arXiv subjects

Timofey Frolov

Publications and source records attributed to Timofey Frolov.

26 records · Page 2Linked to original sources

arXiv2017

Predicting phase behavior of grain boundaries with evolutionary search and machine learning

The study of grain boundary phase transitions is an emerging field until recently dominated by experiments. The major bottleneck in exploration of this phenomenon with atomistic modeling has been the lack of a robust computational tool that can predict interface structure. Here we develop a new computational tool based on evolutionary algorithms that performs efficient grand-canonical grain boundary structure search and we design a clustering analysis that automatically identifies different grain boundary phases. Its application to a model system of symmetric tilt boundaries in Cu uncovers an unexpected rich polymorphism in the grain boundary structures. We find new ground and metastable states by exploring structures with different atomic densities. Our results demonstrate that the grain boundaries within the entire misorientation range have multiple phases and exhibit structural transitions, suggesting that phase behavior of interfaces is likely a general phenomenon.

cond-mat.mtrl-sci↗

arXiv2017

Step free energies at faceted solid surfaces: Theory and atomistic calculations for steps on the Cu(111) surface

A theory for the thermodynamic properties of steps on faceted crystalline surfaces is presented. The formalism leads to the definition of step excess quantities, including an excess step stress that is the step analogy of surface stress. The approach is used to develop a relationship between the temperature dependence of the step free energy ($γ^\mathrm{st}$) and step excess quantities for energy and stress that can be readily calculated by atomistic simulations. We demonstrate the application of this formalism in thermodynamic-integration (TI) calculations of the step free energy, based on molecular-dynamics simulations, considering $\left< 110 \right>$ steps on the $\{111\}$ surface of a classical potential model for elemental Cu. In this application we employ the Frenkel-Ladd approach to compute the reference value of $γ^\mathrm{st}$ for the TI calculations. Calculated results for excess energy and stress show relatively weak temperature dependencies up to a homologous temperature of approximately 0.6, above which these quantities increase strongly and the step stress becomes more isotropic. From the calculated excess quantities we compute $γ^\mathrm{st}$ over the temperature range from zero up to the melting point ($T_\mathrm{m}$). We find that $γ^\mathrm{st}$ remains finite up to $T_\mathrm{m}$, indicating the absence of a roughening temperature for this $\{111\}$ surface facet, but decreases by roughly fifty percent from the zero-temperature value. The strongest temperature dependence occurs above homologous temperatures of approximately 0.6, where the step becomes configurationally disordered due to the formation of point defects and appreciable capillary fluctuations.

cond-mat.mtrl-sci↗

arXiv2015

Segregation-induced phase transformations in grain boundaries

Phase transformations in metallic grain boundaries (GBs) present significant fundamental interest in the context of thermodynamics of low-dimensional physical systems. We report on atomistic computer simulations of the Cu-Ag system that provide direct evidence that GB phase transformations in a single-component GB can continue to exist in a binary alloy. This gives rise to segregation-induced phase transformations with varying chemical composition at a fixed temperature. Furthermore, for such transformations we propose an approach to calculations of free energy differences between different GB phases by thermodynamic integration along a segregation isotherm. This opens the possibility of developing quantitative thermodynamics of GB phases, their transformations to each other, and critical phenomena in the future.

cond-mat.mtrl-sci↗

arXiv2015

Phases, phase equilibria and phase rules in low-dimensional systems

We present a unified approach to thermodynamic description of one, two and three dimensional phases and phase transformations among them. The approach is based on a rigorous definition of a phase applicable to thermodynamic systems of any dimensionality. Within this approach, the same thermodynamic formalism can be applied for the description of phase transformations in bulk systems, interfaces, and line defects separating interface phases. For both lines and interfaces, we rigorously derive an adsorption equation, the phase coexistence equations, and other thermodynamic relations expressed in terms of generalized line and interface excess quantities. As a generalization of the Gibbs phase rule for bulk phases, we derive phase rules for lines and interfaces and predict the maximum number of phases than may coexist in systems of the respective dimensionality.

cond-mat.mtrl-sci↗

arXiv2014

Effect of Interfacial Structural Phase Transitions on the Coupled Motion of Grain Boundaries: A Molecular Dynamics Study

In this work the coupled motion of two different phases of Σ5(210)[001] grain boundaries were investigated by molecular dynamics simulations of fcc Cu. The effect of interfacial structural phase transitions is shown to have a profound effect on both the shear strength and the nature of the coupled motion. Specifically, the motion of the two different phases is described by ideal coupling factors β<100> and β<110> that have different magnitudes and even signs. Additionally, the shear strength for the two inter- facial phases is observed to differ by up to 40 % at the lowest temperatures simulated. The study demonstrates that grain boundary phases transitions may have strong effects on the kinetics of microstructural evolution.

cond-mat.mtrl-sci↗

arXiv2013

Structural phase transformations in metallic grain boundaries

Structural transformations at interfaces are of profound fundamental interest as complex examples of phase transitions in low-dimensional systems. Despite decades of extensive research, no compelling evidence exists for structural transformations in high-angle grain boundaries in elemental systems. Here we show that the critical impediment to observations of such phase transformations in atomistic modeling has been rooted in inadequate simulation methodology. The proposed new methodology allows variations in atomic density inside the grain boundary and reveals multiple grain boundary phases with different atomic structures. Reversible first-order transformations between such phases are observed by varying temperature or injecting point defects into the boundary region. Due to the presence of multiple metastable phases, grain boundaries can absorb significant amounts of point defects created inside the material by processes such as irradiation. We propose a novel mechanism of radiation damage healing in metals which may guide further improvements in radiation resistance of metallic materials through grain boundary engineering.

cond-mat.mtrl-sci↗

arXiv2013

Molecular dynamics modeling of self-diffusion along a triple junction

We propose a computational procedure for creating a stable equilibrium triple junction (TJ) with controlled grain misorientations. We apply this procedure to construct a TJ between a Σ5(210) grain boundary (GB) and two general high-angle GBs in copper, and calculate the diffusion coefficients along the TJ and the GBs using molecular dynamics with an embedded-atom potential. The TJ diffusion is only a factor of two faster than diffusion in the Σ5 GB but significantly faster than diffusion in the general GBs. Both the GBs and the TJ studied here show a premelting behavior near the bulk melting point, where their diffusivities converge to the diffusivity of bulk liquid. Although our results are consistent with the common assumption that TJ diffusion is generally faster than GB diffusion, the difference between the two diffusivities does not appear to be large enough to ensure a significant contribution of TJs to diffusional creep in polycrystals at high temperatures.

cond-mat.mtrl-sci↗

arXiv2012

Step Free Energies at Faceted Solid-Liquid Interfaces from Equilibrium Molecular Dynamics Simulations

In this work a method is proposed for computing step free energies for faceted solid-liquid interfaces based on atomistic simulations. The method is demonstrated in an application to (111) interfaces in elemental Si, modeled with the classical Stillinger-Weber potential. The approach makes use of an adiabatic trapping procedure, and involves simulations of systems with coexisting solid and liquid phases separated by faceted interfaces containing islands with different sizes, for which the corresponding equilibrium temperatures are computed. We demonstrate that the calculated coexistence temperature is strongly affected by the geometry of the interface. We find that island radius is inversely proportional to superheating, allowing us to compute the step free energy by fitting simulation data within the formalism of classical nucleation theory. The approach outlined in this work paves the way to the calculation of step free energies relevant to the solidification of faceted crystals from liquid mixtures, as encountered in nanowire growth by the vapor-liquid-solid mechanism and in alloy casting. The present work also shows that at low undercoolings the Stillinger-Weber interatomic potential for Si tends to crystallize in the wurtzite, rather than the diamond-cubic structure.

cond-mat.mtrl-sci↗