Srujan Rokkam

Phase field modeling of void nucleation and growth in irradiated metals

Motivated by the need to develop a spatially resolved theory of irradiation-induced microstructure evolution in metals, we present a phase field model for void formation in metals with vacancy concentrations exceeding the thermal equilibrium values. This model, which is phenomenological in nature, is cast in the form of coupled Cahn–Hilliard and Allen–Cahn type equations governing the dynamics of the vacancy concentration field and the void microstructure in the matrix, respectively. The model allows for a unified treatment of void nucleation and growth under the condition of random generation of vacancies, which is similar to vacancy generation by collision cascade in irradiated materials. The basic features of the model are illustrated using two-dimensional solutions for the cases of void growth and shrinkage in supersaturated and undersaturated vacancy fields, void–void interactions, as well as the spontaneous nucleation and growth of a large population of voids.

Void nucleation and growth in irradiated polycrystalline metals: a phase-field model

We present a phase-field model for void formation in polycrystalline metals with vacancy concentrations exceeding the thermal equilibrium values. By incorporating a coupled set of Cahn–Hilliard and Allen–Cahn equations, the model captures several relevant processes including vacancy annihilation and nucleation at grain boundaries (GBs), vacancy diffusion toward sinks (including GBs and void surfaces) as well as void nucleation and growth due to vacancy supersaturations occurring in the grain interiors. Illustrative results are presented that characterize the rate of annihilation of the vacancy population at the GB sinks, as well as the formation of void denuded zones adjacent to GBs in bicrystalline and polycrystalline samples, the width of which is found to depend on both the vacancy diffusivity and the vacancy production rate.

Phase-field simulation of thermal conductivity in porous polycrystalline microstructures

Mesoscale computer simulations are used to study the effective thermal conductivity of two-dimensional polycrystalline model microstructures containing finely dispersed stationary voids. The microstructural evolution is captured by phase-field modeling in which the competing mechanisms of curvature-driven grain-boundary (GB) migration and Zener pinning due to the void/grain-boundary interactions control the grain-growth kinetics. We investigate porosity fractions between 0% and 8% by systematically increasing the number of voids in the simulation cell. The temperature distribution throughout the microstructure at progressive instances in time is calculated by solving the solid-state heat-conduction equation. The thermal conductivity of each grid point is assigned a value according to the microstructural feature it represents (grain interiors, GBs, and voids) as determined by the phase-field order parameters. The effective conductivities of the microstructures are analyzed with respect to average grain siz...

Modeling high-temperature diffusion of gases in micro and mesoporous amorphous carbon

In this work, we study diffusion of gases in porous amorphous carbon at high temperatures using equilibrium molecular dynamics simulations. Microporous and mesoporous carbon structures are computationally generated using liquid quench method and reactive force fields. Motivated by the need to understand high temperature diffusivity of light weight gases like H2, O2, H2O, and CO in amorphous carbon, we investigate the diffusion behavior as function of two important parameters: (a) the pore size and (b) the concentration of diffusing gases. The effect of pore size on diffusion is studied by employing multiple realizations of the amorphous carbon structures in microporous and mesoporous regimes, corresponding to densities of 1 g/cm(3) and 0.5 g/cm(3), respectively. A detailed analysis of the effect of gas concentration on diffusion in the context of these two porosity regimes is presented. For the microporous structure, we observe that predominantly, a high diffusivity results when the structure is highly anisotropic and contains wide channels between the pores. On the other hand, when the structure is highly homogeneous, significant molecule-wall scattering leads to a nearly concentration-independent behavior of diffusion (reminiscent of Knudsen diffusion). The mesoporous regime is similar in behavior to the highly diffusive microporous carbon case in that diffusion at high concentration is governed by gas-gas collisions (reminiscent of Fickian diffusion), which transitions to a Knudsen-like diffusion at lower concentration.

ACCELERATED MOLECULAR DYNAMICS METHODS FOR MODELING CHEMICALLY REACTIVE SYSTEMS

Accelerated molecular dynamics (AMD) methods refers to a class of techniques which enable molecular level simulation of nanoscale phenomena for long timescales (of nanoseconds to a few microseconds), by making use of novel statistical principles and high-performance computing algorithms. Typical molecular dynamics (MD) simulation makes use of particle tracking algorithms and integrates stiff Newton’s laws of motion using a small timestep (femtoseconds). Traditionally, MD simulations have been applied to investigate processes that take place on very short timescale (picoseconds to a nanosecond) due to this limitation. However, several thermally activated processes at nanoscale require molecular simulations to longer timescales – a regime which is impractical to achieve via MD. In physical systems that involve chemical reactions, for example, fuel combustion, pyrolysis and polymer degradation, MD simulations for long timescale phenomenon is even more prohibitive. In this work, we discuss a recently developed computational framework which uses AMD principles in conjunction with reactive force fields to simulate chemical reacting systems for longer time. The framework is applicable to simulate combustion chemistry, biochemistry, and material degradation phenomenon at nanoscale. We discuss the developed methodology, algorithmic aspects and application of the resulting toolkit to a sample problem of polymer degradation in ablative heatshield materials.