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Understanding the Mechanistic Role of Grain Boundaries on the Strength and Deformation of Nanocrystalline Metals Using Atomistic Simulations

2019 Grain boundaries
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Release : 2019
Genre : Grain boundaries
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Book Synopsis Understanding the Mechanistic Role of Grain Boundaries on the Strength and Deformation of Nanocrystalline Metals Using Atomistic Simulations by : Satish Rajaram

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. This book was released on 2019. 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.

Atomistic Simulations of Defect Nucleation and Free Volume in Nanocrystalline Materials

2011 Computer simulation
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Release : 2011
Genre : Computer simulation
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Book Synopsis Atomistic Simulations of Defect Nucleation and Free Volume in Nanocrystalline Materials by : Garritt J. Tucker

Download or read book Atomistic Simulations of Defect Nucleation and Free Volume in Nanocrystalline Materials written by Garritt J. Tucker. This book was released on 2011. 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.

Nanocrystalline Alloys

2011
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Release : 2011
Genre :
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Book Synopsis Nanocrystalline Alloys by : Timothy John Rupert

Download or read book Nanocrystalline Alloys written by Timothy John Rupert. This book was released on 2011. 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.

Understanding Grain Boundary and Stress Concentration Effects on Strengthening Mechanisms in Nanotwinned Metals

2020 Finite element method
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Release : 2020
Genre : Finite element method
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Book Synopsis Understanding Grain Boundary and Stress Concentration Effects on Strengthening Mechanisms in Nanotwinned Metals by : Qiongjiali Fang

Download or read book Understanding Grain Boundary and Stress Concentration Effects on Strengthening Mechanisms in Nanotwinned Metals written by Qiongjiali Fang. This book was released on 2020. Available in PDF, EPUB and Kindle. Book excerpt: The superior strength and large tensile plasticity of nanotwinned (nt) face-centered-cubic metals have been explained by different twin size-dependent dislocation mechanisms and their inherent strengthening/softening effects. Grain boundary (GB) plasticity generally induces softening in nanocrystalline metals; however, our current understanding of the role of GBs in plasticity of nt metals remains limited. Contradicting reports exist in literature on how twin size influences stress concentration at GB -- twin boundary (TB) intersections, which facilitates dislocation nucleation. In this thesis, molecular dynamics (MD) simulations and finite element analysis (FEA) were used to study the effects of GB plasticity and stress concentrations, on the mechanical behaviors of four different columnar-grained nt fcc metals. First, the extent of GB plasticity was found to be the same for different TB spacing (TBS). A special GB deformation mechanism, ductile cracking along GB was observed in nt Ni at small TBS, resulting from the higher GB plasticity of nt Ni. In addition, GB plasticity also depends on the strain rate, which contributes to strain rate sensitivity in nt fcc metals. Second, it was found that stress concentration at GB -- TB intersections decreases as TBS decreases in both anisotropic elasticity theory and atomistic simulations, which is contrary to past experimental results. Stress concentration at the intersection of two regions with different TBSs could be estimated simply by geometric average of stress concentrations in adjacent regions with homogeneous CTB distributions, suggesting that the paradox in stress concentration between experiments and our simulation may be related to the uneven TB distribution in experiments. Initial nucleation of twinning partials is predicted to occur at lower strain as TB spacing decreases, when the stress concentrations are smaller, because of a transition from stress-controlled to source-controlled dislocation nucleation. Therefore, this thesis shows that it is necessary to tailor the size and distribution of grains and nanotwins to achieve ideal mechanical properties in columnar-grained nt metals.

Molecular Dynamics Simulation of Nanostructured Materials

2020-05-15 Mathematics
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Release : 2020-05-15
Genre : Mathematics
Kind : eBook
Book Rating : 966/5 ( reviews)

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Book Synopsis Molecular Dynamics Simulation of Nanostructured Materials by : Snehanshu Pal

Download or read book Molecular Dynamics Simulation of Nanostructured Materials written by Snehanshu Pal. This book was released on 2020-05-15. Available in PDF, EPUB and Kindle. Book excerpt: Molecular dynamics simulation is a significant technique to gain insight into the mechanical behavior of nanostructured (NS) materials and associated underlying deformation mechanisms at the atomic scale. The purpose of this book is to detect and correlate critically current achievements and properly assess the state of the art in the mechanical behavior study of NS material in the perspective of the atomic scale simulation of the deformation process. More precisely, the book aims to provide representative examples of mechanical behavior studies carried out using molecular dynamics simulations, which provide contributory research findings toward progress in the field of NS material technology.

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