Download or read book Atomistic Simulation Studies of Grain Boundary Segregation and Strengthening Mechanisms in Nanocrystalline Nanotwinned Silver Copper Alloys written by Xing Ke and published by . This book was released on 2019 with total page 314 pages. Available in PDF, EPUB and Kindle. Book excerpt: Silver (Ag) is a precious metal with a low stacking fault energy that is known to form copious nanoscale coherent twin boundaries during magnetron sputtering synthesis. Nanotwinned Ag metals are potentially attractive for creating new interface-dominated nanomaterials with unprecedented mechanical and physical properties. Grain-boundary segregation of solute elements has been found to increase the stability of interfaces and hardness of nanocrystalline metals. However, heavily alloying inevitably complicates the underlying deformation mechanisms due to the hardening effects of solutes, or a change of stacking fault energies in Ag caused by alloying. For the above reasons, we developed a microalloying (or doping) strategy by carefully selecting Cu as the primary impurity--a solute that is predicted to have no solid-solution strengthening effect in Ag when its content is below 3.0 wt.%. Neither will Cu affect the stacking fault energy of Ag at a concentration

Download or read book Nanocrystalline Alloys written by Timothy John Rupert and published by . This book was released on 2011 with total page 132 pages. Available in PDF, EPUB and Kindle. Book excerpt: Nanocrystalline materials have experienced a great deal of attention in recent years, largely due to their impressive array of physical properties. In particular, nanocrystalline mechanical behavior has been of interest, as incredible strengths are predicted when grain size is reduced to the nanometer range. The vast majority of research to this point has focused on quantifying and understanding the grain size-dependence of strength, leading to the discovery of novel, grain boundary-dominated physics that begin to control deformation at extremely fine grain sizes. With the emergence of this detailed understanding of nanocrystalline deformation mechanisms, the opportunity now exists for studies that explore how other structural features affect mechanical properties in order to identify alternative strengthening mechanisms. In this thesis, we seek to extend our current knowledge of nanocrystalline structure-property relationships beyond just grain size, using combinations of structural characterization, mechanical testing, and atomistic simulations. Controlled experiments on Ni-W are first used to show that solid solution addition and the relaxation of nonequilibrium grain boundary state can dramatically affect the strength of nanocrystalline metals. Next, the sliding wear response of nanocrystalline Ni-W is investigated, to show how alloying and grain boundary structural state affect a more complex mechanical property. This type of mechanical loading also provides a strong driving force for structural evolution, which, in this case, is found to be beneficial. Mechanically-driven grain growth and grain boundary relaxation occur near the surface of the Ni-W samples during sliding, leading to a hardening effect that improves wear resistance and results in a deviation from Archard scaling. Finally, molecular dynamics simulations are performed to confirm that mechanical cycling alone can indeed relax grain boundary structure and strengthen nanocrystalline materials. In all of the cases discuss above, our observations can be directly connected to the unique deformation physics of nanocrystalline materials.

Download or read book Atomistic Simulation Study of Nickel Solute Segregation and Mechanical Behavior in Nanocrystalline Fcc Bcc and Hcp Binary Alloys written by Ève-Audrey Picard and published by . This book was released on 2021 with total page 136 pages. Available in PDF, EPUB and Kindle. Book excerpt: Nanocrystalline metals and alloys have been proven to possess unprecedentedly higher tensile strength than coarse-grained conventional metals. The extreme grain refinement in nanocrystalline metals, however, negatively affects these materials by reducing their ductility through grain-boundary embrittlement and shear localization mechanisms that are promoted by segregation of solute atoms to the interfaces. Different segregation behaviors described in the literature can be divided into either heterogeneous or homogeneous types. Yet current understanding of the impact of solute atom arrangements within grain boundary networks on mechanical properties of cubic and hexagonal nanocrystals remains limited. In this thesis, hybrid Monte-Carlo and molecular dynamics simulations were used to study the segregation behavior of Ni solute atoms in polycrystals made of FCC Ag-Ni, FCC Al-Ni, BCC Nb-Ni, and HCP Zr-Ni alloys. Solute segregation in 4 binary alloys with a constant solute content of 4 at.% Ni was simulated and quantified at the same homologous temperature and at their respective maximum solubility temperature. A spectrum of segregation configurations varying from fully heterogeneous to fully homogeneous was found: Pure heterogeneous segregation (Ag96Ni4 500 K), homogeneous segregation with second-phase precipitates (Al96Ni4 378 K, Nb96Ni4 1110 K), homogeneous segregation with small-scale Ni clusters (Nb96Ni4 1564 K, Zr96Ni4 464 K), and pure homogeneous segregation with amorphous intergranular films (Al96Ni4 913 K, Zr96Ni4 1118 K). These differences in segregation behavior are shown to lead to significant variations in stress-strain response for each alloy. It is found that segregation involving the presence of grain-boundary precipitates with homogeneous segregation behavior promoted the most significant shear localization.

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 Understanding the Mechanistic Role of Grain Boundaries on the Strength and Deformation of Nanocrystalline Metals Using Atomistic Simulations written by Satish Rajaram and published by . This book was released on 2019 with total page 230 pages. Available in PDF, EPUB and Kindle. Book excerpt: Nanocrystalline (NC) materials, defined structurally by having average grain sizes less than 100nm, exhibit a number of enhanced mechanical properties such as ultrahigh strength, improved wear resistance and greater resistance to fatigue crack initiation compared to coarser grained polycrystalline (PC) materials. NC materials exhibit these improved properties, in part, due to the increased grain boundary (GB) volume fraction. NC materials strength increases with decreasing grain size, known as the Hall-Petch (HP) effect often resulting in a peak strength between 10-20nm. Studies have shown that NC materials strength decreases due to the shift from dislocation-dominant to GB-dominant deformation mechanisms in the plastic flow regime as average grain size decreases below 10-20nm. While the potential improved properties are of interest, the application of NC materials are hindered due to microstructural instability i.e., grain growth to reduce the total energy of the system, thus degrading desired mechanical properties. Numerous studies have looked at avenues to stabilize NC microstructure, namely through thermodynamics and kinetics, alloying has been one significant strategy used to stabilize NC materials. As these processes are used to stabilize NC microstructures to thermally-induce grain growth, they add additional uncertainty as the deformation and GB behavior of pure NC materials are still not fully understood. Experimental work on NC materials is difficult due to the length scale being investigated as it is difficult to manufacture and can be time consuming to analyze with current technology. Atomistic simulations have shown the potential to investigate fundamental behavior at the nanoscale and provide important insight in the mechanisms that drive the mechanical behavior of NC materials. This thesis will use atomistic simulations to study the structure-property relationship of face-centered-cubic (fcc) metals by focusing on GBs to investigate the strength of NC nickel. During the course of this thesis, four aspects that govern NC behavior will be studied, yielding, plasticity, thermal effects, and GB disorder to elucidate deeper insight into the underlying deformation mechanisms that control the strength of FCC NC metals i.e., flow stress, in the grain size regime 6 to 20nm.

Download or read book A Resolution of Grain Boundary Strengthening Mechanisms by Nanoindentation Induced Local Mechanical Response written by and published by . This book was released on 2021 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Abstract : Grain boundary segregation is well known to cause significant embrittlement of alloys. But in certain cases, it has also been observed to increase mechanical strength. This project attempts to assess local mechanical behavior of specific grain boundaries with and without segregation in order to understand association between grain boundary chemistry and deformation mechanism utilizing instrumented nanoindentation technique. It is hypothesized that solute segregation strongly affects the grain boundary energy which in turn affects the deformation mechanism processes. This project also utilizes a unique ability provided by the instrumented indentation technique to interrogate local grain boundary strengthening mechanisms proposed by Hall-Petch and Taylor-Ashby using two different indentation geometries. Grain boundary mechanical properties have typically been interpolated from macroscopic mechanical testing on polycrystalline materials, or alternatively, mechanical test procedures carried out on bulk bicrystals. The disadvantages to these types of studies relate to the difficulty in extracting the local response of a particular grain boundary (in the case of polycrystalline materials) or the grain boundary region (in the case of a bicrystal material) from the overall response of the complex interaction between the presence of the grain boundary and the deformation behavior far from the grain boundary. That is, the grain boundary causes a non-local response to the mechanical behavior. This non-local response is particularly evident in bicrystal deformation, where the macroscopic plastic displacement is inconsistent with that observed for single crystal deformation. Moreover, local hardness testing of grain boundary regions in macroscopically deformed materials show that the deformation in the grain boundary region is leads to greater local dislocation density than found in the grain center. This project is designed to use nanoindentation to isolate the mechanical response of the grain boundary as the dependent variable, where indentation geometry, indentation rate, grain boundary misorientation and sample chemistry are the independent experimental variables. It is proposed that this approach can provide insight into long standing hypotheses regarding grain boundary strengthening mechanisms, including the Hall-Petch pile-up theory, grain boundary source theory, grain boundary layer theory and the Ashby-Taylor strain incompatibility theory.

Download or read book Quantification of Grain Boundary Mediated Plasticity Mechanisms in Nanocrystalline Metals written by Jason F. Panzarino and published by . This book was released on 2016 with total page 147 pages. Available in PDF, EPUB and Kindle. Book excerpt: Nanocrystalline metals have been a topic of great discussion over recent years due to their exceptional strengths and novel grain boundary-mediated deformation mechanisms. Their microstructures are known to evolve through dynamic processes such as grain boundary migration and grain rotation, but how the collective interaction of these mechanisms alter the microstructure on a larger scale is not completely understood. In this thesis, we present coupled atomistic modeling and experimental tasks that aim to understand how the grain structure, grain boundaries, and associated grain boundary network change during nanocrystalline plasticity. Due to the complex three-dimensional nature of these mechanisms and the limited spatial and temporal resolution of current in-situ experimental techniques, we turn to atomistic modeling to help understand the dynamics by which these mechanisms unfold. In order to provide a quantitative analysis of this behavior, we develop a tool which fully characterizes nanocrystalline microstructures in atomistic models and subsequently tracks their evolution during molecular dynamics simulations. We then use this algorithm to quantitatively track grain structure and boundary network evolution in plastically deformed nanocrystalline Al, finding that higher testing temperature and smaller average grain size results in increased evolution of grain structure with evidence of larger scale changes to the grain boundary network also taking place. This prompts us to extend our analysis technique to include full characterization of grain boundary networks and rigorous topographical feature identification. We then employ this tool on simulations of Al subject to monotonic tension, cycling loading, and simple annealing, and find that each case results in different evolution of the grain boundary network. Finally, our computational work is complemented synergistically by experimental analyses which track surface microstructure evolution during sliding wear of nanocrystalline Ni-W thin films. These experiments track the development of a surface grain growth layer which evolves through grain boundary mediated plasticity and we are able to make direct connections between this evolution and that which was observed in our simulation work. All of the findings of this thesis are a direct result of the dynamic and collective nature by which nanocrystalline materials deform.

Download or read book A Resolution of Grain Boundary Strengthening Mechanisms by Nanoindentation Induced Local Mechanical Response written by Prasad Pramod Soman and published by . This book was released on 2021 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Grain boundary segregation is well known to cause significant embrittlement of alloys. But in certain cases, it has also been observed to increase mechanical strength. This project attempts to assess local mechanical behavior of specific grain boundaries with and without segregation in order to understand association between grain boundary chemistry and deformation mechanism utilizing instrumented nanoindentation technique. It is hypothesized that solute segregation strongly affects the grain boundary energy which in turn affects the deformation mechanism processes. This project also utilizes a unique ability provided by the instrumented indentation technique to interrogate local grain boundary strengthening mechanisms proposed by Hall-Petch and Taylor-Ashby using two different indentation geometries. Grain boundary mechanical properties have typically been interpolated from macroscopic mechanical testing on polycrystalline materials, or alternatively, mechanical test procedures carried out on bulk bicrystals. The disadvantages to these types of studies relate to the difficulty in extracting the local response of a particular grain boundary (in the case of polycrystalline materials) or the grain boundary region (in the case of a bicrystal material) from the overall response of the complex interaction between the presence of the grain boundary and the deformation behavior far from the grain boundary. That is, the grain boundary causes a non-local response to the mechanical behavior. This non-local response is particularly evident in bicrystal deformation, where the macroscopic plastic displacement is inconsistent with that observed for single crystal deformation. Moreover, local hardness testing of grain boundary regions in macroscopically deformed materials show that the deformation in the grain boundary region is leads to greater local dislocation density than found in the grain center. This project is designed to use nanoindentation to isolate the mechanical response of the grain boundary as the dependent variable, where indentation geometry, indentation rate, grain boundary misorientation and sample chemistry are the independent experimental variables. It is proposed that this approach can provide insight into long standing hypotheses regarding grain boundary strengthening mechanisms, including the Hall-Petch pile-up theory, grain boundary source theory, grain boundary layer theory and the Ashby-Taylor strain incompatibility theory.

Download or read book Atomistic Simulations of Grain Boundary Pinning in CuFe Alloys written by and published by . This book was released on 2005 with total page 11 pages. Available in PDF, EPUB and Kindle. Book excerpt: The authors apply a hybrid Monte Carlo-molecular dynamics code to the study of grain boundary motion upon annealing of pure Cu and Cu with low concentrations of Fe. The hybrid simulations account for segregation and precipitation of the low solubility Fe, together with curvature driven grain boundary motion. Grain boundaries in two different systems, a [Sigma]7+U-shaped half-loop grain and a nanocrystalline sample, were found to be pinned in the presence of Fe concentrations exceeding 3%.

Download or read book The Effect of Solutes on Interface Structure and Microstructural Stability in Binary Nanocrystalline Alloys written by Jonathan Priedeman and published by . This book was released on 2022 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Stabilizing a nanocrystalline material against grain growth is necessary to preserve the nanostructure and associated strength while experiencing conditions experienced during manufacturing and/or service. This work examines the structures and stability provided by solutes in different alloys. First, a platinum-gold alloy thin film is examined during in-situ annealing where observations of a faceted grain boundary structure are made. Atomistic simulations reveal that segregation of gold to that grain boundary stabilize such a structure. Second, a bulk copper-niobium alloy is plastically deformed at elevated temperature. The nanostructure of the alloy is preserved, indicating good stability. The strength of the niobium-doped alloy, however, is more sensitive to temperature than a related copper-tantalum alloy. This sensitivity is found to be a result of the oxidation behavior of the solutes: theoretical calculations of the misfit strengthening provided by these oxide precipitates reveal greater temperature sensitivity than either the niobium or tantalum precipitates. Third, a bulk copper-hafnium alloy is also plastically deformed at elevated temperature, and the nanostructure is also retained. Here the hafnium was processed as a conformal coating over the copper powder prior to consolidation rather than use of an elemental powder-powder mixing. The copper-hafnium alloy has good low-temperature strength, but begins deforming by grain boundary mediated mechanisms at much lower temperatures compared to copper-tantalum and copper-niobium. Again, the precipitates contribute to this behavior, as the hafnium oxidizes to form a monoclinic hafnium dioxide. This oxide provides good resistance to dislocation propagation but does not reinforce grain boundaries due to lack of coherency. Thus, in all three alloys studied, the solute behavior as it seeks to achieve local equilibrium has profound effects on observed structural evolution (and mechanical response).

Download or read book Strengthening Mechanisms in Nanocrystalline Silver Nickel Nanolayered Materials written by Malcolm Ryan Pringle and published by . This book was released on 2021 with total page 110 pages. Available in PDF, EPUB and Kindle. Book excerpt: Among all metals, silver has the highest electrical conductivity but also is one of the softest materials under mechanical deformation. Therefore, developing means and methods for strengthening silver without reducing conductivity is critically important for its use as a conductive electrode material in various engineering applications such as solar cells and touchscreen displays. This thesis presents a molecular-dynamics simulation study of strengthening mechanisms by intercalating nanocrystalline silver films with amorphous nickel layers, characterizing the structure of nanolayered material prototypes obtained by sputtering deposition technique. The objectives of the thesis are three-fold: To study the effects of Ni layer thickness and Ag film grain size on flow stress under compression, to simulate the nanoindentation hardness behavior of Ag-Ni nanolayers, and to elucidate the underlying strengthening mechanisms at atomic scale. It is found that the addition of Ni nanolayers plays a significant strengthening role as the layer thickness increases. However, the Hall-Petch strengthening breakdown observed in the pure nanocrystalline Ag model at an average grain size between 13 and 15 nm, is not observed in the models with the Ni layer. Furthermore, a discovery of dynamic recrystallization was made, that as the strain on the material grew, the Ni nanolayer became less amorphous. Nanoindentation simulations indicate that the proximity of a Ni layer does not change the hardness, and that slip transmission is not a strengthening mechanism in the material. The atomic structure of the deformed models, as a function of applied strain, is analyzed to interpret these observations.

Download or read book Variations in Grain Boundary Segregation for Nanocrystalline Stability and Strength written by Oscar Figueroa (III.) and published by . This book was released on 2012 with total page 39 pages. Available in PDF, EPUB and Kindle. Book excerpt: In the last few decades, nanocrystalline metals have been of increasing interest. Their ability to show increased yield strength and uniform structure show them to be potentially useful in many applications. Additionally, nanocrystalline metals have become more easily manufactured in recent years, allowing for more testing and more use within industrial settings. However, nanocrystalline metals are still highly unstable, mainly due to temperature related growth. Grain boundary segregation is one way in which materials can keep nano length-scale grains. This process involves metal alloys that preferentially segregate the alloying material to the grain boundaries, potentially leading to Grain Boundary Embrittlement (GBE). Using an ideal work of fracture equation, [gamma] = 2[sigma]s - [sigma]g, the energy required to fracture nanocrystalline metal alloys was obtained, and predicted grain stability. Fracture toughness data is also calculated and compared. A contrast between bulk and nanocrystalline alloys is then made, showing benefits to the use of either set of materials for specific alloy functions.

Download or read book Grain Boundary Segregation and Thermal Stability of Nanocrystalline Alloys written by and published by . This book was released on 2015 with total page 18 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Grain Boundary Segregation as a Route to Stabilize Nanocrystalline Alloys written by and published by . This book was released on 2014 with total page 17 pages. Available in PDF, EPUB and Kindle. Book excerpt:

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 Atomistic Simulation of the Mechanical Behavior of Asymetric Tilt Grain Boundaries in AL and CU Bicrystals written by Farzaneh Sharifi and published by . This book was released on 2018 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: GB structural evolution including coarsening and emission of intrinsic stacking fault facets, nucleation of partial dislocation on primary and secondary slip planes, and full dislocation nucleation loops are of predominant features which are observed. Grain boundaries energy and the stress required for dislocation nucleation are also calculated which are found in agreement with similar experimental and simulation works.

Download or read book Strengthening Mechanisms of Sputtered Copper Cobalt and Their Nanocomposites written by Yue Liu and published by . This book was released on 2015 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Low energy planar defects such as twin boundaries have been employed to strengthen materials effectively with insignificant loss of the conductivity and ductility. High density growth twins can be formed in low stacking fault energy (SFE) metals, such as copper (Cu) and silver (Ag). However, low SFE metal cobalt (Co) received little attention due to the complex coexistence of hexagonal close-packed (HCP) and face-centered cubic (FCC) structure. The focus of this research is to identify the strengthening mechanisms of planar defects such as twin boundaries, stacking faults, and layer interfaces in epitaxial FCC/HCP Co, and Cu/Co multilayers. Our studies show that epitaxial Cu/Co multilayers with different texture have drastic different mechanical properties, dictated by the transmission of partial vs. full dislocations across layer interfaces. Furthermore the mechanical properties of epitaxial Co are dominated by high density stacking faults. Moreover, by applying advanced nanoindentation techniques, such as thermal-drift corrected strain-rate sensitivity measurement, the mechanical properties including strain-rate sensitivity is accurately determined. By using in situ nanoindentation under transmission electron microscope (TEM), we determined deformation physics of nanotwinned Cu, including detwinning, dislocation-twin interactions and work hardening. This project provides an important new perspective to investigate mechanical behavior of nanostructured metals with high density stacking faults. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/152621