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Timothy J Rupert - One of the best experts on this subject based on the ideXlab platform.
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effect of grain boundary character on segregation induced structural transitions
Physical Review B, 2016Co-Authors: Timothy J RupertAbstract:Segregation-induced structural transitions in metallic grain boundaries are studied with hybrid atomistic Monte Carlo/molecular dynamics simulations using Cu-Zr as a model system, with a specific emphasis on understanding the effect of grain boundary character. With increasing global composition, the six grain boundary types chosen for this study first form ordered complexions, with the local segregation pattern depending on the grain boundary core structure, then transform into disordered complexions when the grain boundary composition reaches a critical value that is temperature dependent. The tendency for this transition to a disordered interfacial structure consistently depends on the relative solute excess, instead of the grain boundary energy or misorientation angle. Grain boundaries with high relative solute excess go through gradual disordering transitions, whereas those with low relative solute excess remain ordered until high global Zr concentrations but then abruptly transform into thick disordered Films. The results presented here provide a clear picture of the effect of interface character on both dopant segregation patterns and disordered Intergranular Film formation, showing that all grain boundaries are not equal when discussing complexion transitions.
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effect of grain boundary character on segregation induced structural transitions
Physical Review B, 2016Co-Authors: Zhiliang Pan, Timothy J RupertAbstract:Segregation-induced structural transitions in metallic grain boundaries are studied with hybrid atomistic Monte Carlo/molecular dynamics simulations using Cu-Zr as a model system, with a specific emphasis on understanding the effect of grain boundary character. With increasing global composition, the six grain boundary types chosen for this paper first form ordered complexions, with the local segregation pattern depending on the grain boundary core structure, then transform into disordered complexions when the grain boundary composition reaches a critical value that is temperature dependent. The tendency for this transition to a disordered interfacial structure consistently depends on the relative solute excess, instead of the grain boundary energy or misorientation angle. Grain boundaries with high relative solute excess go through gradual disordering transitions, whereas those with low relative solute excess remain ordered until high global Zr concentrations but then abruptly transform into thick disordered Films. The results presented here provide a clear picture of the effect of interface character on both dopant segregation patterns and disordered Intergranular Film formation, showing that all grain boundaries are not equal when discussing complexion transitions.
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amorphous Intergranular Films act as ultra efficient point defect sinks during collision cascades
Scripta Materialia, 2016Co-Authors: Joseph E Ludy, Timothy J RupertAbstract:Atomistic simulations are used to explore the effect of interfacial structure on residual radiation damage. Specifically, an ordered grain boundary is compared to a disordered amorphous Intergranular Film, to investigate how interface thickness and free volume impacts point defect recombination. Collision cascades are simulated and residual point defect populations are analyzed as a function of boundary type and primary knock on atom energy. While ordered grain boundaries easily absorb interstitials, these interfaces are inefficient vacancy sinks. Alternatively, amorphous Intergranular Films act as ultra-efficient, unbiased defect sinks, providing a path for the creation of radiation-tolerant materials.
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amorphous Intergranular Films act as ultra efficient point defect sinks during collision cascades
arXiv: Materials Science, 2015Co-Authors: Joseph E Ludy, Timothy J RupertAbstract:Atomistic simulations are used to explore the effect of interfacial structure on residual radiation damage. Specifically, an ordered grain boundary is compared to a disordered amorphous Intergranular Film, to investigate how interface thickness and free volume impacts point defect recombination. The collision cascades induced by neutron bombardment are simulated and residual point defect populations are analyzed as a function of boundary type and primary knock on atom energy. While ordered grain boundaries easily absorb interstitials, these interfaces are inefficient vacancy sinks. Alternatively, amorphous Intergranular Films act as ultra-efficient, unbiased defect sinks, providing a path for the creation of radiation-tolerant materials.
Stephen H Garofalini - One of the best experts on this subject based on the ideXlab platform.
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molecular dynamics simulations of the effect of the composition of the Intergranular Film on fracture in si3n4
Journal of the American Ceramic Society, 2010Co-Authors: Stephen H Garofalini, Shenghong ZhangAbstract:Molecular dynamics computer simulations were used to study the fracture behavior of silica, multicomponent silicate, and oxynitride Intergranular Films (IGFs) between silicon nitride crystals as a function of composition, Film thickness, and crystallographic orientation. The maximum fracture strength is higher in the IGF between prism surfaces than between basal surfaces. This is caused by the preferential segregation of specific species to the basal surfaces in contrast to the prism surfaces, effectively modifying the composition within the glassy portion of the IGF, with a subsequent effect on strength. The ordering observed in the thinner IGFs causes an increase in strength in the linear portions of the stress/strain curves, effectively increasing fracture stress. The simulations show that the force/atom in the 1 nm SiO2 IGF in the direction of tensile strain is more similar to that in β-cristobalite than to v-SiO2, whereas such data in the 2 nm SiO2 IGF is more similar to v-SiO2. Simulations of the multicomponent silicate IGFs show the expected effect of lowering fracture strength with increasing modifier content in the silicate IGF, similar to that of bulk glasses. Similar to experimental studies, a decrease in strength is observed in silicon oxynitride IGFs with increasing oxygen concentration.
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effect of thickness of the Intergranular Film on fracture in si3n4
Journal of the American Ceramic Society, 2009Co-Authors: Shenghong Zhang, Stephen H GarofaliniAbstract:Molecular dynamics computer simulations were used to study the fracture behavior of silica Intergranular Films (IGFs) between silicon nitride crystals as a function of Film thickness. Results showed a significant increase in fracture stress with decreasing IGF thickness. IGFs that are 2 nm thick fracture similarly to bulk silica glass, while the 1 nm IGF fractured at a much higher value. The simulations show bond rupture and rearrangement of siloxane rings that coalescence to form larger rings, or “voids.” The delineating difference in the systems is the significantly larger concentration of six-membered rings in the 1 nm IGF in comparison to the 2 nm IGF or bulk silica glass. The Si–O bond is more stable in six-membered rings than other ring sizes. Rupture is more catastrophic in the 1 nm IGF than the other systems where stress decays more slowly with strain.
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molecular dynamics simulations of calcium aluminosilicate Intergranular Films on 0001 al2o3 facets
Journal of the American Ceramic Society, 2004Co-Authors: David A Litton, Stephen H GarofaliniAbstract:Molecular dynamics simulations of Intergranular Films (IGF) containing SiO 2 , Al 2 O 3 , and CaO in contact with two surface terminations of the basal plane of Al 2 O 3 were performed to model faceted grain boundaries in sintered Al 2 O 3 . In both the aluminum-terminated and the oxygen-terminated crystal surfaces, cage structures were observed in the Intergranular Film at the interface. Complete epitaxy of aluminum and silicon cations from the IGF was observed on the oxygen termination of the crystal surface. Calcium segregated to specific sites at the interface in all systems studied. Segregation of aluminum ions to the interface was observed from IGFs with high Al 2 O 3 content. High-SiO 2 IGFs impeded the growth of the first of the two aluminum layers parallel to the basal plane, whereas CaO promoted the growth of this layer. However, CaO impeded the growth of the second aluminum layer parallel to the basal plane.
Hansjoachim Kleebe - One of the best experts on this subject based on the ideXlab platform.
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viscous slip along grain boundaries in chlorine doped silicon nitride
Journal of the American Ceramic Society, 2005Co-Authors: Giuseppe Pezzotti, Kenichi Ota, Hansjoachim KleebeAbstract:The effect of chlorine doping on the anelastic-relaxation and torsional-creep behavior of a silicon nitride (Si{sub 3}N{sub 4}) polycrystalline body was studied. Two model polycrystals--one undoped and the other doped with a small fracture of chlorine--were investigated. Their microstructures consisted of equiaxed and well-faceted Si{sub 3}N{sub 4} grains whose boundaries were separated by a continuous, nanometer-sized Film of silica (SiO{sub 2}) glass. The actual presence of chlorine in the doped polycrystal was ascertained by ion chromatography and is thought to be enriched at the grain boundaries. The effect o chlorine on the Intergranular Film structure was characterized by high-resolution electron microscopy. The micromechanical response of the SiO{sub 2} grain boundary under shear stress was monitored up to very high temperatures (i.e., {approximately}2,000 C) by internal-friction and torsional-creep experiments. The presence of the chlorine dopant, which is a network modifier of SiO{sub 2} glass that also causes a widening of the grain-boundary Film, significantly lowered the bulk viscosity of the residual glass. As a consequence of the change in grain-boundary chemistry, the internal-friction curve of the chlorine-doped material shifted toward lower temperatures and the torsional-creep rate markedly increased, as compared to the undoped material. According to a viscoelastic model of themore » Si{sub 3}N{sub 4} polycrystal, the internal-friction data resulted as a superposition of two individual components: (i) a relaxation peak that is related to the anelastic slip mechanism along grain boundaries and (ii) a background component that results from an irreversible diffusional-creep mechanism.« less
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Calcium Concentration Dependence of the Intergranular Film Thickness in Silicon Nitride
Journal of the American Ceramic Society, 1994Co-Authors: Isao Tanaka, Hansjoachim Kleebe, David R Clarke, Michael K. Cinibulk, John Bruley, M RuhleAbstract:High-resolution electron microscopy and nano-beam analytical electron microscopy have been used to characterize both the Intergranular silicate Film thickness and its local composition in a series of high-purity Si3N4 ceramics doped with 0–450 at. ppm Ca. Calcium was detected at both two-grain junctions and triple junctions, even in the 80-ppm-Ca-doped specimen. The thickness of the Intergranular Film at two-grain junctions was found to depend sensitively on Ca content. In undoped material, the thickness was 1.0 ± 0.1 nm. With increasing Ca content, the thickness decreased in the dilute region (80 ppm Ca), but then increased. The variation in Film thickness can be qualitatively understood in terms of the balance of three long-range forces acting normal to the Film, namely the van der Waals dispersion force, a structural “steric” force, and an electrical-double-layer force. By comparing the measured thicknesses to those predicted, estimates for the structural correlation length and the inverse Debye length can be made. These estimates have values of ∼ 0.22 nm and approximately 0.3–0.5 nm, respectively, for the calcia-free and 80 ppm calcia materials.
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high resolution electron microscopy observations of grain boundary Films in silicon nitride ceramics
MRS Proceedings, 1992Co-Authors: Hansjoachim Kleebe, David R Clarke, R M Cannon, Isao Tanaka, Michael K. Cinibulk, Michael J Hoffmann, J Bruley, M RuhleAbstract:Characterization of silicon nitride ceramics by transmission electron microscopy (TEM) provides structural and compositional information on Intergranular phases necessary to elucidate the factors that can influence the presence and thickness of grain-boundary Films. Different TEM techniques can be used for the detection and determination of Intergranular-Film thickness, however, the most accurate results are obtained by high-resolution electron microscopy (HREM). HREM studies were applied, in conjunction with analytical electron microscopy, to investigate the correlation between Intergranular-phase composition and Film thickness. Statistical analyses of a number of grain-boundary Films provided experimental verification of a theoretical equilibrium Film thickness. Model experiments on a high-purity Si 3 N 4 material, doped with low amounts of Ca, suggest the presence of two repulsive forces, a steric force and a force produced by an electrical double layer, that may act to balance the attractive van der Waals force necessary to establish an equilibrium Film thickness.
David R Clarke - One of the best experts on this subject based on the ideXlab platform.
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Calcium Concentration Dependence of the Intergranular Film Thickness in Silicon Nitride
Journal of the American Ceramic Society, 1994Co-Authors: Isao Tanaka, Hansjoachim Kleebe, David R Clarke, Michael K. Cinibulk, John Bruley, M RuhleAbstract:High-resolution electron microscopy and nano-beam analytical electron microscopy have been used to characterize both the Intergranular silicate Film thickness and its local composition in a series of high-purity Si3N4 ceramics doped with 0–450 at. ppm Ca. Calcium was detected at both two-grain junctions and triple junctions, even in the 80-ppm-Ca-doped specimen. The thickness of the Intergranular Film at two-grain junctions was found to depend sensitively on Ca content. In undoped material, the thickness was 1.0 ± 0.1 nm. With increasing Ca content, the thickness decreased in the dilute region (80 ppm Ca), but then increased. The variation in Film thickness can be qualitatively understood in terms of the balance of three long-range forces acting normal to the Film, namely the van der Waals dispersion force, a structural “steric” force, and an electrical-double-layer force. By comparing the measured thicknesses to those predicted, estimates for the structural correlation length and the inverse Debye length can be made. These estimates have values of ∼ 0.22 nm and approximately 0.3–0.5 nm, respectively, for the calcia-free and 80 ppm calcia materials.
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possible electrical double layer contribution to the equilibrium thickness of Intergranular glass Films in polycrystalline ceramics
Journal of the American Ceramic Society, 1993Co-Authors: David R Clarke, Thomas M Shaw, Albert P Philipse, Roger G HornAbstract:The plausibility of the entropic repulsion of electrical double layers acting to stabilize an equilibrium thickness of Intergranular glass Films in polycrystalline ceramics is explored. Estimates of the screening length, surface potential, and surface charge required to provide a repulsive force sufficiently large to balance the attractive van der Waals and capillary forces for observable thicknesses of Intergranular Film are calculated and do not appear to be beyond possibility. However, it has yet to be established whether crystalline particles in a liquid-phase sintering medium possess an electrical double layer at high temperatures. If they do, such a surface charge layer may well have important consequences not only for liquid-phase sintering but also for high-frequency electrical properties and microwave sintering of ceramics containing a liquid phase.
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high resolution electron microscopy observations of grain boundary Films in silicon nitride ceramics
MRS Proceedings, 1992Co-Authors: Hansjoachim Kleebe, David R Clarke, R M Cannon, Isao Tanaka, Michael K. Cinibulk, Michael J Hoffmann, J Bruley, M RuhleAbstract:Characterization of silicon nitride ceramics by transmission electron microscopy (TEM) provides structural and compositional information on Intergranular phases necessary to elucidate the factors that can influence the presence and thickness of grain-boundary Films. Different TEM techniques can be used for the detection and determination of Intergranular-Film thickness, however, the most accurate results are obtained by high-resolution electron microscopy (HREM). HREM studies were applied, in conjunction with analytical electron microscopy, to investigate the correlation between Intergranular-phase composition and Film thickness. Statistical analyses of a number of grain-boundary Films provided experimental verification of a theoretical equilibrium Film thickness. Model experiments on a high-purity Si 3 N 4 material, doped with low amounts of Ca, suggest the presence of two repulsive forces, a steric force and a force produced by an electrical double layer, that may act to balance the attractive van der Waals force necessary to establish an equilibrium Film thickness.
Joseph E Ludy - One of the best experts on this subject based on the ideXlab platform.
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amorphous Intergranular Films act as ultra efficient point defect sinks during collision cascades
Scripta Materialia, 2016Co-Authors: Joseph E Ludy, Timothy J RupertAbstract:Atomistic simulations are used to explore the effect of interfacial structure on residual radiation damage. Specifically, an ordered grain boundary is compared to a disordered amorphous Intergranular Film, to investigate how interface thickness and free volume impacts point defect recombination. Collision cascades are simulated and residual point defect populations are analyzed as a function of boundary type and primary knock on atom energy. While ordered grain boundaries easily absorb interstitials, these interfaces are inefficient vacancy sinks. Alternatively, amorphous Intergranular Films act as ultra-efficient, unbiased defect sinks, providing a path for the creation of radiation-tolerant materials.
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amorphous Intergranular Films act as ultra efficient point defect sinks during collision cascades
arXiv: Materials Science, 2015Co-Authors: Joseph E Ludy, Timothy J RupertAbstract:Atomistic simulations are used to explore the effect of interfacial structure on residual radiation damage. Specifically, an ordered grain boundary is compared to a disordered amorphous Intergranular Film, to investigate how interface thickness and free volume impacts point defect recombination. The collision cascades induced by neutron bombardment are simulated and residual point defect populations are analyzed as a function of boundary type and primary knock on atom energy. While ordered grain boundaries easily absorb interstitials, these interfaces are inefficient vacancy sinks. Alternatively, amorphous Intergranular Films act as ultra-efficient, unbiased defect sinks, providing a path for the creation of radiation-tolerant materials.