The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Yuichi Ikuhara - One of the best experts on this subject based on the ideXlab platform.
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structure of Screw Dislocations in a 0 0 0 1 0 0 0 1 low angle twist grain boundary of alumina α al 2 o 3
Acta Materialia, 2012Co-Authors: Eita Tochigi, Yuki Kezuka, Atsutomo Nakamura, Noriyoshi Shibata, Yuichi IkuharaAbstract:An alumina (α-Al2O3) bicrystal with a (0 0 0 1)/[0 0 0 1] low-angle twist grain boundary was fabricated by bonding two (0 0 0 1) substrates at 1500 °C in air. Dislocation structures in the resultant grain boundary were extensively observed in plan view and cross-sectional direction by transmission electron microscopy (TEM). The grain boundary consisted of a hexagonal dislocation network of 1/3〈12¯10〉 basal Screw Dislocations and wavy belt structures. The atomic core structure of the Screw Dislocations was analyzed in end-on view by high-resolution TEM. It was evident that the 1/3〈12¯10〉 basal Screw dislocation does not dissociate into partial Dislocations, which is in contrast to the 1/3〈12¯10〉 basal edge dislocation that dissociates into 1/3〈11¯00〉 and 1/3〈01¯10〉 partial Dislocations. Also, TEM observation revealed that the wavy belt structures correspond to planar voids. From the morphology of the planar voids, it is considered that they accommodate a slight tilt component of the boundary.
Yumi H Ikuhara - One of the best experts on this subject based on the ideXlab platform.
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structure of Screw Dislocations in a 0 0 0 1 0 0 0 1 low angle twist grain boundary of alumina α al2o3
Acta Materialia, 2012Co-Authors: Eita Tochigi, Yuki Kezuka, Naoya Shibata, Atsutomo Nakamura, Yumi H IkuharaAbstract:An alumina (α-Al2O3) bicrystal with a (0 0 0 1)/[0 0 0 1] low-angle twist grain boundary was fabricated by bonding two (0 0 0 1) substrates at 1500 °C in air. Dislocation structures in the resultant grain boundary were extensively observed in plan view and cross-sectional direction by transmission electron microscopy (TEM). The grain boundary consisted of a hexagonal dislocation network of 1/3〈12¯10〉 basal Screw Dislocations and wavy belt structures. The atomic core structure of the Screw Dislocations was analyzed in end-on view by high-resolution TEM. It was evident that the 1/3〈12¯10〉 basal Screw dislocation does not dissociate into partial Dislocations, which is in contrast to the 1/3〈12¯10〉 basal edge dislocation that dissociates into 1/3〈11¯00〉 and 1/3〈01¯10〉 partial Dislocations. Also, TEM observation revealed that the wavy belt structures correspond to planar voids. From the morphology of the planar voids, it is considered that they accommodate a slight tilt component of the boundary.
V Vitek - One of the best experts on this subject based on the ideXlab platform.
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single crystal yield criterion for chromium based on atomistic studies of isolated 1 2 111 Screw Dislocations
International Journal of Plasticity, 2020Co-Authors: Roman Gröger, V VitekAbstract:Abstract Using the recently developed Bond Order Potential for Cr, we investigate the mobility of isolated 1/2[111] Screw Dislocations under stress at 0 K. These studies provide the dependence of the critical resolved shear stress (i.e., the Peierls stress) on the orientation of the maximum resolved shear stress plane and on the shear stress perpendicular to the slip direction. Similar studies are made for loading in tension and compression to show that the onset of yielding in Cr single crystals can be described using a combination of the shear stresses parallel and perpendicular to the slip direction. Based on these results, we develop a single crystal yield criterion for Cr that includes all non-Schmid contributions to the Peierls stress. This criterion is used to predict the shape of the yield surface and compare it with the regular Tresca's hexagon arising from the Schmid law. For all directions of uniaxial load, the criterion is used to quantify the character and magnitude of the tension-compression asymmetry.
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stress dependence of the peierls barrier of 1 2 1 1 1 Screw Dislocations in bcc metals
Acta Materialia, 2013Co-Authors: Roman Gröger, V VitekAbstract:Abstract The recently formulated constrained nudged elastic band method with atomic relaxations (NEB + r) (Groger R, Vitek V. Model Simul Mater Sci Eng 2012;20:035019) is used to investigate the dependence of the Peierls barrier of 1/2〈1 1 1〉 Screw Dislocations in body-centered cubic metals on non-glide stresses. These are the shear stresses parallel to the slip direction acting in the planes of the 〈1 1 1〉 zone different from the slip plane, and the shear stresses perpendicular to the slip direction. Both these shear stresses modify the structure of the dislocation core and thus alter both the Peierls barrier and the related Peierls stress. Understanding of this effect of loading is crucial for the development of mesoscopic models of thermally activated dislocation motion via formation and propagation of pairs of kinks. The Peierls stresses and related choices of the glide planes determined from the Peierls barriers agree with the results of molecular statics calculations (Groger R, Bailey AG, Vitek V. Acta Mater 2008;56:5401), which demonstrates that the NEB + r method is a reliable tool for determining the variation in the Peierls barrier with the applied stress. However, such calculations are very time consuming, and it is shown here that an approximate approach of determining the stress dependence of the Peierls barrier (proposed in Groger R, Vitek V. Acta Mater 2008;56:5426) can be used, combined with test calculations employing the NEB + r method.
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effect of eshelby twist on core structure of Screw Dislocations in molybdenum atomic structure and electron microscope image simulations
Philosophical Magazine, 2011Co-Authors: Roman Gröger, P B Hirsch, P D Nellist, K J Dudeck, V Vitek, D J H CockayneAbstract:This paper addresses the question as to whether the core structure of Screw Dislocations in Mo in the bulk can be obtained from high-resolution electron microscopy (HREM) images of such Dislocations viewed end-on in a thin foil. Atomistic simulations of the core structure of Screw Dislocations in elastically anisotropic Mo were carried out using bond order potentials. These simulations take account automatically of the effects of the surface relaxation displacements (anisotropic Eshelby twist). They show that the differential displacements of the atoms at the surface are different with components perpendicular to the Burgers vector about five times larger than those in the middle of the foil, the latter being characteristic of the bulk. Nye tensor plots show that the surface relaxation stresses strongly affect the incompatible distortions. HREM simulations of the computed structure reflect the displacements at the exit surface, modified by interband scattering and the microscope transfer function. Nye ten...
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directional versus central force bonding in studies of the structure and glide of 1 2 111 Screw Dislocations in bcc transition metals
Philosophical Magazine, 2009Co-Authors: Roman Gröger, V VitekAbstract:In this paper, we address the differences between Finnis–Sinclair potentials and bond-order potentials (BOPs) when studying 1/2⟨111⟩ Screw Dislocations in bcc transition metals, specifically Mo and W. These two types of potentials differ in that the former is central-force, whereas the latter include angular bonding. The cores of 1/2⟨111⟩ Screw Dislocations have two variants, one invariant with respect to the ⟨101⟩ diad and the other not. Hence, the latter core is degenerate. The BOPs always lead to the invariant type, whereas for Finnis–Sinclair potentials both variants occur. However, the symmetry of the core does not play a decisive role in the glide of Dislocations. It is the description of interatomic forces that governs both the core structure and the glide behaviour of Dislocations. The general characteristics of dislocation glide, the twinning–antitwinning asymmetry and a lower Peierls stress for tension than compression are the same for both types of potentials. Whereas the results obtained with ...
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core structure of Screw Dislocations in body centred cubic metals relation to symmetry and interatomic bonding
Philosophical Magazine, 2004Co-Authors: V VitekAbstract:In this paper we discuss both the crystallographic aspects that govern the general features of the cores of ⟨111⟩ Screw Dislocations in bcc metals and the role of interatomic bonding that is specific to given materials. This analysis is carried out by comparing the results of two atomistic calculations of Dislocations in molybdenum, one performed using many-body central force potentials and the other bond-order potentials that include the angular dependence of interatomic interactions. In both cases the core spreads into three {110} planes of the [111] zone but in one case it is unique and invariant with respect to the ⟨101⟩ type diad, a symmetry operation of the bcc lattice, and in the second case two distinct configurations exist that are related by the diad operation. Which of the structures is found depends on interatomic interactions and it is shown that the γ surface for {110} planes can be used to predict the type of the core spreading. We then demonstrate that both core structures may lead to very...
Eita Tochigi - One of the best experts on this subject based on the ideXlab platform.
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structure of Screw Dislocations in a 0 0 0 1 0 0 0 1 low angle twist grain boundary of alumina α al2o3
Acta Materialia, 2012Co-Authors: Eita Tochigi, Yuki Kezuka, Naoya Shibata, Atsutomo Nakamura, Yumi H IkuharaAbstract:An alumina (α-Al2O3) bicrystal with a (0 0 0 1)/[0 0 0 1] low-angle twist grain boundary was fabricated by bonding two (0 0 0 1) substrates at 1500 °C in air. Dislocation structures in the resultant grain boundary were extensively observed in plan view and cross-sectional direction by transmission electron microscopy (TEM). The grain boundary consisted of a hexagonal dislocation network of 1/3〈12¯10〉 basal Screw Dislocations and wavy belt structures. The atomic core structure of the Screw Dislocations was analyzed in end-on view by high-resolution TEM. It was evident that the 1/3〈12¯10〉 basal Screw dislocation does not dissociate into partial Dislocations, which is in contrast to the 1/3〈12¯10〉 basal edge dislocation that dissociates into 1/3〈11¯00〉 and 1/3〈01¯10〉 partial Dislocations. Also, TEM observation revealed that the wavy belt structures correspond to planar voids. From the morphology of the planar voids, it is considered that they accommodate a slight tilt component of the boundary.
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structure of Screw Dislocations in a 0 0 0 1 0 0 0 1 low angle twist grain boundary of alumina α al 2 o 3
Acta Materialia, 2012Co-Authors: Eita Tochigi, Yuki Kezuka, Atsutomo Nakamura, Noriyoshi Shibata, Yuichi IkuharaAbstract:An alumina (α-Al2O3) bicrystal with a (0 0 0 1)/[0 0 0 1] low-angle twist grain boundary was fabricated by bonding two (0 0 0 1) substrates at 1500 °C in air. Dislocation structures in the resultant grain boundary were extensively observed in plan view and cross-sectional direction by transmission electron microscopy (TEM). The grain boundary consisted of a hexagonal dislocation network of 1/3〈12¯10〉 basal Screw Dislocations and wavy belt structures. The atomic core structure of the Screw Dislocations was analyzed in end-on view by high-resolution TEM. It was evident that the 1/3〈12¯10〉 basal Screw dislocation does not dissociate into partial Dislocations, which is in contrast to the 1/3〈12¯10〉 basal edge dislocation that dissociates into 1/3〈11¯00〉 and 1/3〈01¯10〉 partial Dislocations. Also, TEM observation revealed that the wavy belt structures correspond to planar voids. From the morphology of the planar voids, it is considered that they accommodate a slight tilt component of the boundary.
Roman Gröger - One of the best experts on this subject based on the ideXlab platform.
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single crystal yield criterion for chromium based on atomistic studies of isolated 1 2 111 Screw Dislocations
International Journal of Plasticity, 2020Co-Authors: Roman Gröger, V VitekAbstract:Abstract Using the recently developed Bond Order Potential for Cr, we investigate the mobility of isolated 1/2[111] Screw Dislocations under stress at 0 K. These studies provide the dependence of the critical resolved shear stress (i.e., the Peierls stress) on the orientation of the maximum resolved shear stress plane and on the shear stress perpendicular to the slip direction. Similar studies are made for loading in tension and compression to show that the onset of yielding in Cr single crystals can be described using a combination of the shear stresses parallel and perpendicular to the slip direction. Based on these results, we develop a single crystal yield criterion for Cr that includes all non-Schmid contributions to the Peierls stress. This criterion is used to predict the shape of the yield surface and compare it with the regular Tresca's hexagon arising from the Schmid law. For all directions of uniaxial load, the criterion is used to quantify the character and magnitude of the tension-compression asymmetry.
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stress dependence of the peierls barrier of 1 2 1 1 1 Screw Dislocations in bcc metals
Acta Materialia, 2013Co-Authors: Roman Gröger, V VitekAbstract:Abstract The recently formulated constrained nudged elastic band method with atomic relaxations (NEB + r) (Groger R, Vitek V. Model Simul Mater Sci Eng 2012;20:035019) is used to investigate the dependence of the Peierls barrier of 1/2〈1 1 1〉 Screw Dislocations in body-centered cubic metals on non-glide stresses. These are the shear stresses parallel to the slip direction acting in the planes of the 〈1 1 1〉 zone different from the slip plane, and the shear stresses perpendicular to the slip direction. Both these shear stresses modify the structure of the dislocation core and thus alter both the Peierls barrier and the related Peierls stress. Understanding of this effect of loading is crucial for the development of mesoscopic models of thermally activated dislocation motion via formation and propagation of pairs of kinks. The Peierls stresses and related choices of the glide planes determined from the Peierls barriers agree with the results of molecular statics calculations (Groger R, Bailey AG, Vitek V. Acta Mater 2008;56:5401), which demonstrates that the NEB + r method is a reliable tool for determining the variation in the Peierls barrier with the applied stress. However, such calculations are very time consuming, and it is shown here that an approximate approach of determining the stress dependence of the Peierls barrier (proposed in Groger R, Vitek V. Acta Mater 2008;56:5426) can be used, combined with test calculations employing the NEB + r method.
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effect of eshelby twist on core structure of Screw Dislocations in molybdenum atomic structure and electron microscope image simulations
Philosophical Magazine, 2011Co-Authors: Roman Gröger, P B Hirsch, P D Nellist, K J Dudeck, V Vitek, D J H CockayneAbstract:This paper addresses the question as to whether the core structure of Screw Dislocations in Mo in the bulk can be obtained from high-resolution electron microscopy (HREM) images of such Dislocations viewed end-on in a thin foil. Atomistic simulations of the core structure of Screw Dislocations in elastically anisotropic Mo were carried out using bond order potentials. These simulations take account automatically of the effects of the surface relaxation displacements (anisotropic Eshelby twist). They show that the differential displacements of the atoms at the surface are different with components perpendicular to the Burgers vector about five times larger than those in the middle of the foil, the latter being characteristic of the bulk. Nye tensor plots show that the surface relaxation stresses strongly affect the incompatible distortions. HREM simulations of the computed structure reflect the displacements at the exit surface, modified by interband scattering and the microscope transfer function. Nye ten...
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directional versus central force bonding in studies of the structure and glide of 1 2 111 Screw Dislocations in bcc transition metals
Philosophical Magazine, 2009Co-Authors: Roman Gröger, V VitekAbstract:In this paper, we address the differences between Finnis–Sinclair potentials and bond-order potentials (BOPs) when studying 1/2⟨111⟩ Screw Dislocations in bcc transition metals, specifically Mo and W. These two types of potentials differ in that the former is central-force, whereas the latter include angular bonding. The cores of 1/2⟨111⟩ Screw Dislocations have two variants, one invariant with respect to the ⟨101⟩ diad and the other not. Hence, the latter core is degenerate. The BOPs always lead to the invariant type, whereas for Finnis–Sinclair potentials both variants occur. However, the symmetry of the core does not play a decisive role in the glide of Dislocations. It is the description of interatomic forces that governs both the core structure and the glide behaviour of Dislocations. The general characteristics of dislocation glide, the twinning–antitwinning asymmetry and a lower Peierls stress for tension than compression are the same for both types of potentials. Whereas the results obtained with ...
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Multiscale modeling of plastic deformation of molybdenum and tungsten: II. Yield criterion for single crystals based on atomistic studies of glide of 1/2 Screw Dislocations
arXiv: Materials Science, 2008Co-Authors: Roman Gröger, John L. Bassani, V. Racherla, Vaclav VitekAbstract:Based on the atomistic studies presented in Part I we develop analytical yield criteria for single crystals that capture the effect of shear stresses other than the Schmid stress (non-glide stresses) on the shear stress needed for dislocation glide (Peierls stress). These yield criteria characterize a non-associated plastic flow that originates owing to the complex response of 1/2 Screw Dislocations to an applied stress tensor. Employing these criteria we identify the operative slip systems for tensile/compressive loading along various axes within the standard stereographic triangle and determine the ensuing tension-compression asymmetry. This result is in an excellent qualitative agreement with available experimental data. Moreover, using the constructed yield criteria within the Taylor homogenization procedure, we demonstrate that effects associated with non-planar cores of Screw Dislocations persist in random polycrystals. This affects significantly critical phenomena such as shear localization, which is demonstrated by analyzing the cavitation in a ductile plastic solid.