27 research outputs found
Molecular dynamics simulation of phase competition in terbium
The competition among multiple solid phases determines the final microstructures of a material. Such competition can originate at the very beginning of the solidification process. We report the results of molecular dynamics simulation of the phase competition between the hexagonal close-packed (hcp), face-centered cubic (fcc), and body-centered cubic (bcc) phases during the solidification of pure Tb. We found that the liquid supercooled below the hcp melting temperature has both bcc and hcp/fcc nuclei, but only the bcc nuclei grow such that the liquid always solidifies into the bcc phase, even at temperatures where the hcp phase is more stable. The hcp phase can only form in the last liquid droplet or at the bcc grain boundaries. Depending on the bcc grain orientations, the hcp phase jammed between the bcc grains either completely disappears or slowly grows via a solid-state massive transformation mechanism. Once the hcp phase becomes large enough, the stresses associated with its appearance can trigger a martensitic transformation. Yet, not the entire bcc phase is consumed by the martensitic transformation and the remaining bcc phase is transformed into the hcp phase via the solid-state massive transformation mechanism. Finally, if the supercooling is too large, the nucleation becomes almost barrier free and the liquid solidifies into a structure consisting of ultra-fine hcp and bcc grains after which the bcc phase quickly disappears
Molecular Dynamics Study of Self-Diffusion in Zr
We employed a recently developed semi-empirical Zr potential to determine the
diffusivities in the hcp and bcc Zr via molecular dynamics simulation. The
point defect concentration was determined directly from MD simulation rather
than from theoretical methods using T=0 calculations. We found that the
diffusion proceeds via the interstitial mechanism in the hcp Zr and both the
vacancy and interstitial mechanisms give contribution in diffusivity in the bcc
Zr. The agreement with the experimental data is excellent for the hcp Zr and
for the bcc Zr it is rather good at high temperatures but there is a
considerable disagreement at low temperatures
Asperity contacts at the nanoscale: comparison of Ru and Au
We develop and validate an interatomic potential for ruthenium based on the
embedded atom method framework with the Finnis/Sinclair representation. We
confirm that the new potential yields a stable hcp lattice with reasonable
lattice and elastic constants and surface and stacking fault energies. We
employ molecular dynamics simulations to bring two surfaces together; one flat
and the other with a single asperity. We compare the process of asperity
contact formation and breaking in Au and Ru, two materials currently in use in
micro electro mechanical system switches. While Au is very ductile at 150 and
300 K, Ru shows considerably less plasticity at 300 and 600 K (approximately
the same homologous temperature). In Au, the asperity necks down to a single
atom thick bridge at separation. While similar necking occurs in Ru at 600 K,
it is much more limited than in Au. On the other hand, at 300 K, Ru breaks by a
much more brittle process of fracture/decohesion with limited plastic
deformation.Comment: 10 pages, 13 figure
Effect of Samarium doping on the nucleation of fcc-Aluminum in undercooled liquids
The effect of Sm doping on the fcc-Al nucleation was investigated in Al-Sm
liquids with low Sm concentrations (xSm) with molecular dynamics simulations.
The nucleation in the moderately undercooled liquid is achieved by the recently
developed persistent-embryo method. Systematically computing the nucleation
rate with different xSm (xSm=0%, 1%, 2%, 3%, 5%) at 700 K, we found Sm dopant
reduces the nucleation rate by up to 25 orders of magnitudes with only 5%
doping concentration. This effect is mostly associated with the increase in the
free energy barrier with a minor contribution from suppression of the
attachment to the nucleus caused by Sm doping.Comment: 4 figure
Development of a semi-empirical potential for simulation of Ni solute segregation into grain boundaries in Ag
An Ag–Ni semi-empirical potential was developed to simulate the segregation of Ni solutes at Ag grain boundaries (GBs). The potential combines a new Ag potential fitted to correctly reproduce the stable and unstable stacking fault energies in this metal and the existing Ni potential from Mendelev et al (2012 Phil. Mag. 92 4454–69). The Ag–Ni cross potential functions were fitted to ab initio data on the liquid structure of the Ag80Ni20 alloy to properly incorporate the Ag–Ni interaction at small atomic separations, and to the Ni segregation energies at different sites within a high-energy Σ9 (221) symmetric tilt GB. By deploying this potential with hybrid Monte Carlo/molecular dynamics simulations, it was found that heterogeneous segregation and clustering of Ni atoms at GBs and twin boundary defects occur at low Ni concentrations, 1 and 2 at%. This behavior is profoundly different from the homogeneous interfacial dispersion generally observed for the Cu segregation in Ag. A GB transformation to amorphous intergranular films was found to prevail at higher Ni concentrations (10 at%). The developed potential opens new opportunities for studying the selective segregation behavior of Ni solutes in interface-hardened Ag metals and its effect on plasticity
Competitive B2 and B33 Nucleation during Solidification of Ni50Zr50 Alloy: Molecular Dynamics Simulation and Classical Nucleation Theory
We investigated the homogenous nucleation of the stoichiometric B2 and B33
phases in the Ni50Zr50 alloy using the persistent embryo method and the
classical nucleation theory. The two phases become very close competitors at
large supercoolings, which is consistent with the experimental observations. In
the case of the B2 phase, the linear temperature dependence of the solid-liquid
interface (SLI) free energy extrapolated to the melting temperature leads to
the same value as the one obtained from the capillarity fluctuation method
(CFM). In the case of the B33 phases, the SLI free energy is also a linear
function of temperature at large supercoolings but the extrapolation to the
melting temperature leads to a value which is considerably different from the
CFM value. This is consistent with the large anisotropy of the SLI properties
of the B33 phase nearby the melting temperature observed in the simulation of
the nominally flat interface migration
Temperature dependence of the solid-liquid interface free energy of Ni and Al from molecular dynamics simulation of nucleation
The temperature dependence of the solid-liquid interfacial free energy,
{\gamma}, is investigated for Al and Ni at the undercooled temperature regime
based on a recently developed persistent-embryo method. The atomistic
description of the nucleus shape is obtained from molecular dynamics
simulations. The computed {\gamma} shows a linear dependence on the
temperature. The values of {\gamma} extrapolated to the melting temperature
agree well with previous data obtained by the capillary fluctuation method.
Using the temperature dependence of {\gamma}, we estimate the nucleation free
energy barrier in a wide temperature range from the classical nucleation
theory. The obtained data agree very well with the results from the brute-force
molecular dynamics simulations