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R.c. Pond - One of the best experts on this subject based on the ideXlab platform.
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a study of the accommodation of coherency strain by interfacial defects at a grain boundary in gold
Philosophical Magazine, 2006Co-Authors: R.c. Pond, D. L. Medlin, A SerraAbstract:The structure of a ⟨ 110 ⟩ 90° grain boundary in Au is investigated using high-resolution transmission electron microscopy (HRTEM) and atomistic simulation. It consists of coherent segments, exhibiting the extended 9R configuration described by Medlin et al. 10, with superimposed line defects to accommodate the coherency strain. Two types of defects are observed, Crystal Dislocations and disconnections, where the latter exhibit step nature in addition to dislocation character. Both types of defect are identified by HRTEM in combination with circuit mapping, and their parameters are shown to be consistent with the topological theory of interfacial defects 7. Moreover, the misfit-relieving function of observed defect arrays, their influence on interface orientation and the relative rotation of the adjacent Crystals is elucidated. During observation, defect decomposition is observed in a manner which conserves Burgers vector and step height. One of the decomposition products is glissile, consistent with the ...
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twins as barriers to basal slip in hexagonal close packed metals
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2002Co-Authors: A Serra, D J Bacon, R.c. PondAbstract:The boundary structure of {10 $$\bar 1$$ 1}, {10 $$\bar 1$$ 2}, {11 $$\bar 2$$ 1} and, {11 $$\bar 2$$ 2} twins in hexagonal-close-packed (hcp) metals and the interaction of Crystal Dislocations with the first two twin types have been studied previously using atomic-scale computer simulation. The interaction of Crystal Dislocations with {11 $$\bar 2$$ 1} and {11 $$\bar 2$$ 2} twin boundaries is described here and compared with the results for {10 $$\bar 1$$ 1} and {10 $$\bar 1$$ 2} twins. These four twins are found to create barriers to the motion of Crystal Dislocations gliding on the basal plane, and the strength of the barrier depends in a relatively complex manner on Crystallographic parameters and details of the atomic structures of the interfaces. In some circumstances, Crystal Dislocations can be transmitted through the twin boundary, thereby creating twinning Dislocations.
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Dislocations in interfaces in the h.c.p. metals - I. Defects formed by absorption of Crystal Dislocations
Acta Materialia, 1999Co-Authors: Alberto Serra, D J Bacon, R.c. PondAbstract:Atomic-scale computer simulation techniques have been used to investigate the interaction of Crystal Dislocations with two interfaces in hexagonal-close-packed (h.c.p.) metals, namely the {101??2} twin boundary and a ???12??10???/90?? tilt boundary which is incommensurate in the direction perpendicular to the tilt axis. Crystal Dislocations are always found to be absorbed in the tilt boundary with concomitant reconstruction of their cores. In the twin boundary, a broader range of interactions is observed, including defect transmission from matrix to twin and decomposition in the interface into discrete defects. The easy generation of mobile twinning Dislocations facilitates the latter behaviour. The simulations demonstrate that the core structures of localized interfacial defects exhibit preferred riser configurations. For the twin, the favoured structure is the `basal-on-prism' configuration, whereas risers in the tilt boundary resemble {101??2} twin forms. By comparing interaction processes in two interfaces, this investigation elucidates the role of Crystallographic considerations and interfacial structure. It also illustrates that the core structure of interfacial defects can be complex and contributes significantly to total defect energy.
D J Bacon - One of the best experts on this subject based on the ideXlab platform.
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interaction of a moving twin boundary with perfect Dislocations and loops in a hcp metal
Philosophical Magazine, 2010Co-Authors: Anna Serra, D J BaconAbstract:Atomic-scale computer simulation is used to investigate the interaction of a moving {1012} twin boundary in a hcp metal with either a straight 1/3 dislocation lying perpendicular to the direction of twinning shear or a periodic row of perfect dislocation loops. The screw dislocation does not decompose in the moving interface and has no effect on its motion. The 60°-mixed dislocation is attracted by the boundary and decomposes into twinning Dislocations and a disconnection (an interfacial defect with both step and dislocation character): the sign of the Crystal dislocation determines the form of the disconnection and, thus, its effect on twin boundary motion. Boundary reactions with Crystal Dislocations are likely to be important for assisting the twinning process. Loops with Burgers vector, b , parallel to the interface are reformed in the other Crystal after the twin boundary has passed through. The boundary attracts both interstitial and vacancy dislocation loops with inclined b , but is not transparent...
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twins as barriers to basal slip in hexagonal close packed metals
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2002Co-Authors: A Serra, D J Bacon, R.c. PondAbstract:The boundary structure of {10 $$\bar 1$$ 1}, {10 $$\bar 1$$ 2}, {11 $$\bar 2$$ 1} and, {11 $$\bar 2$$ 2} twins in hexagonal-close-packed (hcp) metals and the interaction of Crystal Dislocations with the first two twin types have been studied previously using atomic-scale computer simulation. The interaction of Crystal Dislocations with {11 $$\bar 2$$ 1} and {11 $$\bar 2$$ 2} twin boundaries is described here and compared with the results for {10 $$\bar 1$$ 1} and {10 $$\bar 1$$ 2} twins. These four twins are found to create barriers to the motion of Crystal Dislocations gliding on the basal plane, and the strength of the barrier depends in a relatively complex manner on Crystallographic parameters and details of the atomic structures of the interfaces. In some circumstances, Crystal Dislocations can be transmitted through the twin boundary, thereby creating twinning Dislocations.
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Atomic-scale modeling of Dislocations and related properties in the hexagonal-close-packed metals
2002Co-Authors: D J Bacon, Vaclav VitekAbstract:Metals with the hcp Crystal structure have a wide variety of mechanical and physical properties, and understanding the links between atomic processes, microstructure, and properties can open the way for new applications. Computer modeling can provide much of the information required. This article reviews recent progress in atomic-scale computer simulation in three important areas. The first is the core structure of Dislocations responsible for the primary slip modes, where modeling has revealed the variety of core states that can arise in pure, elemental metals and ordered alloys. While most research has successfully employed many-body, central-force interatomic potentials, they are inadequate for metals which have an unfilled d-electron band, such as alpha-Ti and alpha-Zr, and the resulting noncentral character of the atomic bonding is shown to have subtle yet significant effects on dislocation properties. Deformation twinning is an important process in plasticity of the hcp metals, and modeling has been used to investigate the factors that control the structure and mobility of twinning Dislocations. Furthermore, simulation shows that twinning Dislocations are actually generated, in some cases, following the interaction of Crystal Dislocations with twin boundaries; this can lead to the very mobile boundaries observed experimentally. The final area concerns the nature and properties of the defects created by radiation damage. Computer simulation has been used to determine the number and arrangement of defects produced in primary, displacement-cascade damage in several hcp metals. The number is similar to that found in cubic metals and is considerably smaller than that expected from earlier models. Many self-interstitial atoms cluster in cascades to form highly glissile dislocation loops, and, so, contribute to two-dimensional material transport in damage evolution.
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Dislocations in interfaces in the h.c.p. metals - I. Defects formed by absorption of Crystal Dislocations
Acta Materialia, 1999Co-Authors: Alberto Serra, D J Bacon, R.c. PondAbstract:Atomic-scale computer simulation techniques have been used to investigate the interaction of Crystal Dislocations with two interfaces in hexagonal-close-packed (h.c.p.) metals, namely the {101??2} twin boundary and a ???12??10???/90?? tilt boundary which is incommensurate in the direction perpendicular to the tilt axis. Crystal Dislocations are always found to be absorbed in the tilt boundary with concomitant reconstruction of their cores. In the twin boundary, a broader range of interactions is observed, including defect transmission from matrix to twin and decomposition in the interface into discrete defects. The easy generation of mobile twinning Dislocations facilitates the latter behaviour. The simulations demonstrate that the core structures of localized interfacial defects exhibit preferred riser configurations. For the twin, the favoured structure is the `basal-on-prism' configuration, whereas risers in the tilt boundary resemble {101??2} twin forms. By comparing interaction processes in two interfaces, this investigation elucidates the role of Crystallographic considerations and interfacial structure. It also illustrates that the core structure of interfacial defects can be complex and contributes significantly to total defect energy.
J W Morris - One of the best experts on this subject based on the ideXlab platform.
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Nanomechanical Testing of Gum Metal
Experimental Mechanics, 2010Co-Authors: E. A. Withey, A. M. Minor, S. Kuramoto, D. C. Chrzan, J W MorrisAbstract:“Gum Metal” is a newly developed β-Ti alloy that, in the cold-worked condition, has exceptional elastic elongation and high strength. The available evidence suggests that Gum Metal does not yield until the applied stress approaches the ideal strength, and then deforms by mechanisms that do not involve conventional Crystal Dislocations. To study its behavior, submicron-sized pillars of solution-treated and cold-worked Gum Metal were compressed in situ in a quantitative compression stage in a transmission electron microscope. Solution-treated specimens and half of the cold-worked specimens exhibited essentially monotonic hardening during compression, but with serrated load-deflection curves that included periodic partial relaxations of the stress. The other cold-worked specimens exhibited pronounced shear instability. These samples deformed by a stick-slip motion along a well-defined shear plane, with a serrated load-deflection curve demonstrating partial stress relaxation at each sliding event. The pattern of deformation is consistent with prior work showing deformation by the formation and growth of shear bands and faults in a matrix that is densely decorated with defects.
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the deformation of gum metal in nanoindentation
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2008Co-Authors: E Withey, Shigeru Kuramoto, A. M. Minor, D. C. Chrzan, M Jin, J W MorrisAbstract:Abstract “Gum Metal” describes a newly developed set of alloys with nominal composition Ti–24(Nb + V + Ta)–(Zr,Hf)–O. In the cold-worked condition these alloys have exceptional elastic elongation and high-strength; the available evidence suggests that they do not yield until the applied stress approaches the ideal strength of the alloy, and then deform by mechanisms that do not involve conventional Crystal Dislocations. The present paper reports research on the nanoindentation of this material in both the cold-worked and annealed conditions. Nanoindentation tests were conducted in situ in a transmission electron microscope (TEM) stage that allows the deformation process to be observed in real time, and ex situ in a Hysitron nanoindenter, with samples subsequently extracted for high-resolution TEM study. The results reveal unusual deformation patterns beneath the nanoindenter that are, to our knowledge, unique to this material. In the cold-worked alloy deformation is confined to the immediate neighborhood of the indentation, with no evidence of dislocation, twin or fault propagation into the bulk. The deformed volume is highly inhomogeneous; the deformation is accomplished by a series of incremental rotations that are ordinarily resolved into discrete nanodomains. The annealed material deforms in a similar way within the nanoindentation pit, but Dislocations emanate from the pit boundary. These are pinned by microstructural barriers only a few nanometers apart, a condition that recent theory suggests is necessary for the material to achieve ideal strength.
Wojtowytsch Stephan - One of the best experts on this subject based on the ideXlab platform.
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The effect of forest Dislocations on the evolution of a phase-field model for plastic slip
'Springer Science and Business Media LLC', 2019Co-Authors: Kurzke Matthias, Dondl Patrick, Wojtowytsch StephanAbstract:We consider the gradient flow evolution of a phase-field model for Crystal Dislocations in a single slip system in the presence of forest Dislocations. The model is based on a Peierls-Nabarro type energy penalizing non-integer slip and elastic stress. Forest Dislocations are introduced as a perforation of the domain by small disks where slip is prohibited. The Γ-limit of this energy was deduced by Garroni and Müller (2005 and 2006). Our main result shows that the gradient flows of these Γ-convergent energy functionals do not approach the gradient flow of the limiting energy. Indeed, the gradient flow dynamics remains a physically reasonable model in the case of non-monotone loading. Our proofs rely on the construction of explicit sub- and super-solutions to a fractional Allen-Cahn equation on a flat torus or in the plane, with Dirichlet data on a union of small discs. The presence of these obstacles leads to an additional friction in the viscous evolution which appears as a stored energy in the Γ-limit, but it does not act as a driving force. Extensions to related models with soft pinning and non-viscous evolutions are also discussed. In terms of physics, our results explain how in this phase field model the presence of forest Dislocations still allows for plastic as opposed to only elastic deformation
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The Effect of Forest Dislocations on the Evolution of a Phase-Field Model for Plastic Slip
'Springer Science and Business Media LLC', 2017Co-Authors: Dondl, Patrick W., Kurzke, Matthias W., Wojtowytsch StephanAbstract:We consider the gradient flow evolution of a phase-field model for Crystal Dislocations in a single slip system in the presence of forest Dislocations. The model consists of a Peierls-Nabarro type energy penalizing non-integer slip and elastic stress. Forest Dislocations are introduced as a perforation of the domain by small disks where slip is prohibited. The $\Gamma$-limit of this energy was deduced by Garroni and M\"uller (2005 and 2006). Our main result shows that the gradient flows of these $\Gamma$-convergent energy functionals do not approach the gradient flow of the limiting energy. Indeed, the gradient flow dynamics remains a physically reasonable model in the case of non-monotone loading. Our proofs rely on the construction of explicit sub- and super-solutions to a fractional Allen-Cahn equation on a flat torus or in the plane, with Dirichlet data on a union of small discs. The presence of these obstacles leads to an additional friction in the viscous evolution which appears as a stored energy in the $\Gamma$-limit, but it does not act as a driving force. Extensions to related models with soft pinning and non-viscous evolutions are also discussed. In terms of physics, our results explain how in this phase field model the presence of forest Dislocations still allows for plastic as opposed to only elastic deformation
Mildred S. Dresselhaus - One of the best experts on this subject based on the ideXlab platform.
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electron energy can oscillate near a Crystal dislocation
New Journal of Physics, 2017Co-Authors: Mingda Li, Mildred S. DresselhausAbstract:Crystal Dislocations govern the plastic mechanical properties of materials but also affect the electrical and optical properties. However, a fundamental and quantitative quantum field theory of a dislocation has remained undiscovered for decades. Here we present an exactly-solvable one-dimensional quantum field theory of a dislocation, for both edge and screw Dislocations in an isotropic medium, by introducing a new quasiparticle which we have called the 'dislon'. The electron-dislocation relaxation time can then be studied directly from the electron self-energy calculation, which is reducible to classical results. In addition, we predict that the electron energy will experience an oscillation pattern near a dislocation. Compared with the electron density's Friedel oscillation, such an oscillation is intrinsically different since it exists even with only single electron is present. With our approach, the effect of Dislocations on materials' non-mechanical properties can be studied at a full quantum field theoretical level.
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canonical quantization of Crystal dislocation and electron dislocation scattering in an isotropic medium
arXiv: Materials Science, 2015Co-Authors: Wenping Cui, Mildred S. Dresselhaus, Gang ChenAbstract:Crystal Dislocations govern the plastic mechanical properties of materials but also affect the electrical and optical properties. However, a fundamental and quantitative quantum-mechanical theory of dislocation remains undiscovered for decades. Here we present an exactly solvable quantum field theory of dislocation, for both edge and screw Dislocations in an isotropic medium by introducing a new quasiparticle "dislon". With this approach, the electron-dislocation relaxation time is studied from electron self-energy which can be reduced to classical results. Moreover, a fundamentally new type of electron energy Friedel oscillation near dislocation core is predicted, which can occur even with single electron at present. For the first time, the effect of Dislocations on materials' non-mechanical properties can be studied at a full quantum field theoretical level.