[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 electron-transfer 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.
[Show abstract][Hide abstract] ABSTRACT: Time-domain 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 beam-offset TDTR to establish the thermal conductivities of Si and ${\mathrm{Si}}_{0.991}{\mathrm{Ge}}_{0.009}$ across the semiconductor-metal 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 Wiedemann-Franz law to derive the associated electrical resistivity, and found it consistent with the Bloch-Gr\"uneisen model.
Physical Review B 05/2015; 91(20). DOI:10.1103/PhysRevB.91.205104 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Transport coefficients, the elements of the so-called 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 strain-dependent Onsager matrix. Using an analytical method\char22{}the self-consistent mean field method\char22{}we compute analytical expressions of the Onsager matrix describing vacancy-mediated diffusion of impurities in face-centered cubic structures under elementary strains. Also, we compute the derivatives of the Onsager matrix with respect to strain\char22{}the elasto-diffusion 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 non-Arrhenian, as well as their derivative with respect to strain. In this case, nonlinear effects leading to a solute drag reduction are identified.
Physical Review B 11/2014; 90(18). DOI:10.1103/PhysRevB.90.184301 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The width and energy of low-index interfacial boundaries (IFBs) in Ni–Ni3Al are calculated using first-principles methods for temperatures ranging from 0 to 1300 K. The low-temperature, coherent and chemically sharp (1 0 0), (1 1 0) and (1 1 1) IFBs are studied using conventional spin-polarized 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 long-range order profiles of these defects as a function of temperature. The simple face-centered cubic-based 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 high-temperature 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 temperature-dependent diffusivity D(T) of hydrogen solute atoms trapped at dislocations-dislocation pipe diffusion of hydrogen-in 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 well-annealed 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 first-principles 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.
Physical Review B 07/2014; 90(2):024306. DOI:10.1103/PhysRevB.90.024306 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Transport coefficients, the elements of the so-called Onsager matrix, are essential quantities for modeling solid-state kinetics controlled by diffusion. In a face-centered-cubic structure, drag of solute atoms by vacancies can be caused by solute-vacancy binding at nearest neighbors. In order to investigate solute drag in alloys with interactions up to the third-nearest-neighbor sites, we extend an analytic method: the self-consistent 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 low-temperature 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.
Physical Review B 03/2014; 89(14). DOI:10.1103/PhysRevB.89.144202 · 3.74 Impact Factor
[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 (10-12) twin boundary and (10-10)
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
oxygen-dislocation 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: Small-angle 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 without-hydrogen measurement subtracted from with-hydrogen 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 long-range 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 inter-hydrogen 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].
Journal of Alloys and Compounds 11/2013; 577:189–191. DOI:10.1016/j.jallcom.2013.04.082 · 3.00 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Transport coefficients, the elements of the so-called Onsager matrix, are essential quantities for modeling solid-state kinetics controlled by diffusion. Focusing on diffusion in binary alloys with a body-centered 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 self-consistent 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 10/2013; 88(13):134201. DOI:10.1103/PhysRevB.88.134201 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Stress introduces anisotropy in the transport coefficients in materials, affecting diffusion. Using first-principles quantum-mechanical methods for activation barriers of atomic jumps, combined with the extended self-consistent mean-field theory to compute transport coefficients with strain-reduced symmetry, we predict significant stress-induced anisotropy for Si impurity diffusion in nickel. This causes complex spatial- and temperature-dependent 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(13):134108. DOI:10.1103/PhysRevB.88.134108 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We calculate first-principles interaction energies between substitutional solutes and oxygen interstitials in α-titanium 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 density-functional 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: Density-functional theory energies, forces, and elastic constants determine the parametrization of an empirical, modified embedded-atom method potential for molybdenum. The accuracy and transferability of the potential are verified by comparison to experimental and density-functional 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 low-energy 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.
[Show abstract][Hide abstract] ABSTRACT: We develop a geometry-based model from first-principles data for the interaction of solutes with a prismatic screw dislocation core, and predict the thermally activated cross-slip 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 cross-slip 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 cross-slip stress than pure Mg. In particular, low concentrations of Y are predicted to significantly decrease the cross-slip stress in Mg.
[Show abstract][Hide abstract] 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 long-range
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
force-constant 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 two-dimensional
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). DOI:10.1103/PhysRevE.85.066706 · 2.81 Impact Factor
[Show abstract][Hide abstract] 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 initial-position-dependent 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 higher-energy core always reconstructs into the lower-energy one independent of the applied strain direction. Transformation from low- to high-energy core was not observed. Therefore, at T = 0 K, only the low-energy core is stable under applied strain.
[Show abstract][Hide abstract] ABSTRACT: We determine the stability and properties of interfaces of low-index 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 added-row 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 09/2011; 115:17799. DOI:10.1021/jp2017133 · 4.77 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 unstable--leaving no consistent picture of the diffusion pathways. Using first-principles quantum-mechanical methods, we find three oxygen interstitial sites in titanium, and quantify the multiple interpenetrating networks for oxygen diffusion. Surprisingly, all transitions contribute to diffusion.
[Show abstract][Hide abstract] ABSTRACT: We develop a first-principles model of thermally-activated cross-slip 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 solutes---K, Na, and Sc---that lower the Mg prismatic stacking
fault energy to improve formability. To connect to thermally activated
cross-slip, 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 cross-slip
softening for a solute as a function of temperature. The direct interaction
calculations predict lowered forming temperatures for Mg-0.7at.%Sc,
Mg-0.4at.%K, and Mg-0.6at.%Na of approximately 250C.