Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session D23: Diffusion and Transport Properties |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Duane Johnston, University of Illinois at Urbana-Champaign Room: 325 |
Monday, March 16, 2009 2:30PM - 2:42PM |
D23.00001: A multiscale study of hydrogen embrittlement of metals: Revisiting the hydrogen enhanced local plasticity (HELP) mechanism Johann von Pezold, J\"{o}rg Neugebauer The embrittlement of metals by H is a long-standing problem, whose underlying mechanisms are still largely unclear. Here we consider the atomistic basis of the HELP mechanism, which asserts that H mobilises dislocations by shielding elastic dislocation-dislocation interactions. Using a combination of density-functional theory calculations, semiempirical EAM potentials and an effective lattice gas Hamiltonian we determine the interaction of H with the strain field around edge dislocations in Ni. Our results reveal an attractive, but short ranged interaction between H interstitials, which leads above a critical concentration to the formation of a precursor hydride phase. The increased lattice parameter of this hydride phase induces significant misfit stress, but localises the dislocation stress field due to a localisation of the dislocation strain field. For H concentrations of up to 0.25 at. \%, the stress localisation is found to dominate and thus effectively shields the stress field of the dislocation, pointing towards a novel atomistic basis for the HELP mechanism. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D23.00002: DFT energetics of hydrogen binding to point defects in iron and steels William Counts, Chris Wolverton, Ron Gibala It is well known that hydrogen degrades the properties of iron and steel. One proposed mechanism of hydrogen attack is the concept of ``hydrogen traps'', which are generally microstructural material defects that bind hydrogen atoms. The stability and energetics of a number of potential traps (like vacancies, interstitials, and substitutional alloying elements) in bcc and fcc iron are investigated using density functional theory (DFT). We find that hydrogen is very sensitive to its local environment. For example, inIn a perfect bcc iron lattice, hydrogen prefers to sit in the tetrahedral site rather than the octahedral site or a substitutional site, and there is a repulsive interaction between carbon and hydrogen. In the presence of a vacancy, the energy barrier between the octahedral and tetrahedral sites disappears and hydrogen only resides in the octahedral sites. The hydrogen-vacancy binding energy changes as more hydrogen atoms bind to the vacancy, but the binding energy does not significantly change in the presence of carbon. We also find that H$_{2}$ molecules are not stable inside a vacancy: they dissociate spontaneously (without a barrier) into octahedral positions. Details about vacancy and other potential traps are presented and their roles in hydrogen embrittlement are discussed. [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D23.00003: First-principles studies of helium in palladium tritides: PdT$_{x}$ (0$\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle=}\vphantom{_x}}$}} $x$\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle=}\vphantom{_x}}$}} $1) Pei Lin, Yan Wang, M.Y. Chou Helium bubbles have significant impact on the stability and mechanical properties of materials used in nuclear-energy systems. Theoretical studies on the behavior of He in various metals have been reported in the past. However, few studies have taken into account the effect of the coexistence of He and tritium ($^{3}$H) in the metal lattice. We have performed first-principles calculations of He inside palladium tritides with various H concentrations. Instead of interacting with He impurities directly, the interstitial H mainly modifies the electronic structure of the metal lattice. The Pd d-orbital near the Fermi-level shifts downward as H atoms occupy the interstitial sites, which in turn promotes the interaction between He 2-s and Pd 4-d states. We have examined the changes in the formation energy of a He impurity with various H concentrations. The effect will further modify the energetics of trapping multiple He atoms to form a cluster inside the hydrogenated Pd system. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D23.00004: Calculating self diffusion in Mo using the AM05 density functional Thomas R. Mattsson, Nils Sandberg, Rickard Armiento, Ann E. Mattsson Vacancy diffusion is a major mechanism of mass transport in solids. While the motion of vacancies and interstitials is largely understood for fcc metals like aluminum [1], important questions remain for bcc metals. We present first principles and model potential simulations of self-diffusion in Mo, compare the results to available experimental data, and discuss the magnitude and origin of different contributions to the diffusion. The density functional AM05 [2] is employed to calculate formation energies and diffusion barrier for vacancy migration. AM05 has been successfully applied to a wide range of different solids [3] and is shown to perform well also for Mo. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [1] N. Sandberg, B. Magyari-Kope, and T.R. Mattsson, PRL 89, 065901 (2002). [2] R. Armiento and A. E. Mattsson, PRB 72, 085108 (2005). [3] A.E. Mattsson, et al. JCP 128, 084714 (2008). [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D23.00005: Prediction and Modeling of Atomic Mobility in Alloys Zi-Kui Liu Atomic diffusion is a common and important non-equilibrium process in solids that takes place at finite temperatures. To computationally simulate atomic diffusion processes, the thermodynamic and atomic mobility databases of the materials of interest are needed. The modeling technique of atomic mobility databases and related software has been becoming more and more matured in the last decades. However, the input data for the modeling is exclusively taken from experimentally measured tracer and chemical diffusion coefficients. In this presentation, our recent progress in predicting self and dilute diffusion coefficients by quantum mechanics calculations will be discussed [1]. Our approach to the unstable vibrational mode of transition states during diffusion will be presented. The contribution to phenomenological modeling of atomic mobility will be briefed. \\[4pt] [1] M. Mantina, Y. Wang, R. Arroyave, L. Q. Chen, Z. K. Liu and C. Wolverton, ``First-principles calculation of self-diffusion coefficients,'' Phys. Rev. Lett., Vol.100, 2008, 215901. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D23.00006: Analysis of Cd jump rates among the two Ga sublatices in Ga$_7$Pd$_3$ using a stochastic model of hyperfine interactions M. O. Zacate, W. E. Evenson, G. S. Collins Atomic jump rates of Cd among the two inequivalent Ga sublattices in Ga$_7$Pd$_3$ were measured using perturbed angular correlation spectroscopy (PAC). Atomic jumps result in a reorientation and/or change in strength of electric field gradients experienced by the Cd PAC probes. Spectra were fitted to model functions generated using a stochastic model for fluctuating EFGs under the assumption that Cd probes can jump to nearest neighbor sites of each Ga sublattice. Jump activation energies were determined by fitting spectra collected at different temperatures simultaneously under the constraint that jump rates obey Arrhenius behavior. The following activation energies were determined for intra- sublattice jumps: 0.52(1) eV for Ga(3)$\rightarrow$Ga(3) and 0.25(4) eV for Ga(4)$\rightarrow$Ga(4) and for inter-sublattice jumps: 0.6(3) eV for Ga(3)$\rightarrow$Ga(4) and 0.47(9) eV for Ga(4)$\rightarrow$Ga(3), in which Ga(4) denotes the site with .3m point symmetry. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D23.00007: Vanadium diffusion in ferrite Sungho Kim, Jeffrey Houze, Seong-Gon Kim, Mark Horstemeyer Vanadium is a very important additive in high strength steel alloys. Small amount of vanadium addition strengthens the steel alloys. As a first step to study the effect of vanadium addition in steel alloys we calculated the diffusion path and the migration energy of vanadium atom in ferrite crystal using first principles calculations. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D23.00008: Anisotropic physical properties of RSn$_{2}$ (R = Gd - Lu) single crystals Emilia Morosan, Michael Mehlman, Liang Zhao, Thomas Sanders A wealth of magnetic and electronic properties (including metamagnetism, spin glass, non Fermi liquid behavior etc) has been observed in rare earth intermetallic compounds. The availability of these systems in single crystal form is imperious for characterizing their complex behavior. We are reporting the anisotropic physical properties of single crystals of the orthorhombic RSn$_{2}$ compounds (R = Gd - Lu). Complex anisotropic H -- T phase diagrams are observed in GdSn$_{2}$, which has an antiferromagnetic ground state below 27.5 K, with two more transitions at lower temperatures. The magnetic field induces metamagnetic phase transitions in several of the R members of the series. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D23.00009: Magnetostructural properties of RCo$_{2}$ (R = rare earth) compounds Durga Paudyal, Ya. Mudryk, V. K. Pecharsky, K. A. Gschneidner, Jr. First principles electronic structure calculations show that the cubic Laves phase (C15) is stable for GdCo$_{2}$ and TbCo$_{2}$, while the ground state structures of DyCo$_{2}$, HoCo$_{2}$, and ErCo$_{2}$ are tetragonal distortions of the cubic C15, orthorhombic, and rhombohedral structures, respectively. The R and Co moments exhibit antiparallel coupling, forming a ferrimagnetic state in every ground state structure of RCo$_{2}$. The predicted magnetostructural properties of RCo$_{2}$ are in good agreement with x-ray powder diffraction and magnetic measurements. The spin splitting of the conduction electrons due to the indirect R-R exchange plays a crucial role in the magnetostructural transformations in the R = Dy, Ho, and Er RCo$_{2}$ phases, and accounts for its absence in the monomorphic GdCo$_{2}$ and TbCo$_{2}$ compounds. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D23.00010: Transition in NiMnSn and NiMnIn C.P. Opeil, J.C. Lashley, J.L. Smith, T. Planes, L. Manosa Magneto-transport, specific heat, magneto-striction and temperature dependent UV photoemission are used to explore the martensite transition of the ferromagnetic shape memory alloys Ni$_{x}$Mn$_{y}$In$_{z}$ and Ni$_{x}$Mn$_{y}$Sn$_{z}$. Comparisons will be made to a previous work\footnote{Opeil, et al. \textit{Physical Review Letters} \textbf{100}, 165703 (2008).} on the stoichiometric single crystal Ni$_{2}$MnGa which reveals a temperature (235 \underline {$>$} T \underline {$>$} 190 K) and field dependent (0 -- 1 T) positive/negative magneto-resistance slope. Our experimental results will be discussed in light of a possible pseudo-gap formation coincident with the martensite transition in the two off-stoichiometric alloys. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D23.00011: Measurements of persistent currents in normal metal rings with cantilever torsional magnetometry William Shanks, Ania Bleszynski Jayich, Bruno Peaudecerf, Jack Harris We have measured the magnetization of arrays of micron-scale aluminum rings at low temperatures and in high magnetic fields using cantilever torsional magnetometry. We see clear evidence of normal state persistent currents in these rings. The current's dependence on magnetic field, temperature and the rings' circumference is consistent with theoretical predictions (due to Riedel and von Oppen) for non-interacting electrons in the diffusive regime. To fully characterize these samples, we also measured the magnetization of codeposited rings in the superconducting state and the magnetoresistance of codeposited wires. Together these measurements provide an especially clear picture of the normal-state persistent currents. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D23.00012: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 4:54PM - 5:06PM |
D23.00013: AC-condutance of a quantum chaotic cavity (semiclassical approach) C. Petitjean, D. Waltner, J. Kuipers, I. Adagideli, K.R. Richter Due to the progress made in the control and the manipulation of mesoscopic structures driven by high frequency periodic voltages, the ac regime has been recently experimentally investigated [1] and consequently its theoretical interest has been renewed. We consider here, a quantum chaotic cavity that is coupled via tunnel barriers and gates to a macroscopic circuit which contains ac-sources [2]. By extending to the ac-transport, the recent trajectory-based semiclassical theory of quantum chaotic transport in presence of tunnel barrier [3], we derive for arbitrary tunneling rates and arbitrary positive Ehrenfest time, the averaged and the weak-localization correction to the screened conductance. Then we use these results to investigate the effect of dephasing on the relaxation resistance of a chaotic capacitor in the linear low frequency regime. This last investigation are in principle relevant to the recent measure of the admittance at zero magnetic flux of a mesoscopic capacitor [1,4].\\ {\bf References}\\ \newcommand\itm[2]{\parbox[t]{1cm}{#1}\parbox[t]{14cm}{#2}} \itm{[1]} {J. Gabelli et al., Science {\bf 313}, 499 (2006).}\\ \itm{[2]} {C. Petitjean et al, {\it in preparation } (2008).}\\ \itm{[3]} {R.S.~Whitney, Phys. Rev. {\bf B, 75}, 235404 (2007).}\\ \itm{[4]} {S.~Nigg and M.~B\"uttiker, Phys. Rev. {\bf B 77}, 085312 (2008).} [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D23.00014: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 5:18PM - 5:30PM |
D23.00015: Evidence for Conditioning as a Progenitor of Double-C Transformation Mechanisms in Pu-Ga Alloys. J.R. Jeffries, K.J.M. Blobaum, M.A. Wall, A.J. Schwartz By alloying Pu with Ga, the fcc $\delta$ phase can be retained down to room temperature. This metastable $\delta $ phase is realized due to slow Ga diffusion, which prevents the $\delta$ phase from decomposing into the equilibrium mixed phase structures. The metastable $\delta$ phase in a Pu-1.9 at.{\%} Ga alloy, however, does yield to chemical driving forces by undergoing the $\delta \quad \to \quad \alpha $' isothermal martensitic transformation below M$_{s} \quad \approx $ -100 \r{ }C. This transformation exhibits poorly understood double-C behavior in the time-temperature-transformation diagram. Recently, a ``conditioning'' treatment---which entails isothermally holding a specimen at sub-anneal temperatures but above M$_{s}$ ---has been shown to dramatically affect the amount of $\alpha $' phase formed at low temperature. We report evidence that the conditioning treatment induces the lower-C of the double-C curve, and we implicate the classical nucleation of equilibrium phases as the underlying mechanism behind conditioning in Pu-Ga alloys. [Preview Abstract] |
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