Publications (72)282.44 Total impact
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ABSTRACT: The microplasticity of the Mg alloy AZ31 is explored through highenergy Xray diffraction (HEXD). Through cyclic loading of the sample, a softening response is found to follow the resolved shear stress for basal slip. Stress relaxation is studied by applying an incremental elongation increase while continuously collecting images from a detector array. The rate exponent associated with a particular reflection is developed by evaluating the average lattice strain in the loading direction from the composite detector image collected at a particular point in time, and then following the time evolution. A distinct rate exponent is identified for grain orientations that have a propensity for slip on secondorder pyramidal planes, highlighting the capability to capture the simultaneous action of different deformation mechanisms through HEXD. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. 
Article: Energetics of Rutile TiO 2 Vicinal Surfaces With 〈001〉 Steps From the Energy Density Method
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ABSTRACT: Rutile TiO2 vicinal surfaces with ⟨001⟩ steps are investigated using the energy density method (EDM) based on density functional theory. In this approach, EDM provides the energy for each atom so that we can determine the stability of different step configurations correctly. Even though the energy variation due to the step is localized around the step edge, the step–step interaction is long ranged. The finitesize effect in the step–step interaction is identified using EDM. The oxygen vacancy at the step edge explains the atomic structure of the step observed by STM.  [Show abstract] [Hide abstract]
ABSTRACT: We study the influence of coordination number and bond length on the phase stability and orbital occupation in Ti using density functional theory. In particular, Ti under a wide range of conditions (equilibrium state, hydrostatic pressure, anisotropic strain, and phase transformations) is systematically investigated allowing us to derive generic energetic and electronic trends. Our analysis of the correlations between electronic structure and the atomic geometry reveals that the most suitable descriptors of the system are an effective coordination number and an effective bond length. Utilizing these descriptors, we show that (i) the phase stability of Ti increases with coordination number, because of the increased number of interatomic bonds; (ii) the occupation number of the d (s and p) orbital decreases (increases) with increasing the bond length, because of the localized (delocalized) character of the d (p and s) orbital. These dependencies are particularly evident after applying a simple harmonic strain correction to the energy and an electrontransfer correction within the phase. The physical picture derived from pure Ti is used to explain the phase stability and orbital occupation of Ti–Nb and Ti–Zr alloys, which reveals the underlying mechanisms for various experimental phenomena in Ti alloys. 
Article: Thermal transport across highpressure semiconductormetal transition in Si and Si 0.991 Ge 0.009
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ABSTRACT: Timedomain thermoreflectance (TDTR) can be applied to metallic samples at high pressures in the diamond anvil cell and provide noncontact measurements of thermal transport properties. We have performed regular and beamoffset TDTR to establish the thermal conductivities of Si and ${\mathrm{Si}}_{0.991}{\mathrm{Ge}}_{0.009}$ across the semiconductormetal phase transition and up to 45 GPa. The thermal conductivities of metallic Si and Si(Ge) are comparable to aluminum and indicative of predominantly electronic heat carriers. Metallic Si and Si(Ge) have an anisotropy of approximately 1.4, similar to that of beryllium, due to the primitive hexagonal crystal structure. We used the WiedemannFranz law to derive the associated electrical resistivity, and found it consistent with the BlochGr\"uneisen model.  [Show abstract] [Hide abstract]
ABSTRACT: Transport coefficients, the elements of the socalled Onsager matrix, are required for accurate mesoscopic simulations of kinetics or to predict macroscopic diffusion kinetic behavior. These coefficients can be significantly affected by strain. At the atomic scale, the effect of strain on atomic jump frequencies can be computed using density functional theory calculations. The present work shows how these results can be used to compute the straindependent Onsager matrix. Using an analytical method\char22{}the selfconsistent mean field method\char22{}we compute analytical expressions of the Onsager matrix describing vacancymediated diffusion of impurities in facecentered cubic structures under elementary strains. Also, we compute the derivatives of the Onsager matrix with respect to strain\char22{}the elastodiffusion tensor\char22{}to investigate strain sensitivity of transport. We show that the atomic scale symmetry breaking induced by strain changes diffusion behavior qualitatively. This phenomenon is shown for the Ni(Si) alloy under tetragonal strain. The terms of the Onsager matrix are found to be nonArrhenian, as well as their derivative with respect to strain. In this case, nonlinear effects leading to a solute drag reduction are identified.  [Show abstract] [Hide abstract]
ABSTRACT: The width and energy of lowindex interfacial boundaries (IFBs) in Ni–Ni3Al are calculated using firstprinciples methods for temperatures ranging from 0 to 1300 K. The lowtemperature, coherent and chemically sharp (1 0 0), (1 1 0) and (1 1 1) IFBs are studied using conventional spinpolarized density functional methods. Cluster expansion methods, as implemented in the ATAT software suite, are used to estimate the interfacial excess free energies (IEFEs) and composition and longrange order profiles of these defects as a function of temperature. The simple facecentered cubicbased cluster expansion produces interfacial widths in the range of 1.5–3.0 nm at 1000 K. Interfacial widths double in size with an increase in temperature of 500 K. We also find that the IEFEs for the (1 0 0), (1 1 0) and (1 1 1) IFBs are strongly temperature dependent, decreasing by 90% as temperature increases from 0 to 1000 K. While vibrational and electronic entropic contributions were also considered, changes in free energy are dominated by the configurational entropy. The predicted hightemperature IEFE is approximately 10 mJ m−2 which is in excellent agreement with previous fits to experimentally measured coarsening rates.  [Show abstract] [Hide abstract]
ABSTRACT: The temperaturedependent diffusivity D(T) of hydrogen solute atoms trapped at dislocationsdislocation pipe diffusion of hydrogenin deformed polycrystalline PdH_{x} (x∼10^{3} [H]/[Pd]) has been quantified with quasielastic neutron scattering between 150 and 400 K. We observe diffusion coefficients for trapped hydrogen elevated by one to two orders of magnitude above bulk diffusion. Arrhenius diffusion behavior has been observed for dislocation pipe diffusion and regular bulk diffusion, the latter in wellannealed polycrystalline Pd. For regular bulk diffusion of hydrogen in Pd we find D(T)=D_{0}exp(E_{a}/kT)=0.005exp(0.23 eV/kT) cm^{2}/s, in agreement with the known diffusivity of hydrogen in Pd. For hydrogen dislocation pipe diffusion we find D(T)≃10^{5}exp(E_{a}/kT) cm^{2}/s, where E_{a}=0.042 and 0.083 eV for concentrations of 0.52×10^{3} and 1.13×10^{3}[H]/[Pd], respectively. Ab initio computations provide a physical basis for the pipe diffusion pathway and confirm the reduced barrier height.  [Show abstract] [Hide abstract]
ABSTRACT: We perform a firstprinciples study of the effect of strain on the migration of Si atoms in Ni. For that purpose, migration barriers are computed using the nudged elastic band method and attempt frequencies are computed using the direct force method. Good agreement is found with tracer diffusion experiments. We used the elastic dipole model to calculate effects of strain on migration barriers by performing calculations on unstrained cells, therefore reducing significantly the computing time. We validate this approach by comparing results with migration barriers calculated on strained cells and obtain an excellent agreement up to a strain of 1%. Computing all the jump frequencies in the neighborhood of Si solutes, the effect of strain is found to be nearly independent of the relative position of the solute atom. A simple elastic analysis models the changes in the vacancy jump with strain; this correlates with the changes in geometry for the "cage" of atoms surrounding the hopping atom at the saddle point.  [Show abstract] [Hide abstract]
ABSTRACT: Transport coefficients, the elements of the socalled Onsager matrix, are essential quantities for modeling solidstate kinetics controlled by diffusion. In a facecenteredcubic structure, drag of solute atoms by vacancies can be caused by solutevacancy binding at nearest neighbors. In order to investigate solute drag in alloys with interactions up to the thirdnearestneighbor sites, we extend an analytic method: the selfconsistent mean field method. With this method, we calculate the Onsager matrix of model alloys to identify kinetic effects arising from individual and collective jump frequencies and assess the results on select cases using atomic kinetic Monte Carlo simulations. Using preexisting density functional theory data from various sources, we show that many impurities have lowtemperature solute drag before changing to solute exchange at high temperatures. We evaluate the transition temperature for these alloys between these two regimes and compare the results with available experimental data. Some disagreement is found, which can be due both to experimental and numerical shortcomings. In order to guide diffusion calculations, the sensitivity of the Onsager matrix to the range of the kinetic correlation and to the input density functional theory data is studied.  [Show abstract] [Hide abstract]
ABSTRACT: Oxygen greatly affects the mechanical properties of titanium. In addition, dislocations and twin boundaries influence the plastics deformation of hcp metals. As part of a systematic study of defects interactions in Ti, we investigate the interactions of oxygen with (1012) twin boundary and (1010) prism plane stacking fault. The energetics of four interstitial sites in the twin geometry are compared with the bulk octahedral site. We show that two of these sites located at the twin boundary are more attractive to oxygen than bulk while the sites away from the boundary are repulsive. Moreover, we study the interaction of oxygen with the prismatic stacking fault to approximate oxygendislocation interaction. We show that oxygen increases the stacking fault energy and therefore is repelled by the faulted geometry and consequently a dislocation core.  [Show abstract] [Hide abstract]
ABSTRACT: Smallangle neutron scattering (SANS) measurements of hydrogen segregation at dislocations in heavily deformed single crystal Pd have been performed at very low solute concentration (PdH0.0016) at equilibrium with respect to hydrogen gas at 295 K. The net (the withouthydrogen measurement subtracted from withhydrogen measurement) absolute differential macroscopic scattering cross section has been fit with a cylindrical form factor to represent the Cottrell atmosphere, yielding local trapped concentration δ ∼ 0.06 [H]/[Pd], local volumetric dilatation f ∼ 1.01, and trapping radius R ∼ 4 Å of the segregated hydrogen. This measurement augments SANS results below ambient temperature [B.J. Heuser, H. Ju, Phys. Rev. B 83 (2011) 094103]. The temperature dependence of the measured radius is confirmed by a Fourier transform of hydrogen occupation at dislocations based on an elastic continuum treatment [Trinkle et al., Phys. Rev. B 83 (2011) 174116]. The measured trapping parameters are consistent with a depopulation of weak longrange dislocation strain fields at ambient temperature; hydrogen binding to stronger core dislocation sites persists at 295 K and results in the measured net scattering. The local solute concentration and trapping radius (less than two Burgers vectors in Pd), are both too small to support optic mode dispersion due to interhydrogen interactions. This result supports the conclusion that trapped hydrogen undergoes a hydride to solid solution phase transformation between 200 and 300 K based on hydrogen vibration density of states measurements using incoherent inelastic neutron scattering [Trinkle et al., Phys. Rev. B 83 (2011) 174116, Ju et al., Nucl. Instr. Meth. Phys. Res. A 654, (2011) 522, and Heuser et al., Phys. Rev. B 78 (2008) 214101]. 
Article: Stressinduced anisotropic diffusion in alloys: Complex Si solute flow near a dislocation core in Ni
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ABSTRACT: Stress introduces anisotropy in the transport coefficients in materials, affecting diffusion. Using firstprinciples quantummechanical methods for activation barriers of atomic jumps, combined with the extended selfconsistent meanfield theory to compute transport coefficients with strainreduced symmetry, we predict significant stressinduced anisotropy for Si impurity diffusion in nickel. This causes complex spatial and temperaturedependent fluxes; as an example, the heterogenous strain field of a dislocation creates unusual flow patterns that affect mechanical and segregation behavior.  [Show abstract] [Hide abstract]
ABSTRACT: Transport coefficients, the elements of the socalled Onsager matrix, are essential quantities for modeling solidstate kinetics controlled by diffusion. Focusing on diffusion in binary alloys with a bodycentered cubic crystal structure, we investigate the drag of solute atoms by vacancies, an effect induced by kinetic correlations. To accomplish this, an analytic method—the selfconsistent mean field method—is extended to take into account interactions between the solute atom and a vacancy up to the third nearest neighbor sites. We identify kinetic effects involving one or more frequencies. Analytic results are compared with select atomic kinetic Monte Carlo simulations. We show that (1) solute drag is a more general phenomena than previously assumed, (2) it can induced by association and dissociation exchanges, and (3) we identify the mechanisms involved.  [Show abstract] [Hide abstract]
ABSTRACT: Weighted least squares fitting to a database of quantum mechanical calculations can determine the optimal parameters of empirical potential models. While algorithms exist to provide optimal potential parameters for a given fitting database of structures and their structure property functions, and to estimate prediction errors using Bayesian sampling, defining an optimal fitting database based on potential predictions remains elusive. A testing set of structures and their structure property functions provides an empirical measure of potential transferability. Here, we propose an objective function for fitting databases based on testing set errors. The objective function allows the optimization of the weights in a fitting database, the assessment of the inclusion or removal of structures in the fitting database, or the comparison of two different fitting databases. To showcase this technique, we consider an example LennardJones potential for Ti, where modeling multiple complicated crystal structures is difficult for a radial pair potential. The algorithm finds different optimal fitting databases, depending on the objective function of potential prediction error for a testing set.  [Show abstract] [Hide abstract]
ABSTRACT: We calculate firstprinciples interaction energies between substitutional solutes and oxygen interstitials in αtitanium and predict the effect of solutes on oxygen diffusion from those interactions. Interaction between 45 solutes across the periodic table and three oxygen interstitial sites are calculated with densityfunctional theory. The interaction energies show distinct trends across the periodic table corresponding to both atomic radii and the period. Changes in diffusion barrier due to solutes are modeled with the kinetically resolved activation barrier approximation. Solute effects at infinite dilution are numerically calculated and show both accelerated and reduced oxygen diffusivity.  [Show abstract] [Hide abstract]
ABSTRACT: Densityfunctional theory energies, forces, and elastic constants determine the parametrization of an empirical, modified embeddedatom method potential for molybdenum. The accuracy and transferability of the potential are verified by comparison to experimental and densityfunctional data for point defects, phonons, thermal expansion, surface and stacking fault energies, and ideal shear strength. Searching the energy landscape predicted by the potential using a genetic algorithm verifies that it reproduces not only the correct bcc ground state of molybdenum but also all lowenergy metastable phases. The potential is also applicable to the study of plastic deformation and used to compute energies, core structures, and Peierls stresses of screw and edge dislocations. 
Article: Prediction of thermal crossslip stress in magnesium alloys from a geometric interaction model
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ABSTRACT: We develop a geometrybased model from firstprinciples data for the interaction of solutes with a prismatic screw dislocation core, and predict the thermally activated crossslip stress above room temperature in Mg alloys. Electronic structure methods provide data for the change in prismatic stacking fault energy for different possible fault configurations for 29 different solutes. The direct solute–dislocation interaction energies for solutes that produce stable prismatic screw dislocation cores (K, Na, Sc and Ca) is correlated with stacking fault misfits. This geometric interaction model produces similar prediction errors for all 29 solutes. The model predicts alloys with crossslip stresses lower than pure Mg for three previously considered solutes (K, Na and Sc) and three new solutes (Ca, Y and Zr). The model also qualitatively confirms the experimental observation that Mg–Li alloys have lower crossslip stress than pure Mg. In particular, low concentrations of Y are predicted to significantly decrease the crossslip stress in Mg. 
Article: Direct calculation of lattice Green function with arbitrary interactions for general crystals
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ABSTRACT: Efficient computation of lattice defect geometries such as point defects, dislocations, disconnections, grain boundaries, interfaces and free surfaces requires accurate coupling of displacements near the defect to the longrange elastic strain. Flexible boundary condition methods embedded a defect in infinite harmonic bulk through the lattice Green function. We demonstrate an efficient and accurate calculation of the lattice Green function from the forceconstant matrix for general crystals with an arbitrary basis by extending a method for Bravais lattices. New terms appear due to the presence of optical modes and the possible loss of inversion symmetry. By separately treating poles and discontinuities in reciprocal space, numerical accuracy is controlled at all distances. We compute the lattice Green function for a twodimensional model with broken symmetry to elucidate the role of different coupling terms. The algorithm is generally applicable in two and three dimensions, to crystals with arbitrary number of atoms in the unit cell, symmetry, and interactions. 
Article: Core structure of a screw dislocation in Ti from density functional theory and classical potentials
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ABSTRACT: Previous density functional theory (DFT) studies of the 1/3 〈12¯10〉 screw dislocation in titanium have shown metastable core structures depending on the initial position of the dislocation line. We investigate this problem by modeling a screw dislocation with two initial positions using both DFT and a modified embedded atom (MEAM) potential for Ti with flexible boundary conditions. Both DFT and MEAM produce initialpositiondependent core structures. The MEAM potential stacking fault energies and core structures are in good agreement with DFT. MEAM potential computes the core energies and shows the behavior of both cores under applied strain. We found that the higherenergy core always reconstructs into the lowerenergy one independent of the applied strain direction. Transformation from low to highenergy core was not observed. Therefore, at T = 0 K, only the lowenergy core is stable under applied strain.  [Show abstract] [Hide abstract]
ABSTRACT: Fundamental properties of molten Nibased alloys are calculated using ab initio molecular dynamics simulations based on Density Functional Theory. In this work predictions for density inversion, an increase in liquid metal density with increasing temperature, are made for a model NiAlW alloy and the Nibased superallloys RENEN4 and CMSX4. Previous calculations on simple, binary and ternary Ni alloys illustrated proof of concept of this method, predicting liquidphase molar volumes (V(c,T)) within 0.61.8% of available experimental data. Simulation cells of 500 atoms, run for approximately 10 psec, were adequate to converge liquid metal densities and diffusion rates. Predicted liquid metal densities for NiAlW alloys, for target compositions and temperatures consistent with 0 and 0.5 solid volume fractions in the melt, produce approximately a 2% density inversion. The liquid metal density for a Ni based superalloy, RENEN4, is calculated at the liquidus and solidus temperatures expected in the mushy zone. A series of 500 atom instantiations of the superalloy melt produce selfconsistent densities that are well within expected numerical error. This illustrates that simulations of moderate spatial and temporal scales sample enough degrees of freedom to properly represent the configuration entropy found in a liquid metal superalloy. Density inversions are predicted for both RENEN4 and CMSX4 for chemistries and temperatures consistent with 0 and 0.4 solid volume fractions in the melt.
Publication Stats
965  Citations  
282.44  Total Impact Points  
Top Journals
Institutions

20082015

University of Illinois, UrbanaChampaign
 Department of Materials Science and Engineering
Urbana, Illinois, United States


20052008

WrightPatterson Air Force Base
Dayton, Ohio, United States


20032008

The Ohio State University
 Department of Physics
Columbus, OH, United States


2006

Air Force Research Laboratory
Washington, Washington, D.C., United States


2001

Los Alamos National Laboratory
 Theoretical Division
Los Alamos, CA, United States
