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Katsuyo Thornton - One of the best experts on this subject based on the ideXlab platform.
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continuum simulations of the formation of kirkendall effect induced hollow cylinders in a binary Substitutional Alloy
Acta Materialia, 2009Co-Authors: Huichia Yu, Dong Hee Yeon, Xiaofan Li, Katsuyo ThorntonAbstract:Abstract We examine binary Substitutional diffusion in cylindrical diffusion couples in which free surfaces are considered explicit vacancy sources and sinks. The central region of the cylinder is initially occupied by an atomic species with a larger hop frequency, while the outer region is occupied by another atomic species with a smaller hop frequency. Equilibrium vacancy concentration is maintained at free surfaces that serve as vacancy sources and sinks. In the crystal, diffusion is governed by the standard diffusion equations with analytically evaluated diffusion coefficients. The void growth dynamics and hollow cylinder formation stemming from the Kirkendall effect are simulated. Our results show that the Kirkendall void growth involves two competing factors. One is the net inward vacancy flux that favors void growth. The other is the Gibbs–Thomson effect that favors void shrinkage. We compute the critical initial radius for void growth above which the Kirkendall effect dominates over the Gibbs–Thomson effect. The fully grown void radius and the elapsed time to the fully grown size are also predicted for different fast-diffuser volume fractions and fast-to-slow diffuser atomic hop frequency ratios.
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Substitutional diffusion and kirkendall effect in binary crystalline solids containing discrete vacancy sources and sinks
Acta Materialia, 2007Co-Authors: Huichia Yu, Dong Hee Yeon, Katsuyo ThorntonAbstract:We investigate vacancy-mediated diffusion in a binary Substitutional Alloy by explicitly accounting for discrete vacancy sources and sinks. The regions between sources and sinks are treated as binary crystals with a perfect lattice structure containing a dilute concentration of vacancies. The sources and sinks are assumed ideal, maintaining an equilibrium vacancy concentration in their immediate vicinity. Diffusion within the perfect lattice is described with a diffusion-coefficient matrix determined by kinetic Monte Carlo simulations for a binary, thermodynamically ideal Alloy in which the components have different vacancy-exchange frequencies. Continuum simulations are performed for diffusion couples with discrete grain boundaries acting as vacancy sources and sinks. Effective grain coarsening due to the Kirkendall effect is observed even in the absence of Gibbs-Thomson driving forces. As in standard ternary systems, uphill diffusion is observed. We also find that the drift of the lattice frame of reference as a result of the Kirkendall effect increases with the source/sink density. Upon increasing the density of vacancy sources and sinks, we recover the conventional treatment of Substitutional diffusion, which assumes a dense and uniform distribution of vacancy sources and sinks that maintain an equilibrium vacancy concentration throughout the solid. The inverse Kirkendall effect, where the slower component segregates at grain boundaries acting as vacancy sinks, is also observed in the simulations.
Huichia Yu - One of the best experts on this subject based on the ideXlab platform.
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continuum simulations of the formation of kirkendall effect induced hollow cylinders in a binary Substitutional Alloy
Acta Materialia, 2009Co-Authors: Huichia Yu, Dong Hee Yeon, Xiaofan Li, Katsuyo ThorntonAbstract:Abstract We examine binary Substitutional diffusion in cylindrical diffusion couples in which free surfaces are considered explicit vacancy sources and sinks. The central region of the cylinder is initially occupied by an atomic species with a larger hop frequency, while the outer region is occupied by another atomic species with a smaller hop frequency. Equilibrium vacancy concentration is maintained at free surfaces that serve as vacancy sources and sinks. In the crystal, diffusion is governed by the standard diffusion equations with analytically evaluated diffusion coefficients. The void growth dynamics and hollow cylinder formation stemming from the Kirkendall effect are simulated. Our results show that the Kirkendall void growth involves two competing factors. One is the net inward vacancy flux that favors void growth. The other is the Gibbs–Thomson effect that favors void shrinkage. We compute the critical initial radius for void growth above which the Kirkendall effect dominates over the Gibbs–Thomson effect. The fully grown void radius and the elapsed time to the fully grown size are also predicted for different fast-diffuser volume fractions and fast-to-slow diffuser atomic hop frequency ratios.
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Substitutional diffusion and kirkendall effect in binary crystalline solids containing discrete vacancy sources and sinks
Acta Materialia, 2007Co-Authors: Huichia Yu, Dong Hee Yeon, Katsuyo ThorntonAbstract:We investigate vacancy-mediated diffusion in a binary Substitutional Alloy by explicitly accounting for discrete vacancy sources and sinks. The regions between sources and sinks are treated as binary crystals with a perfect lattice structure containing a dilute concentration of vacancies. The sources and sinks are assumed ideal, maintaining an equilibrium vacancy concentration in their immediate vicinity. Diffusion within the perfect lattice is described with a diffusion-coefficient matrix determined by kinetic Monte Carlo simulations for a binary, thermodynamically ideal Alloy in which the components have different vacancy-exchange frequencies. Continuum simulations are performed for diffusion couples with discrete grain boundaries acting as vacancy sources and sinks. Effective grain coarsening due to the Kirkendall effect is observed even in the absence of Gibbs-Thomson driving forces. As in standard ternary systems, uphill diffusion is observed. We also find that the drift of the lattice frame of reference as a result of the Kirkendall effect increases with the source/sink density. Upon increasing the density of vacancy sources and sinks, we recover the conventional treatment of Substitutional diffusion, which assumes a dense and uniform distribution of vacancy sources and sinks that maintain an equilibrium vacancy concentration throughout the solid. The inverse Kirkendall effect, where the slower component segregates at grain boundaries acting as vacancy sinks, is also observed in the simulations.
Stephen M Foiles - One of the best experts on this subject based on the ideXlab platform.
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embedded atom method effective pair interaction study of the structural and thermodynamic properties of cu ni cu ag and au ni solid solutions
Physical Review B, 1996Co-Authors: Mark Asta, Stephen M FoilesAbstract:The structural and thermodynamic properties of Cu-Ni, Cu-Ag, and Au-Ni solid solutions have been studied using a computational approach which combines an embedded-atom-method (EAM) description of Alloy energetics with a second-order-expansion (SOE) treatment of compositional and displacive disorder. It is discussed in detail how the SOE approach allows the EAM expression for the energy of a Substitutional Alloy to be cast in the form of a generalized lattice-gas Hamiltonian containing effective pair interactions with arbitrary range. Furthermore, we show how the SOE-EAM method can be combined with either mean-field or Monte Carlo statistical mechanics techniques in order to calculate short-range-order (SRO) parameters, average nearest-neighbor bond lengths, and Alloy thermodynamic properties which include contributions from static displacive relaxations and dynamic atomic vibrations. We demonstrate that the contributions to Alloy heats of mixing arising from displacive relaxations can be sizeable, and that the neglect of these terms can lead to large overestimations of calculated phase-transition temperatures. The effects of vibrational free-energy contributions on the results of composition-temperature phase diagram calculations are estimated to be relatively small for the phase-separating Alloy systems considered in this study. It is shown that within the SOE approach displacive effects can act only to displace the peakmore » in the Fourier-transformed SRO parameter away from Brillouin-zone-boundary special points and towards the origin. Consistent with this result, we show that the unusual SRO observed in diffuse scattering experiments for Au-Ni solid solutions can be understood as arising from a competition between chemical and displacive driving forces which favor ordering and clustering, respectively. {copyright} {ital 1996 The American Physical Society.}« less
Li Rongbin - One of the best experts on this subject based on the ideXlab platform.
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The strengthening effect of boron interstitial supersaturated solid solution on aluminum films
Materials Letters, 2017Co-Authors: Hailong Shang, Kaicheng Shi, Li RongbinAbstract:Abstract A series of aluminum-boron (Al-B) Alloy films with the addition of various levels of B content were synthesized by magnetron co-sputtering of Al and B targets. The strengthening effect of B content was investigated through characterization of the microstructures and microhardness of the films. The results showed that Al-B films are a single-phase interstitial supersaturated solid solution when B content is less than 1.89 at% studied. With the addition of the B content from 0.06 at% to 1.89 at%, the average grain size of the films decreased continuously from ∼100 nm of pure Al to less than 10 nm, and the microhardness of the films increased continuously from 3.3 GPa to 6.0 GPa. Compared to Substitutional Alloy elements (such as Cr, Fe, Ti, Mg and Cu), the strengthening effect of B per unit was found to be approximately ten times greater, which is attributed to the larger lattice distortion caused by interstitial solid solution of B.
Dongliang Chen - One of the best experts on this subject based on the ideXlab platform.
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local structural changes during the disordered Substitutional Alloy transition in bi2te3 by high pressure xafs
Journal of Applied Physics, 2018Co-Authors: Zhiying Guo, Hailiang Zhu, Juncai Dong, Q J Jia, Yu Gong, Yan Wang, Dongliang Yang, Yidong Zhao, Haiying Xing, Dongliang ChenAbstract:A2B3-type 3D topological insulators, Bi2Te3 and Sb2Te3, have been reported to transform into disordered Substitutional Alloys under high pressure. However, γ → δ phase transition and the local structure changes around Bi during the formation of the disordered Bi-Te binary Alloy in Bi2Te3 still remain unclear. Here, high-pressure X-ray absorption fine structure (XAFS) combined with high-pressure X-ray diffraction has been used to explore the local structural transformations in the three structural phase transitions of Bi2Te3. The Bi L3-X-ray absorbing near edge structure (XANES) spectra of δ-Bi2Te3 clearly showed that a new absorption feature at energy of about 13 465 eV would emerge during the γ → δ phase transition. Through simulation of the XANES spectra by varying the cluster size, we confirmed that the new absorption peak arises from the medium-range order in bcc structure. The Bi L3-EXAFS results in δ phase reveal that the Bi atoms in the third shell exhibit abnormal elongations with pressure until merging with the shrinking Te shell. Our findings indicate a two-step structural transition of the disordered Substitutional Alloys via an ordered bcc intermediate phase.A2B3-type 3D topological insulators, Bi2Te3 and Sb2Te3, have been reported to transform into disordered Substitutional Alloys under high pressure. However, γ → δ phase transition and the local structure changes around Bi during the formation of the disordered Bi-Te binary Alloy in Bi2Te3 still remain unclear. Here, high-pressure X-ray absorption fine structure (XAFS) combined with high-pressure X-ray diffraction has been used to explore the local structural transformations in the three structural phase transitions of Bi2Te3. The Bi L3-X-ray absorbing near edge structure (XANES) spectra of δ-Bi2Te3 clearly showed that a new absorption feature at energy of about 13 465 eV would emerge during the γ → δ phase transition. Through simulation of the XANES spectra by varying the cluster size, we confirmed that the new absorption peak arises from the medium-range order in bcc structure. The Bi L3-EXAFS results in δ phase reveal that the Bi atoms in the third shell exhibit abnormal elongations with pressure until m...
