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Size Effect On Micro Metal Strength Simulated By Discrete Dislocation And Dislocation Density Function Dynamics
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Book Synopsis Size Effect on Micro-metal Strength Simulated by Discrete Dislocation and Dislocation Density-function Dynamics by : 梁素芯
Download or read book Size Effect on Micro-metal Strength Simulated by Discrete Dislocation and Dislocation Density-function Dynamics written by 梁素芯 and published by . This book was released on 2015 with total page 105 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis SIZE EFFECT ON MICRO-METAL STR by : So-Sum Leung
Download or read book SIZE EFFECT ON MICRO-METAL STR written by So-Sum Leung and published by Open Dissertation Press. This book was released on 2017-01-26 with total page 192 pages. Available in PDF, EPUB and Kindle. Book excerpt: This dissertation, "Size Effect on Micro-metal Strength Simulated by Discrete Dislocation and Dislocation Density-function Dynamics" by So-sum, Leung, 梁素芯, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: The pronounced smaller-being-stronger size effect of nano- and micron-metals has been extensively explored by experimental approaches and simulations in recent decades. In this study, 2D dislocation dynamics (2D DD) was used to simulate the tensile deformation of micron-sized polycrystalline fcc metals. When the grain size is constant, changing the specimen thickness increases the yield strength either when the thickness-to-grain size ratio (t-to-d ratio) is Apart from the size effect in poly-crystals, single crystals are also known to exhibit pronounced size effect. The size effect of single crystals can be expressed in a power-law σ D-m, in which the exponent m ranges from 0.3 to 1.0. Dislocation-density function dynamics was used in this study to explore the relationship between the size dependence of the yield strength and the dislocation microstructure in different specimen sizes. The "post-mortem" dislocation structure was analyzed to work out a length scale governing the size effect. It was found that the initial dislocation structure has a significant effect on the yield strength of single crystals. For the dislocation microstructure studied, specimens of sizes 4000 b and 8000 b are more significantly affected by the stochasticity of the initial dislocation micron structure, whereas the initial dislocation structure could be easily disentangled in even smaller specimens and thus the strength depend less on the initial microstructure. For larger specimen ( 16000 b), there is less stochasticity in the initial microstructure, since the specimen size is larger than the characteristic length describing the dislocation mean free path of the specimen. DDFD is also used to study the tensile deformation of tri-crystals. The grain size effect on the strength was studied. The simulated relationship between the 0.2% proof stress and the inverse square root obeys the Hall-Petch relation. In addition, the internal length scale was computed by applying a strain gradient plasticity theory (SG) to the 1D strain profile derived from DDFD results. To shed light on the identity of the internal length scale (l), l is compared with two characteristic lengths that describe the tri-crystal microstructure: the mean dislocation spacing 1/√p and the dislocation pile-up length. The internal length scale in general approximates to the pile-up length. Exception is found for the largest grain size L=500 nm when the strain is large in which l approximates to the mean dislocation spacing. This suggest that in general plasticity is governed by a length scale related to the dislocation-pileup length, whereas for the exceptional case the bowing out of dislocations through forest dislocations is the governing factor for plasticity. Subjects: Dislocations in metals
Book Synopsis The Plasticity of Metals at the Sub-micrometer Scale and Dislocation Dynamics in a Thin Film by : Seok Woo Lee
Download or read book The Plasticity of Metals at the Sub-micrometer Scale and Dislocation Dynamics in a Thin Film written by Seok Woo Lee and published by Stanford University. This book was released on 2011 with total page 186 pages. Available in PDF, EPUB and Kindle. Book excerpt: Nanotechnology has played a significant role in the development of useful engineering devices and in the synthesis of new classes of materials. For the reliable design of devices and for structural applications of materials with micro- or nano-sized features, nanotechnology has always called for an understanding of the mechanical properties of materials at small length scales. Thus, it becomes important to develop new experimental techniques to allow reliable mechanical testing at small scales. At the same time, the development of computational techniques is necessary to interpret the experimentally observed phenomena. Currently, microcompression testing of micropillars, which are fabricated by focused-ion beam (FIB) milling, is one of the most popular experimental methods for measuring the mechanical properties at the micrometer scale. Also, dislocation dynamics codes have been extensively developed to study the local evolution of dislocation structures. Therefore, we conducted both experimental and theoretical studies that shed new light on the factors that control the strength and plasticity of crystalline materials at the sub-micrometer scale. In the experimental work, we produced gold nanopillars by focused-ion beam milling, and conducted microcompression tests to obtain the stress-strain curves. Firstly, the size effects on the strength of gold nanopillars were studied, and "Smaller is Stronger" was observed. Secondly, we tried to change the dislocation densities to control the strength of gold nanopillars by prestraining and annealing. The results showed that prestraining dramatically reduces the flow strength of nanopillars while annealing restores the strength to the pristine levels. Transmission electron microscopy (TEM) revealed that the high dislocation density (~1015 m-2) of prestrained nanopillars significantly decreased after heavy plastic deformation. In order to interpret this TEM observation, potential dislocation source structures were geometrically analyzed. We found that the insertion of jogged dislocations before relaxation or enabling cross-slip during plastic flow are prerequisites for the formation of potentially strong natural pinning points and single arm dislocation sources. At the sub-micron scale, these conditions are most likely absent, and we argue that mobile dislocation starvation would occur naturally in the course of plastic flow. Two more outstanding issues have also been studied in this dissertation. The first involves the effects of FIB milling on the mechanical properties. Since micropillars are made by FIB milling, the damage layer at the free surface is always formed and would be expected to affect the mechanical properties at a sub-micron scale. Thus, pristine gold microparticles were produced by a solid-state dewetting technique, and the effects of FIB milling on both pristine and prestrained microparticles were examined via microcompression testing. These experiments revealed that FIB milling significantly reduces the strength of pristine microparticles, but does not alter that of prestrained microparticles. Thus, we confirmed that if there are pre-existing mobile-dislocations present in the crystal, FIB milling does not affect the mechanical properties. The second issue is the scaling law commonly used to describe the strength of micropillars as a function of sample size. For the scaling law, the power-law approximation has been widely used without understanding fundamental physics in it. Thus, we tried to analyze the power-law approximation in a quantitative manner with the well-known single arm source model. Material parameters, such as the friction stress, the anisotropic shear modulus, the magnitude of Burgers vector and the dislocation density, were explored to understand their effects on the scaling behavior. Considering these effects allows one to rationalize the observed material-dependent power-law exponents quantitatively. In another part of the dissertation, a computational study of dislocation dynamics in a free-standing thin film is described. We improved the ParaDiS (Parallel Dislocation Simulator) code, which was originally developed at the Lawrence Livermore National Laboratory, to deal with the free surface of a free-standing thin film. The spectral method was implemented to calculate the image stress field in a thin film. The faster convergence in the image stress calculation were obtained by employing Yoffe's image stress, which removes the singularity of the traction at the intersecting point between a threading dislocation and free surface. Using this newly developed code, we studied the stability of dislocation junctions and jogs, which are the potential dislocation sources, in a free standing thin film of a face-centered-cubic metal and discussed the creation of a dislocation source in a thin film. In summary, we have performed both microcompression tests and dislocation dynamics simulations to understand the dislocation mechanisms at the sub-micron scale and the related mechanical properties of metals. We believe that these experimental and computational studies have contributed to the enhancement of our fundamental knowledge of the plasticity of metals at the sub-micron scale.
Book Synopsis The Plasticity of Metals at the Sub-micrometer Scale and Dislocation Dynamics in a Thin Film by : Seok Woo Lee
Download or read book The Plasticity of Metals at the Sub-micrometer Scale and Dislocation Dynamics in a Thin Film written by Seok Woo Lee and published by . This book was released on 2011 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Nanotechnology has played a significant role in the development of useful engineering devices and in the synthesis of new classes of materials. For the reliable design of devices and for structural applications of materials with micro- or nano-sized features, nanotechnology has always called for an understanding of the mechanical properties of materials at small length scales. Thus, it becomes important to develop new experimental techniques to allow reliable mechanical testing at small scales. At the same time, the development of computational techniques is necessary to interpret the experimentally observed phenomena. Currently, microcompression testing of micropillars, which are fabricated by focused-ion beam (FIB) milling, is one of the most popular experimental methods for measuring the mechanical properties at the micrometer scale. Also, dislocation dynamics codes have been extensively developed to study the local evolution of dislocation structures. Therefore, we conducted both experimental and theoretical studies that shed new light on the factors that control the strength and plasticity of crystalline materials at the sub-micrometer scale. In the experimental work, we produced gold nanopillars by focused-ion beam milling, and conducted microcompression tests to obtain the stress-strain curves. Firstly, the size effects on the strength of gold nanopillars were studied, and "Smaller is Stronger" was observed. Secondly, we tried to change the dislocation densities to control the strength of gold nanopillars by prestraining and annealing. The results showed that prestraining dramatically reduces the flow strength of nanopillars while annealing restores the strength to the pristine levels. Transmission electron microscopy (TEM) revealed that the high dislocation density (~1015 m-2) of prestrained nanopillars significantly decreased after heavy plastic deformation. In order to interpret this TEM observation, potential dislocation source structures were geometrically analyzed. We found that the insertion of jogged dislocations before relaxation or enabling cross-slip during plastic flow are prerequisites for the formation of potentially strong natural pinning points and single arm dislocation sources. At the sub-micron scale, these conditions are most likely absent, and we argue that mobile dislocation starvation would occur naturally in the course of plastic flow. Two more outstanding issues have also been studied in this dissertation. The first involves the effects of FIB milling on the mechanical properties. Since micropillars are made by FIB milling, the damage layer at the free surface is always formed and would be expected to affect the mechanical properties at a sub-micron scale. Thus, pristine gold microparticles were produced by a solid-state dewetting technique, and the effects of FIB milling on both pristine and prestrained microparticles were examined via microcompression testing. These experiments revealed that FIB milling significantly reduces the strength of pristine microparticles, but does not alter that of prestrained microparticles. Thus, we confirmed that if there are pre-existing mobile-dislocations present in the crystal, FIB milling does not affect the mechanical properties. The second issue is the scaling law commonly used to describe the strength of micropillars as a function of sample size. For the scaling law, the power-law approximation has been widely used without understanding fundamental physics in it. Thus, we tried to analyze the power-law approximation in a quantitative manner with the well-known single arm source model. Material parameters, such as the friction stress, the anisotropic shear modulus, the magnitude of Burgers vector and the dislocation density, were explored to understand their effects on the scaling behavior. Considering these effects allows one to rationalize the observed material-dependent power-law exponents quantitatively. In another part of the dissertation, a computational study of dislocation dynamics in a free-standing thin film is described. We improved the ParaDiS (Parallel Dislocation Simulator) code, which was originally developed at the Lawrence Livermore National Laboratory, to deal with the free surface of a free-standing thin film. The spectral method was implemented to calculate the image stress field in a thin film. The faster convergence in the image stress calculation were obtained by employing Yoffe's image stress, which removes the singularity of the traction at the intersecting point between a threading dislocation and free surface. Using this newly developed code, we studied the stability of dislocation junctions and jogs, which are the potential dislocation sources, in a free standing thin film of a face-centered-cubic metal and discussed the creation of a dislocation source in a thin film. In summary, we have performed both microcompression tests and dislocation dynamics simulations to understand the dislocation mechanisms at the sub-micron scale and the related mechanical properties of metals. We believe that these experimental and computational studies have contributed to the enhancement of our fundamental knowledge of the plasticity of metals at the sub-micron scale.
Book Synopsis Computer Simulations of Dislocations by : Vasily Bulatov
Download or read book Computer Simulations of Dislocations written by Vasily Bulatov and published by Oxford University Press. This book was released on 2006-11-02 with total page 301 pages. Available in PDF, EPUB and Kindle. Book excerpt: The book presents a variety of methods for computer simulations of crystal defects in the form of "numerical recipes", complete with computer codes and analysis tools. By working through numerous case studies and problems, this book provides a useful starter kit for further method development in the computational materials sciences.
Book Synopsis Finite element simulation of dislocation based plasticity and diffusion in multiphase materials at high temperature by : Albiez, Jürgen
Download or read book Finite element simulation of dislocation based plasticity and diffusion in multiphase materials at high temperature written by Albiez, Jürgen and published by KIT Scientific Publishing. This book was released on 2019-05-22 with total page 222 pages. Available in PDF, EPUB and Kindle. Book excerpt: A single-crystal plasticity model as well as a gradient crystal plasticity model are used to describe the creep behavior of directionally solidi?ed NiAl based eutectic alloys. To consider the transition from theoretical to bulk strength, a hardening model was introduced to describe the strength of the reinforcing phases. Moreover, to account for microstructural changes due to material ?ux, a coupled diffusional-mechanical simulation model was introduced.
Book Synopsis Size Effects in Plasticity by : George Voyiadjis
Download or read book Size Effects in Plasticity written by George Voyiadjis and published by Academic Press. This book was released on 2019-08-01 with total page 408 pages. Available in PDF, EPUB and Kindle. Book excerpt: Size Effects in Plasticity: From Macro to Nano provides concise explanations of all available methods in this area, from atomistic simulation, to non-local continuum models to capture size effects. It then compares their applicability to a wide range of research scenarios. This essential guide addresses basic principles, numerical issues and computation, applications and provides code which readers can use in their own modeling projects. Researchers in the fields of computational mechanics, materials science and engineering will find this to be an ideal resource when they address the size effects observed in deformation mechanisms and strengths of various materials. Provides a comprehensive reference on the field of size effects and a review of mechanics of materials research in all scales Explains all major methods of size effects simulation, including non-local continuum models, non-local crystal plasticity, discrete dislocation methods and molecular dynamics Includes source codes that readers can use in their own projects
Book Synopsis Crystal Plasticity Finite Element Methods by : Franz Roters
Download or read book Crystal Plasticity Finite Element Methods written by Franz Roters and published by John Wiley & Sons. This book was released on 2011-08-04 with total page 188 pages. Available in PDF, EPUB and Kindle. Book excerpt: Written by the leading experts in computational materials science, this handy reference concisely reviews the most important aspects of plasticity modeling: constitutive laws, phase transformations, texture methods, continuum approaches and damage mechanisms. As a result, it provides the knowledge needed to avoid failures in critical systems udner mechanical load. With its various application examples to micro- and macrostructure mechanics, this is an invaluable resource for mechanical engineers as well as for researchers wanting to improve on this method and extend its outreach.
Book Synopsis Atomistic Modeling of Materials Failure by : Markus J. Buehler
Download or read book Atomistic Modeling of Materials Failure written by Markus J. Buehler and published by Springer Science & Business Media. This book was released on 2008-08-07 with total page 547 pages. Available in PDF, EPUB and Kindle. Book excerpt: This is an introduction to molecular and atomistic modeling techniques applied to fracture and deformation of solids, focusing on a variety of brittle, ductile, geometrically confined and biological materials. The overview includes computational methods and techniques operating at the atomic scale, and describes how these techniques can be used to model cracks and other deformation mechanisms. The book aims to make new molecular modeling techniques available to a wider community.
Book Synopsis Dislocation Mechanism-Based Crystal Plasticity by : Zhuo Zhuang
Download or read book Dislocation Mechanism-Based Crystal Plasticity written by Zhuo Zhuang and published by Academic Press. This book was released on 2019-04-12 with total page 450 pages. Available in PDF, EPUB and Kindle. Book excerpt: Dislocation Based Crystal Plasticity: Theory and Computation at Micron and Submicron Scale provides a comprehensive introduction to the continuum and discreteness dislocation mechanism-based theories and computational methods of crystal plasticity at the micron and submicron scale. Sections cover the fundamental concept of conventional crystal plasticity theory at the macro-scale without size effect, strain gradient crystal plasticity theory based on Taylar law dislocation, mechanism at the mesoscale, phase-field theory of crystal plasticity, computation at the submicron scale, including single crystal plasticity theory, and the discrete-continuous model of crystal plasticity with three-dimensional discrete dislocation dynamics coupling finite element method (DDD-FEM). Three kinds of plastic deformation mechanisms for submicron pillars are systematically presented. Further sections discuss dislocation nucleation and starvation at high strain rate and temperature effect for dislocation annihilation mechanism. Covers dislocation mechanism-based crystal plasticity theory and computation at the micron and submicron scale Presents crystal plasticity theory without size effect Deals with the 3D discrete-continuous (3D DCM) theoretic and computational model of crystal plasticity with 3D discrete dislocation dynamics (3D DDD) coupling finite element method (FEM) Includes discrete dislocation mechanism-based theory and computation at the submicron scale with single arm source, coating micropillar, lower cyclic loading pillars, and dislocation starvation at the submicron scale
Book Synopsis Dislocation-based Multi-scale Modeling for Size-dependent Plasticity of Heterogeneous Materials by : Hao Lyu
Download or read book Dislocation-based Multi-scale Modeling for Size-dependent Plasticity of Heterogeneous Materials written by Hao Lyu and published by . This book was released on 2016 with total page 182 pages. Available in PDF, EPUB and Kindle. Book excerpt: First, this framework is applied to study the size effect and deformation mechanisms of dual phase steel. Here, only the strain gradient theory is employed to predict the response of dual phase steel under constant strain rate tensile test. The predicted mechanical behavior and texture evolution are in good agreement with experimental works.
Book Synopsis Dislocation Mechanics of Metal Plasticity and Fracturing by : Ronald W. Armstrong
Download or read book Dislocation Mechanics of Metal Plasticity and Fracturing written by Ronald W. Armstrong and published by MDPI. This book was released on 2020-11-03 with total page 188 pages. Available in PDF, EPUB and Kindle. Book excerpt: The modern understanding of metal plasticity and fracturing began about 100 years ago, with pioneering work; first, on crack-induced fracturing by Griffith and, second, with the invention of dislocation-enhanced crystal plasticity by Taylor, Orowan and Polanyi. The modern counterparts are fracture mechanics, as invented by Irwin, and dislocation mechanics, as initiated in pioneering work by Cottrell. No less important was the breakthrough development of optical characterization of sectioned polycrystalline metal microstructures started by Sorby in the late 19th century and leading eventually to modern optical, x-ray and electron microscopy methods for assessments of crystal fracture surfaces, via fractography, and particularly of x-ray and electron microscopy techniques applied to quantitative characterizations of internal dislocation behaviors. A major current effort is to match computational simulations of metal deformation/fracturing behaviors with experimental measurements made over extended ranges of microstructures and over varying external conditions of stress-state, temperature and loading rate. The relation of such simulations to the development of constitutive equations for a hoped-for predictive description of material deformation/fracturing behaviors is an active topic of research. The present collection of articles provides a broad sampling of research accomplishments on the two subjects.
Book Synopsis The Investigation of Plastic Behavior by Discrete Dislocation Dynamics for Single Crystal Pillar at Submicron Scale by : Yinan Cui
Download or read book The Investigation of Plastic Behavior by Discrete Dislocation Dynamics for Single Crystal Pillar at Submicron Scale written by Yinan Cui and published by Springer. This book was released on 2016-10-26 with total page 141 pages. Available in PDF, EPUB and Kindle. Book excerpt: This thesis transports you to a wonderful and fascinating small-scale world and tells you the origin of several new phenomena. The investigative tool is the improved discrete dislocation-based multi-scale approaches, bridging the continuum modeling and atomistic simulation. Mechanism-based theoretical models are put forward to conveniently predict the mechanical responses and defect evolution. The findings presented in this thesis yield valuable new guidelines for microdevice design, reliability analysis and defect tuning.
Book Synopsis A Contribution to the Modeling of Metal Plasticity and Fracture by : Shyam Mohan Keralavarma
Download or read book A Contribution to the Modeling of Metal Plasticity and Fracture written by Shyam Mohan Keralavarma and published by . This book was released on 2012 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The objective of this dissertation is to further the understanding of inelastic behavior in metallic materials. Despite the increasing use of polymeric composites in aircraft structures, high specific strength metals continue to be used in key components such as airframe, fuselage, wings, landing gear and hot engine parts. Design of metallic structures subjected to thermomechanical extremes in aerospace, automotive and nuclear applications requires consideration of the plasticity, creep and fracture behavior of these materials. Consideration of inelasticity and damage processes is also important in the design of metallic components used in functional applications such as thin films, flexible electronics and micro electro mechanical systems. Fracture mechanics has been largely successful in modeling damage and failure phenomena in a host of engineering materials. In the context of ductile metals, the Gurson void growth model remains one of the most successful and widely used models. However, some well documented limitations of the model in quantitative prediction of the fracture strains and failure modes at low triaxialities may be traceable to the limited representation of the damage microstructure in the model. In the first part of this dissertation, we develop an extended continuum model of void growth that takes into account details of the material microstructure such as the texture of the plastically deforming matrix and the evolution of the void shape. The need for such an extension is motivated by a detailed investigation of the effects of the two types of anisotropy on the materials' effective response using finite element analysis. The model is derived using the Hill-Mandel homogenization theory and an approximate limit analysis of a porous representative volume element. Comparisons with several numerical studies are presented towards a partial validation of the analytical model. Inelastic phenomena such as plasticity and creep result from the collective behavior of a large number of nano and micro scale defects such as dislocations, vacancies and grain boundaries. Continuum models relate macroscopically observable quantities such as stress and strain by coarse graining the discrete defect microstructure. While continuum models provide a good approximation for the effective behavior of bulk materials, several deviations have been observed in experiments at small scales such as an intrinsic size dependence of the material strength. Discrete dislocation dynamics (DD) is a mesoscale method for obtaining the mechanical response of a material by direct simulation of the motion and interactions of dislocations. The model incorporates an intrinsic length scale in the dislocation Burgers vector and potentially allows for size dependent mechanical behavior to emerge naturally from the dynamics of the dislocation ensemble. In the second part of this dissertation, a simplified two dimensional DD model is employed to study several phenomena of practical interest such as strain hardening under homogeneous deformation, growth of microvoids in a crystalline matrix and creep of single crystals at elevated temperatures. These studies have been enabled by several recent enhancements to the existing two-dimensional DD framework described in Chapter V. The main contributions from this research are: (i) development of a fully anisotropic continuum model of void growth for use in ductile fracture simulations and (ii) enhancing the capabilities of an existing two-dimensional DD framework for large scale simulations in complex domains and at elevated temperatures.
Book Synopsis Thermally Activated Mechanisms in Crystal Plasticity by : D. Caillard
Download or read book Thermally Activated Mechanisms in Crystal Plasticity written by D. Caillard and published by Elsevier. This book was released on 2003-09-08 with total page 453 pages. Available in PDF, EPUB and Kindle. Book excerpt: KEY FEATURES: A unified, fundamental and quantitative resource. The result of 5 years of investigation from researchers around the world New data from a range of new techniques, including synchrotron radiation X-ray topography provide safer and surer methods of identifying deformation mechanisms Informing the future direction of research in intermediate and high temperature processes by providing original treatment of dislocation climb DESCRIPTION: Thermally Activated Mechanisms in Crystal Plasticity is a unified, quantitative and fundamental resource for material scientists investigating the strength of metallic materials of various structures at extreme temperatures. Crystal plasticity is usually controlled by a limited number of elementary dislocation mechanisms, even in complex structures. Those which determine dislocation mobility and how it changes under the influence of stress and temperature are of key importance for understanding and predicting the strength of materials. The authors describe in a consistent way a variety of thermally activated microscopic mechanisms of dislocation mobility in a range of crystals. The principles of the mechanisms and equations of dislocation motion are revisited and new ones are proposed. These describe mostly friction forces on dislocations such as the lattice resistance to glide or those due to sessile cores, as well as dislocation cross-slip and climb. They are critically assessed by comparison with the best available experimental results of microstructural characterization, in situ straining experiments under an electron or a synchrotron beam, as well as accurate transient mechanical tests such as stress relaxation experiments. Some recent attempts at atomistic modeling of dislocation cores under stress and temperature are also considered since they offer a complementary description of core transformations and associated energy barriers. In addition to offering guidance and assistance for further experimentation, the book indicates new ways to extend the body of data in particular areas such as lattice resistance to glide.
Book Synopsis Mesoscale Models by : Sinisa Mesarovic
Download or read book Mesoscale Models written by Sinisa Mesarovic and published by Springer. This book was released on 2018-11-19 with total page 344 pages. Available in PDF, EPUB and Kindle. Book excerpt: The book helps to answer the following questions: How far have the understanding and mesoscale modeling advanced in recent decades, what are the key open questions that require further research and what are the mathematical and physical requirements for a mesoscale model intended to provide either insight or a predictive engineering tool? It is addressed to young researchers including doctoral students, postdocs and early career faculty,
Book Synopsis Multiscale Modeling of Dislocation and Grain Boundary Mechanics in Small Scale Metals by : Jamie Gravell
Download or read book Multiscale Modeling of Dislocation and Grain Boundary Mechanics in Small Scale Metals written by Jamie Gravell and published by . This book was released on 2021 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Metals are of great importance for structural applications due to their high yield strength and fracture toughness. In recent years, efforts have been undertaken to further improve these properties, accelerated by advances in materials research and manufacturing processes. The conventional strategy to achieve high strength is to reduce the average grain size, but this is inevitably followed by the loss of ductility. Deformation mechanisms for plastic flow and ductility are largely dependent on microscopic defects such as dislocations, grain boundaries (GBs), and triple junctions (TJs). It is necessary to obtain a fundamental understanding of the correlation between defect mechanics and macroscopic properties across a variety of time and length scales so as to overcome the strength-ductility trade-off. With this motivation, a computational and theoretical approach has been taken to investigate the complex interplay between defects and macroscopic material response. In the first part of this dissertation (Chapters 2-3), dislocation mechanics within single crystals are examined to understand the role of sample size, crystallographic orientation, and loading conditions on the mechanism response. The focus is drawn to the plastic deformation which occurs at the mesoscale, wherefrom material properties are determined. Chapter 2 reports on DD simulations conducted to examine plastic deformation in single crystalline Cu micropillars subjected to two types of combined loading conditions: tension after torsion and torsion after tension. These combined loadings are then compared with simple tension and pure torsion, respectively. In metallic materials, the activation of one slip system increases the flow strength of other slip systems, which is a phenomenon known as latent hardening. This latent hardening behavior has been understood by the “forest hardening” mechanism arising from mutual dislocation interactions at the continuum length scale. As the size of a sample decreases to the submicron scale, the interactions between dislocations become increasingly sparse, so plastic deformation is instead governed mainly by dislocation sources. We find that there exists a transition from latent hardening to latent softening in intermediately-sized 600 nm samples undergoing the combined tension after torsion loading. The systematic computational and theoretical model described here suggests explosive multiplication causes dislocation density to greatly increase, giving rise to latent softening in those micropillars under tension after torsion. At the continuum length scale, mechanical properties of metals show relatively weak orientation dependence; however, Chapter 3 shows how strong anisotropic behaviors are exhibited as the size of sample decreases to micron and nanometer length scales. DD simulations are performed to investigate the orientation-dependent plasticity in submicron face-centered cubic (FCC) micropillars subjected to torsion. Accommodating results from atomistic modeling, updated surface nucleation schemes in DD models have been developed for three orientations ([001], [101], and [111]), allowing investigation of the dislocation microstructure evolution and the corresponding anisotropic mechanical response upon torsional loading and unloading. The DD simulation results show that the coaxial and hexagonal dislocation networks formed in [101]- and [111]-oriented nanopillars, respectively, exhibited excellent plastic recovery, while the rectangular dislocation network formed in the [001] crystal orientation was more stable and did not experience as much plastic recovery. Following work on isolated dislocation mechanics within a single crystal, the second part of this dissertation, Chapter 4, transitions into the exploration of defect mechanisms within bicrystals. Mechanical properties of metals such as strength and toughness are strongly correlated to complex interactions between various defects in the crystalline structure. While elementary interactions between these defects have been investigated using recent micro- and nano-characterization techniques, understanding of the detailed interaction mechanisms has hardly been obtained. To model plasticity in polycrystals at larger time and length scales, it is necessary to formulate a general guideline to predict both the interaction type (transmission or reflection) and the dislocation’s subsequent slip system after the interaction. Many criteria based on the geometric alignment of the defects have been developed to predict this phenomenon, but these have not been found to be accurate when applied to general data sets of grain boundaries (GBs). With this motivation, we conduct a systematic study using molecular dynamics (MD) models of bicrystals to analyze defect interaction process between a prismatic dislocation loop and eleven different grain boundaries of the following character: three tilt, three twist, and five mixed. Based on the MD observations, two new prediction methods are developed: the first is a new data-driven parametric score function based on the classical geometric criteria, and the second is by applying Gaussian process machine learning methods to find the probability distribution of a hidden function. The proposed methods could pave a new way to predict the unit interaction of dislocation with various GBs, which could show much higher accuracy compared to pre-existing geometric criteria. Finally, additional work on paving the way to polycrystalline modeling at the mesoscale is detailed, followed by an overall summary in Chapter 5.
