresponding transition mechanism are analysed in detail. The bcc (011) planes are transited from the fcc (111) plane and the (111) plane. We suggest that the transition mechanism consists mainly of compression, shear, slid and rotation of the lattice. In addition, our radial distribution function analysis explicitly indicates the phase transition of Al from fcc phase to bcc structure.

https://iopscience.iop.org/article/10.1088/1674-1056/21/2/026402/pdf

Atomistic simulation of fcc—bcc phase transition in single crystal Al under uniform compression

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Atomistic simulation of fcc—bcc phase transition in single crystal Al under uniform compression