Publications (62)180.43 Total impact
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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.Physical Review Letters 07/2014; 113(2):025504. · 7.73 Impact Factor  Phys. Rev. B. 07/2014; 90(2):024306.
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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.03/2014; 89(14).  [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.Acta Materialia. 03/2014; 76.  [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.Acta Materialia. 01/2014; 75:60–70. 
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.Physical Review B 10/2013; 88:134108. · 3.66 Impact Factor  [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.Journal of Applied Physics 06/2013; 113(22). · 2.21 Impact Factor  [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.Physical Review B 01/2013; 88:134201. · 3.66 Impact Factor  [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.Physical review. B, Condensed matter 06/2012; 85(21). · 3.77 Impact Factor 
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.Acta Materialia. 03/2012; 60(5):2350–2358. 
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.Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics 02/2012; 85(6). 
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.Acta Materialia 01/2012; 60(3):1287  1292. · 3.94 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: How impurity atoms move through a crystal is a fundamental and recurrent question in materials. The previous models of oxygen diffusion in titanium relied on interstitial lattice sites that were recently found to be unstableleaving no consistent picture of the diffusion pathways. Using firstprinciples quantummechanical methods, we find three oxygen interstitial sites in titanium, and quantify the multiple interpenetrating networks for oxygen diffusion. Surprisingly, all transitions contribute to diffusion.Physical Review Letters 07/2011; 107(4):045504. · 7.73 Impact Factor 
Article: Prediction of thermal crossslip stress in magnesium alloys from direct first principles data
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ABSTRACT: We develop a firstprinciples model of thermallyactivated crossslip in magnesium in the presence of a random solute distribution. Electronic structure methods provide data for the interaction of solutes with prismatic dislocation cores and basal dislocation cores. Direct calculations of interaction energies are possible for solutesK, Na, and Scthat lower the Mg prismatic stacking fault energy to improve formability. To connect to thermally activated crossslip, we build a statistical model for the distribution of activation energies for double kink nucleation, barriers for kink migration, and roughness of the energy landscape to be overcome by an athermal stress. These distributions are calculated numerically for a range of concentrations, as well as alternate approximate analytic expressions for the dilute limit. The analytic distributions provide a simplified model for the maximum crossslip softening for a solute as a function of temperature. The direct interaction calculations predict lowered forming temperatures for Mg0.7at.%Sc, Mg0.4at.%K, and Mg0.6at.%Na of approximately 250C.05/2011;  04/2011: pages 13  15; , ISBN: 9781118062029
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ABSTRACT: Hydrogen arranges at dislocations in palladium to form nanoscale hydrides, changing the vibrational spectra. An ab initio hydrogen potential energy model versus Pd neighbor distances allows us to predict the vibrational excitations for H from absolute zero up to room temperature adjacent to a partial dislocation and with strain. Using the equilibrium distribution of hydrogen with temperature, we predict excitation spectra to explain new incoherent inelastic neutronscattering measurements. At 0K, dislocation cores trap H to form nanometersized hydrides, while increased temperature dissolves the hydrides and disperses H throughout bulk Pd.Physical review. B, Condensed matter 04/2011; 83. · 3.77 Impact Factor 
Article: Thermal conductivity of compressed H_ {2} O to 22 GPa: A test of the LeibfriedSchlömann equation
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ABSTRACT: The LeibfriedSchlömann (LS) equation, a commonly assumed model for the pressure dependence of thermal conductivity Λ, is tested by measurements on compressed H2O using a combination of the timedomain thermoreflectance method with the diamond anvil cell technique. The thermal conductivity of ice VII increases by an order of magnitude between 2 and 22 GPa, reaching Λ≈25 W m1 K1. Over a large compression range of ≈4%–33%, the LS equation describes the pressure dependence of Λ of ice VII to better than 20%.Physical review. B, Condensed matter 04/2011; 83(13). · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We propose an efficient, accurate method to integrate the basins of attraction of a smooth function defined on a general discrete grid and apply it to the Bader charge partitioning for the electron charge density. Starting with the evolution of trajectories in space following the gradient of charge density, we derive an expression for the fraction of space neighboring each grid point that flows to its neighbors. This serves as the basis to compute the fraction of each grid volume that belongs to a basin (Bader volume) and as a weight for the discrete integration of functions over the Bader volume. Compared with other gridbased algorithms, our approach is robust, more computationally efficient with linear computational effort, accurate, and has quadratic convergence. Moreover, it is straightforward to extend to nonuniform grids, such as from a meshrefinement approach, and can be used to both identify basins of attraction of fixed points and integrate functions over the basins.The Journal of Chemical Physics 01/2011; 134(6):064111. · 3.12 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We determine the stability and properties of interfaces of lowindex Au surfaces adhered to TiO2(110), using density functional theory energy density calculations. We consider Au(100) and Au(111) epitaxies on rutile TiO2(110) surface, as observed in experiments. For each epitaxy, we consider several different interfaces: Au(111)//TiO2(110) and Au(100)//TiO2(110), with and without bridging oxygen, Au(111) on 1x2 addedrow TiO2(110) reconstruction, and Au(111) on a proposed 1x2 TiO reconstruction. The density functional theory energy density method computes the energy changes on each of the atoms while forming the interface, and evaluates the work of adhesion to determine the equilibrium interfacial structure.The Journal of Physical Chemistry C 01/2011; 115:17799. · 4.84 Impact Factor 
Article: Energy density in density functional theory: Application to crystalline defects and surfaces
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ABSTRACT: We propose a method to decompose the total energy of a supercell containing defects into contributions of individual atoms, using the energy density formalism within density functional theory. The spatial energy density is unique up to a gauge transformation, and we show that unique atomic energies can be calculated by integrating over Bader and chargeneutral volumes for each atom. Numerically, we implement the energy density method in the framework of the Vienna ab initio simulation package (VASP) for both normconserving and ultrasoft pseudopotentials and the projector augmented wave method, and use a weighted integration algorithm to integrate the volumes. The surface energies and point defect energies can be calculated by integrating the energy density over the surface region and the defect region, respectively. We compute energies for several surfaces and defects: the (110) surface energy of GaAs, the monovacancy formation energies of Si, the (100) surface energy of Au, and the interstitial formation energy of O in the hexagonal closepacked Ti crystal. The surface and defect energies calculated using our method agree with sizeconverged calculations of the difference between the total energies of the system with and without the defect. Moreover, the convergence of the defect energies with size can be found from a single calculation.Physical review. B, Condensed matter 11/2010; 83. · 3.77 Impact Factor
Publication Stats
287  Citations  
180.43  Total Impact Points  
Top Journals
Institutions

2010–2014

University of Illinois, UrbanaChampaign
 • Department of Materials Science and Engineering
 • Department of Physics
Urbana, Illinois, United States 
Brown University
 School of Engineering
Providence, RI, United States


2003–2008

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


2006

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


2005–2006

WrightPatterson Air Force Base
Dayton, Ohio, United States


2001

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