The kinetic interplay between helium (He) segregation, He cluster formation and curvature-driven grain boundary migration in bcc iron (alpha-Fe) has been investigated using molecular dynamics simulations. He atoms that segregate to the migrating boundary are found to be trapped in vacant substitutional sites emitted by the migrating boundary. He atoms that form clusters in the bulk restrain the boundary migration via a pinning mechanism. The pinning pressure of He clusters is proportional to the density and the squared radius of each cluster. A cluster pinning model has been developed by taking into account the two-fold effect of clusters on the boundary migration: (1) reducing the boundary mobility and (2) acting as pinning objects that delay or even completely halt the boundary migration. The model is found to be in agreement with the simulation results.
Molecular dynamics simulations were performed to investigate the migration of curved and planar boundaries. The reduced mobilities of capillarity driven U-shaped half-loop twin boundaries in b.c.c iron (Fe) were computed between 800 and 1200 K. To rationalize these results, simulations were also performed for planar twin boundaries of different inclination to determine their absolute mobilities and grain boundary energies. The variation of these properties with inclination was integrated into a continuum model which was found to produce the steady-state shapes of curved boundaries consistent with those from simulations. A further extension of the continuum model enabled estimations of the reduced mobility that were in good agreement with simulation results. It was also identified that atomistic events governing the migration of curved and planar boundaries shared a number of similarities. Overall, the analyses of the shapes, mobilities and atomic-scale migration mechanisms of curved and planar boundaries presented here provide a correlation between the migration of these types of twin boundaries.
A series of molecular dynamics simulations was performed in this work to investigate the kinetic interaction between helium clusters and grain boundaries in iron. Helium cluster formation and size distributions were found to be markedly different in the bulk compared to the region of a stationary boundary. Upon reaching a steady-state cluster distribution, the spatial fluctuation of cluster-enriched boundaries was analyzed to determine the grain boundary mobility using the random walk method. Segregated clusters reduced the boundary mobility, the drag effect of clusters increasing as the bulk solute concentration increases. The drag effect was further rationalized by employing Cahn's solute drag model using the effective binding energy of He clusters and the grain boundary diffusivity of a single He atom, their magnitudes having been determined from the segregation level and from monitoring the trajectory of a solute atom in the investigated grain boundaries, respectively. The model is found to provide a satisfactory explanation of the simulation results in the zero velocity limit.
A three-dimensional atomistic Kinetic Monte Carlo (aKMC) model was developed and used to study the interaction between mobile solutes and a migrating interface. While the model was developed with a simplified energetic and topological description, it was also constructed to capture, in the absence of solute, the Burke–Turnbull model for interface migration and, in the presence of solutes, solute segregation to different types of interface sites. After parameterizing the model, simulations were performed to study the relationship between average interface velocity and imposed driving pressure for varying solute concentration and solute diffusivity. Despite significant differences in the underlying assumptions of numerical and analytical solute drag models, the latter was found to be a phenomenological tool that adequately captures the trends observed by the aKMC simulations (e.g. the effect of solute concentration on solute drag pressure). One trend that could not be adequately explained was the observed dependence of maximum drag pressure on solute diffusivity. This effect is attributed to the coupling between the structure of a migrating interface and the ability for solute to remain segregated to the interface.
This document is a note on the Cahn solute drag model based on recent work in the literature. The solute drag models, in general, predict the magnitude of retardation of grain boundary migration due to solute segregation. An overview of classical solute drag model by Cahn is first presented. This is followed by some notes on the model, including a modified phenomenological model proposed by the author and a discussion on the solute drag model for the case of solutes with inhibited diffusion at the boundary. A C code that was used to solve the Cahn model numerically is included and available for download on github.
To identify atoms in a bicrystal cell based on the grain to which they belong, LAMMPS is equipped with a feature called order parameters, i.e. the fix orient/fcc command. The command has been adapted for b.c.c crystals in author's doctoral thesis. This document aims to describe the order parameter calculation in detail, including its implementation in LAMMPS for b.c.c and f.c.c crystals.
This document presents the anatomy of an EAM potential file for modelling the behaviour of binary iron-helium (Fe-He) system via molecular dynamics (MD) simulation using LAMMPS. The potentials discussed in this document are the Ackland-04, the Aziz-95 and the Gao-11 potentials, describing the Fe-Fe, He-He, and Fe-He interaction, respectively. While the emphasis here is placed upon these potentials, this document can still be used as a guide for constructing a LAMMPS-compatible potential file for EAM binary systems from the literature.
This document describes the numerical technique implemented to characterize the size of helium clusters from molecular dynamics simulation, i.e. Chapters 7 and 8 of author's doctoral thesis (here).
Microstructure evolution during material processing is determined by a number of factors, such as the kinetics of grain boundary migration in the presence of impurities, which can take form of solid solution, second-phase precipitates or clusters. The dynamic interaction between grain boundaries and clusters has not been explored. In this work, a variety of simulation tools are utilized to approach this problem from an atomistic perspective. Atomistic simulations are first implemented to explore the parameter space of the solute drag problem, i.e. grain boundary migration in a binary ideal solid solution system, via a kinetic Monte Carlo framework. Depending on their diffusivity, solute atoms are capable of modifying the structure of a migrating boundary, leading to a diffusion-dependent drag pressure. A phenomenological model adapted from the Cahn model is proposed to explain the simulation results. The interaction between clusters and a migrating grain boundary is studied next using molecular dynamics simulations. The iron helium (Fe-He) system is chosen as the object of the study. A preliminary step towards such a study is to investigate the grain boundary migration in pure bcc Fe. An emphasis is placed upon demonstrating the correlation between the migration of curved and planar boundaries. Evidence that verifies such a correlation is established, based on the analyses on the shapes, the kinetics and the migration mechanism of both types of boundaries. Next, the formation of He clusters in the bulk and grain boundaries of Fe is examined. The cluster formation at the boundary occurs at a lower rate relative to that in the bulk. This is attributed to the boundary being a slow diffusion channel for interstitial He atoms. The overall effect of clusters on the boundary migration is twofold. Clusters reduce the boundary mobility via segregation; the magnitude of their effect can be rationalized using the Cahn model in the zero velocity limit. Clusters also act as pinning sources, delaying or even completely halting the boundary migration. A phenomenological model adapted from the Zener pinning model is used to discuss the role of clusters on grain boundary migration.
The role of depolarizing field and surface energy in the domain evolution of a ferroelectric system is investigated by means of phase-field simulation method using the time-dependent Ginzburg Landau equation. It is found that the presence of depolarizing field in the ferroelectric system can lead to the development of multidomains from the initially uniform-polarization state when the thickness of the ferroelectric decreases below a certain critical value. The increasingly relevant surface energy in thin film ferroelectrics may further aggravate the monodomain stability and contribute to promoting the multidomain state if the gradient polarization across the film thickness becomes increasingly significant. The project aims to study and analyze the individual contribution of both effects as well as the interplay among them. The domain characteristics resulting from the presence of both effects will also be studied and then evaluated using the available theoretical models and experimental results.
Flash memory, the current leading technology for non-volatile memory (NVM), is projected by many to run obsolete in the face of future miniaturization trend in the semiconductor devices due to some of its technical limitations. Several different technologies have been developed in attempt for replacing Flash memory as the most dominant NVM technology; none of which seems to indicate significant success at the moment. Among these technologies is RRAM (Resistive Random Access Memory), a novel type of memory technology which has only recently emerged to join the race. The underlying principle of an RRAM device is based on the colossal electroresistance (CER) effect, i.e. the resistance switching behavior upon application of voltage of varying polarity and/or magnitude. This thesis aims to investigate the CER effect and how it can be designed to be a non-volatile memory as well as other novel application, e.g. memristor. The various technical aspects pertaining to this phenomenon, including the materials and the physical basis, are explored and analyzed. Additionally, the market potential of the RRAM technology is assessed, including a market study of memory industry, current intellectual property (IP) landscape and some of the relevant business strategies. The production strategy (i.e. the production cost, initial investment, and pricing strategy) is derived from the technical and market analysis evaluated earlier.