Download or read book Atomistic Simulations of the Role of Dopant Atoms in Grain Growth and Deformation in Nanocrystalline Materials written by Paul Christopher Millett and published by . This book was released on 2006 with total page 264 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Atomistic Simulations of Defect Nucleation and Free Volume in Nanocrystalline Materials written by Garritt J. Tucker and published by . This book was released on 2011 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Atomistic simulations are employed in this thesis to investigate defect nucleation and free volume of grain boundaries and nanocrystalline materials. Nanocrystalline materials are of particular interest due to their improved mechanical properties and alternative strain accommodation processes at the nanoscale. These processes, or deformation mechanisms, within nanocrystalline materials are strongly dictated by the larger volume fraction of grain boundaries and interfaces due to smaller average grain sizes. The behavior of grain boundaries within nanocrystalline materials is still largely unknown. One reason is that experimental investigation at this scale is often difficult, time consuming, expensive, or impossible with current resources. Atomistic simulations have shown the potential to probe fundamental behavior at these length scales and provide vital insight into material mechanisms. Therefore, work conducted in this thesis will utilize atomistic simulations to explore structure-property relationships of face-centered-cubic grain boundaries, and investigate the deformation of nanocrystalline copper as a function of average grain size. Volume-averaged kinematic metrics are formulated from continuum mechanics theory to estimate nonlocal deformation fields and probe the nanoscale features unique to strain accommodation mechanisms in nanocrystalline metals. The kinematic metrics are also leveraged to explore the tensile deformation of nanocrystalline copper at 10K. The distribution of different deformation mechanisms is calculated and we are able to partition the role of competing mechanisms in the overall strain of the nanocrystalline structure as a function of grain size. Grain boundaries are observed to be influential in smaller grained structures, while dislocation glide is more influential as grain size increases. Under compression, however, the resolved compressive normal stress on interfaces hinders grain boundary plasticity, leading to a tension-compression asymmetry in the strength of nanocrystalline copper. The mechanisms responsible for the asymmetry are probed with atomistic simulations and the volume-averaged metrics. Finally, the utility of the metrics in capturing nonlocal nanoscale deformation behavior and their potential to inform higher-scaled models is discussed.

Download or read book Atomistic Simulation of Nanocrystalline Materials written by and published by . This book was released on 1995 with total page 3 pages. Available in PDF, EPUB and Kindle. Book excerpt: Atomistic simulations show that high-energy grain boundaries in nanocrystalline copper and nanocrystalline silicon are highly disordered. In the case of silicon the structures of the grain boundaries are essentially indistinguishable from that of bulk amorphous silicon. Based on a free-energy argument, we suggest that below a critical grain size nanocrystalline materials should be unstable with respect to the amorphous phase.

Download or read book Quantifying the Influence of Twin Boundaries on the Deformation of Nanocrystalline Copper Using Atomistic Simulations written by and published by . This book was released on 2014 with total page 15 pages. Available in PDF, EPUB and Kindle. Book excerpt: Over the past decade, numerous efforts have sought to understand the influence of twin boundaries on the behavior of polycrystalline materials. Early results suggested that twin boundaries within nanocrystalline face-centered cubic metals have a considerable effect on material behavior by altering the activated deformation mechanisms. In this work, we employ molecular dynamics simulations to elucidate the role of twin boundaries on the deformation of 100 columnar nanocrystalline copper at room temperature under uniaxial strain. We leverage non-local kinematic metrics, formulated from continuum mechanics theory, to compute atomically-resolved rotational and strain fields during plastic deformation. These results are then utilized to compute the distribution of various nanoscale mechanisms during straining, and quantitatively resolve their contribution to the total strain accommodation within the microstructure, highlighting the fundamental role of twin boundaries. Our results show that nanoscale twins influence nanocrystalline copper by altering the cooperation of fundamental deformation mechanisms and their contributed role in strain accommodation, and we present new methods for extracting useful information from atomistic simulations. The simulation results suggest a tension-compression asymmetry in the distribution of deformation mechanisms and strain accommodation by either dislocations or twin boundary mechanisms. In highly twinned microstructures, twin boundary migration can become a significant deformation mode, in comparison to lattice dislocation plasticity in non-twinned columnar microstructures, especially during compression.

Download or read book Influence of Grain Misorientation on Grain Growth in Nanocrystalline Metals written by Justin Glen Brons and published by . This book was released on 2013 with total page 152 pages. Available in PDF, EPUB and Kindle. Book excerpt: It is well known that the grain size of a material controls its properties, including mechanical strength, electrical conduction, and corrosion resistance. Typically, a fine grain size is desirable, since it allows for these properties to be increased. Nanocrystalline materials have been engineered in order to maximize the benefits associated with this fine grain size. Unfortunately, the high density of grain boundaries for a given volume of the material leads to an increase in the excess energy that is associated with grain boundaries. This excess energy can act as a driving force for grain growth, which causes these nanocrystalline structures to be unstable, with this grain growth often times being detrimental to the material properties. This research investigated the influence of grain boundary mobility and the applied driving force on grain growth in nanocrystalline metal films by focusing on the role grain boundary misorientation plays in regulating grain growth. The was be accomplished by completing two types of studies: (i) Annealing sputter-deposited thin films to study mobility in a case where the driving force is assumed to be dominated by grain boundary curvature and (ii) Mechanically indenting thin films with different microstructural features while submerged in liquid nitrogen. In terms of the latter study, the mobility was expected to be extremely low due to the cryogenic temperatures. Both sets of films were then characterized using precession-enhanced diffraction-based orientation analysis in the transmission electron microscope to quantify the evolution in grain size, grain morphology, and in the grain-to-grain misorientation.

Download or read book Thermodynamic Contribution on Grain Growth in Nanocrystalline MgAl2O4 Spinel written by Dereck Nills Ferreira Muche and published by . This book was released on 2018 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Fully dense materials consisting of nanostructures can exhibit very interesting and, in many cases, enhanced properties, mostly as a result of an unconventionally high ratio of atoms on interfaces, or grain boundaries (GBs) compared to bulk materials. In nanocrystalline MgAl2O4 for instance, retaining grains in the nanoscale bring benefits, such as a pronounced grain boundary strengthening (or Hall-Petch strengthening) improving mechanical performance of the material in applications. Several processes and approaches have been proposed to achieve high densities while maintaining relatively small grains, however after manufacturing, the samples are still subjected to coarsening if exposed to high temperature losing their nanostructure due to grain growth. With the goal of understanding processing and stability of ultrafine grain sizes, the work presented here is divided in 4 main topics: I) proposal of a synthesis approach for metal oxide nanoparticles based on an aqueous precipitation method, with which very small grains can be obtained with nanopowders showing minimal agglomeration. II) A novel sintering processing technique is introduced, so-called Deformable Punch Spark Plasma Sintering, DP-SPS, in which nanoparticles are sintered under high pressures (~2 GPa) during densification at moderate temperatures (720–870 °C), significantly inhibiting grain growth. The method was applied in MgAl2O4 and highly transparent samples were characterized mechanically where hardness increases from 17.2 to 28.4 GPa when grain sizes are refined from 188 nm to 7.1 nm, respectively. III) An atomistic description and quantification of the thermodynamic and kinetic contributions of grain growth for a better understanding and control of the microstructural evolution was carried out in the produced nanocrystalline MgAl2O4. The grain growth experiments carried out in Differential Scanning Calorimetry and systematic investigation of cation disorder showed faster growth behavior for smaller grains, combined with an increase in excess energies as a result of an increase in cationic inversion. IV) Finally, a thermodynamic investigation of MgAl2O4 nanoparticles shows the effect of segregation of dopants at grain boundaries and surfaces promoting decrease in excess interfacial energy that results in coarsening inhibition.

Download or read book A Quantized Crystal Plasticity Model for Nanocrystalline Metals written by Lin Li and published by . This book was released on 2011 with total page 156 pages. Available in PDF, EPUB and Kindle. Book excerpt: Abstract: Nanocrystalline (NC) metals, which consist of grains or crystallites with sizes less than 100 nm, have exhibited unique mechanical and physical properties, in comparison to coarse-grained (CG) counterparts. The appealing mechanical properties, for instance, include extremely high strengths, very extended elastic-plastic transitions, and unprecedented magnitudes of recoverable plastic strain. Further, footprints of inter-granular stresses measured from diffraction experiments are distinct for NC metals vs. CG metals. In particular, recent in-situ synchrotron measurements reveal that residual lattice strains change rather modestly even after imposing macro plastic strains to ~1%. Remarkably, over the same regime, the corresponding residual peak widths decrease. These phenomena are in sharp contrast to CG metals, for which residual lattice strain and peak widths both increase with deformation. In this dissertation, a quantized crystal plasticity (QCP) model is developed to explore the aforementioned unique NC features. The QCP model employs a crystallographic description of dislocation slip plasticity; in particular, single slip events across nano scale grains impart large (~1%) increments in grain-averaged plastic shear. Therefore, plasticity does not proceed in a smooth, continuous fashion but rather via strain jumps, imparting violent grain-to-grain redistribution in stress. This discrete feature is consistent with recent Molecular Dynamics (MD) simulations, which illustrate a dramatic jump in grain-averaged shear strain when a dislocation spontaneously transverses a nano grain interior after depinning from grain boundary (GB) ledges. Finite element simulations implementing this quantized plasticity approach predict the experimental properties of enhanced strength, extended elastic-plastic strain, and recoverable plastic strain, as well as the trends in residual lattice strain and peak width mentioned, but only under certain conditions. First, the grain-to-grain distribution of critical stress for slip activation is very different from that for CG materials. In particular, no events occur below a rather large threshold stress ~ 1/grain size; and above this threshold, a very asymmetric distribution predominates, signifying that a relatively large number of easier-to-slip grains are balanced by a minority of harder-to-slip grains. Second, there exists a large residual stress state, which can be removed via post deformation. The quantized crystal plasticity provides an alternate view of NC deformation, compared to hypotheses in the literature that are centered on GB sliding or deformation of a GB phase separated from grain interior. The QCP model is capable of bridging the disparity in length and time scales between MD simulations and physical experiments, and as well establishes an insightful connection between them.

Download or read book Grain Growth in Polycrystalline Materials written by G. Abbruzzese and published by . This book was released on 1992 with total page 496 pages. Available in PDF, EPUB and Kindle. Book excerpt: The volumes present investigations on grain growth phenomena and their observation in various materials: metals and alloys, ceramics, sintered materials, thin films, etc.; normal and abnormal grain growth including twinning, texture, particle and other drag effects as well as analysis of topological aspects and grain size and grain orientation correlations; grain boundary structure, mobility and interaction with particles and impurity atoms. Experimental methods applicable to measurements of grain size, orientation of individual grains, etc.

Download or read book Two phase Nanocrystalline amorphous Simulations of Anisotropic Grain Growth Using Q state Monte Carlo written by Jeffrey B. Allen and published by . This book was released on 2015 with total page 16 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Science at the Interface written by and published by . This book was released on 2009 with total page 104 pages. Available in PDF, EPUB and Kindle. Book excerpt: Interfaces are a critical determinant of the full range of materials properties, especially at the nanoscale. Computational and experimental methods developed a comprehensive understanding of nanograin evolution based on a fundamental understanding of internal interfaces in nanocrystalline nickel. It has recently been shown that nanocrystals with a bi-modal grain-size distribution possess a unique combination of high-strength, ductility and wear-resistance. We performed a combined experimental and theoretical investigation of the structure and motion of internal interfaces in nanograined metal and the resulting grain evolution. The properties of grain boundaries are computed for an unprecedented range of boundaries. The presence of roughening transitions in grain boundaries is explored and related to dramatic changes in boundary mobility. Experimental observations show that abnormal grain growth in nanograined materials is unlike conventional scale material in both the level of defects and the formation of unfavored phases. Molecular dynamics simulations address the origins of some of these phenomena.

Download or read book Computational Modelling of the Mechanical Behavior of Nanocrystalline Metals Based on the Deformation Mechanisms and Their Transitions written by Baozhi Zhu and published by . This book was released on 2006 with total page 152 pages. Available in PDF, EPUB and Kindle. Book excerpt: There has been a growing research interest in understanding the mechanical behaviors and the deformation mechanisms of nanocrystalline metals and alloys in the past a few decades, due to their extraordinary mechanical prosperities, such as high strength, hardness, and wear resistance, which have great potentials in engineering applications. As grain sizes in crystalline metals and alloys transit down to the lower end of the nanometer range, the plastic deformations are no longer dominated by the intragrain dislocation activities. Instead deformations assisted by grain boundary start to play a more important role in deciding the mechanical response of the bulk materials, as the interfacial volume fraction increases with the reduction of grain sizes. A polycrystalline constitutive theory is developed in the form of the extend aggregate Taylor model of Asaro and Needleman for the nanocrystalline metals. The plastic deformation description is based on the Asaro, Krysl and Kad (AKK) model, which considers deformation mechanisms such as the emission of perfect, partial dislocations and deformation twins from grain boundary and grain boundary sliding when the grain size is sufficiently small in the nanometer regime (less than 100nm), and their transitions are governed by the factors such as grain size, stacking fault energy, temperature, and strain rate, etc. Therefore the effect of grain size distributions in addition to the mean grain size is considered important on the mechanical response in this constitutive theory. The grain size distributions can be simulated with the experimentally determined lognormal distributions for the electro-deposited nanocrystalline metals for example. Numerical simulations are carried out for nanocrystalline Ni, Cu, Al and Pd, and the simulated phenomena include the mechanical response of these materials when subjected to uniaxial tension and compression under different deformation rates, texture development under high pressure torsion (HPT), and the grain growth effect during nanoindentation, etc, where the contribution of each deformation mechanism is carefully studied. The obtained numerical results are in reasonably good agreement with the experiments. Due to the fact that the deformation mechanisms in nanostructured materials are not yet fully understood, this constitutive theory will need to be further improved with the future findings of deformation mechanisms, which this theory has the flexibility to easily incorporate.

Download or read book Effect of Dopants at the Grain Boundary on Thermal Transport in Beta SiC at High Temperatures written by Nipun Goel and published by . This book was released on 2016 with total page 162 pages. Available in PDF, EPUB and Kindle. Book excerpt: Advanced semiconductor materials for thermoelectric applications often comprise of nanostructured grains in order to take advantage of phonon scattering phenomenon at the grain boundaries and thus increase the thermoelectric figure of merit for the material. Opportunities for further improvements in the figure of merit are available via usage of appropriate dopants. In this report, thermal transport across low-angle, symmetric tilt grain boundaries in beta-SiC is studied and the influence of dopants, introduced at these grain boundaries, on the phononic transmission across the grain boundary is investigated. Non-equilibrium molecular dynamics (NEMD) simulation are used to gain insights into the impact of grain-boundary segregation on Kapitza resistance of doped beta-SiC at high-temperature. In particular, the role of dopant concentration and dopant/matrix interaction strength in determining the resistance is assessed. Dopants that adhere to the matrix material with the same strength as they adhere to other dopant atoms are determined to spread out across the grain boundary cross-section forming a layered structure and resulted in a concomitant gradual increase in resistance with increase in dopant concentration. Whereas, for relatively weak dopant/matrix interaction strengths, dopant clustering predominates, and the Kapitza resistance increases significantly for small changes in dopant concentration. The different interaction strength regimes are investigated by mapping the spatial distribution of temperature at the grain boundary cross-section and calculating the degree of structural disorder. It was found that the dopant clusters lead to a heat flux parallel to the grain boundary plane and a significant increase in boundary disorder, partly explaining the observed increase in Kapitza resistance at the boundary. A comparison of the local vibrational density of states for the weak and strong dopant/matrix interaction strength cases is performed and a subset of modes that are significant for thermal transport in this system are identified. It is determined that for the nano-structures studied, the loss of optical phonon modes that have typically been ignored for thermal transport analyses, resulted in a more significant increase in Kapitza resistance at the grain boundary. This analysis is complemented by calculations of the projected density of states and a corresponding eigenmode analysis of the dynamical matrix that highlight important phonon polarizations and propagation directions. We also examine the dependence of the Kapitza resistance on temperature, dopant mass and dopant/matrix interaction strength, the latter parameter affecting grain-boundary structure and, hence, phonon scattering. The study concludes with an investigation into the effects of grain boundary orientation and the local grain boundary energy on phonon scattering at the boundaries. More specifically, the impact of dopants on the interface resistance is examined for these boundaries. It is observed that for the methodology used to create the grain boundary systems, the interface resistance was independent of the grain boundary orientation irrespective of the dopant concentration. However, grain boundary energy had distinct effects on the interface resistance. Layering dopant caused an increase in disorder at the grain boundaries in higher energy system resulting in an increase in phonon scattering and therefore higher interface resistance.

Download or read book Grain Boundary Influence on Radiation Induced Defect Evolution in Nanocrystalline Metals written by James Nathaniel (II) and published by . This book was released on 2019 with total page 175 pages. Available in PDF, EPUB and Kindle. Book excerpt: The development of materials that can better withstand the operating environment within nuclear reactors is of critical importance for the longevity of existing and the robustness of future nuclear energy systems. It is conjectured that nanocrystalline materials should exhibit significant reductions in radiation damage, however despite extensive studies, fundamental questions remain regarding defect evolution and migration to grain boundaries. Specifically, the role of grain size and grain boundary properties must be understood to develop insights into how to create new material microstructures that have enhanced radiation tolerance. The work presented makes use of in situ and ex situ experimental approaches to examine the role of grain size and grain boundary character in response to radiation damage in model FCC metals. Heavy ion irradiation experiments were carried out on metal foils under varying experimental conditions followed by post-irradiation analysis of grains ranging from 10 nm to 200 nm in size. Transmission electron microscopy (TEM) and related techniques were used to evaluate defect densities (dislocations and cavities) and defect cluster size as a function of grain size and grain boundary misorientation. Phenomena related to grain boundary response to damage absorption and sink efficiency are examined as well. The goal of this work is to contribute to building a fundamental framework for microstructural design to fabricate more radiation tolerant materials.

Download or read book Deformation and Microrotation in the Vicinity of Grain Boundaries written by and published by . This book was released on 2011 with total page 18 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Grain Boundary Segregation in Metals written by Pavel Lejcek and published by Springer Science & Business Media. This book was released on 2010-07-20 with total page 249 pages. Available in PDF, EPUB and Kindle. Book excerpt: Grain boundaries are important structural components of polycrystalline materials used in the vast majority of technical applications. Because grain boundaries form a continuous network throughout such materials, their properties may limit their practical use. One of the serious phenomena which evoke these limitations is the grain boundary segregation of impurities. It results in the loss of grain boundary cohesion and consequently, in brittle fracture of the materials. The current book deals with fundamentals of grain boundary segregation in metallic materials and its relationship to the grain boundary structure, classification and other materials properties.

Download or read book Metals Abstracts written by and published by . This book was released on 1992 with total page 608 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Wide Bandgap Semiconductors written by Kiyoshi Takahashi and published by Springer Science & Business Media. This book was released on 2007-04-12 with total page 481 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book offers a comprehensive overview of the development, current state, and future prospects of wide bandgap semiconductor materials and related optoelectronics devices. With 901 references, 333 figures and 21 tables, this book will serve as a one-stop source of knowledge on wide bandgap semiconductors and related optoelectronics devices.