Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session L35: Metals: Alloys and Impurities |
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Sponsoring Units: DCMP Chair: Michael Mehl, Naval Research Laboratory Room: C140 |
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L35.00001: First Principles Stability and Coherency Strain in Mg3RE (RE=rare earth) D019 Metastable Precipitates in Mg A. Issa, J. Saal, C. Wolverton As the need for strong yet lightweight materials intensifies, magnesium alloys have become increasingly important. Although lightweight, these alloys exhibit low strength, particularly in comparison to aluminum alloys. The potential to greatly strengthen magnesium alloys has driven current research, with a recent focus on strengthening precipitates, particularly involving rare earth (RE) dopants. The morphology of these precipitates dictates their effect on the strength of the alloy, and quantifying the coherency strain between the precipitates and the Mg matrix is key to determining the morphology of the precipitate. The large size of the potential composition space makes a systematic experimental study costly and time consuming. Therefore, we apply density functional theory (DFT) to systematically predict the formation energies and coherency strains of D019 precipitates in Mg-RE systems along several crystallographic directions. In particular, we look for D019 precipitates that favorably form plate-shaped morphologies along non-basal planes, as this morphology should be effective obstacles to plastic deformation. These Mg-RE systems also provide an interesting testing ground for the accuracy of DFT methods for intermetallic compounds containing f- electrons. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L35.00002: Chemical Correlations in Atomic Size-Mismatch Disordered Alloys Predicted from KKR-DCA D.A. Biava, D.D. Johnson The dynamical cluster approximation (DCA) has been implemented in a Korringa-Kohn-Rostoker (KKR) electronic-structure method to predict electronic and structural properties of disordered alloys, in particular, chemical short-range order (SRO). We adapted an optimal-basis method\footnote{A. Alam and D.D. Johnson, Phys. Rev. B 80, 125123 (2009)} to the KKR-DCA to account for variations in atomic size due to different configurations present in size-mismatch alloys. In comparison to experiment, we find excellent agreement for predicted lattice constants and SRO, with origins identified in the electronic structure and affecting mechanical properties at finite temperatures. We also show how coarse-grained symmetry of the DCA can be exploited to reduce memory and computation time, allowing us to perform for the first time self-consistent KKR-DCA calculations with $2^{16}$ or more configurations (and atoms) on a single compute node. [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L35.00003: Modified Embedded Atom Method potential for Fe-C system Laalitha Liyanage, Jeff Houze, Seong-Gon Kim, Mark Tschopp, Sungho Kim, Mike Baskes, Mark Horstemeyer A Modified Embedded Atom Method potential for the Fe-C alloy system was developed. Pair parameters were constructed based on the structural and elastic properties of element pairs in the L12 reference structure from ab-initio simulations and then adjusted to reproduce heat of formation and elastic constants of cementite, and the interstitial energies for iron. The single element potential of carbon correctly predicts graphite and diamond as the two minimum energy structures.The potential parameters were optimized using an optimization method combining Latin hypercube sampling of the N-dimensional parameter space and multi-objective optimization. The potential was tested for stability of cementite by molecular dynamic simulation at room temperature. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L35.00004: The unified mechanism of aging effects in both martensite and parent phase for shape-memory alloys: atomic-level simulations J. Deng, X. Ding, T. Suzuki, K. Otsuka, T. Lookman, A. Saxena, J. Sun, X. Ren Most shape-memory alloys (SMAs) subject to the aging effects not only in the martensite phase but also in the parent phase. These aging effects have been attracted much attention as they strongly affect the practical applications of SMAs. So far, the intrinsic mechanism of them has remained controversial due to the difficulty in visualization of what happens in atomic scale. In the present study, by using a combination of molecular dynamics method and Monte-Carlo method [1], we investigate the aging effects in both martensite and parent phase. We successfully reproduced the thermal behaviors of aging effects for SMAs, i.e., the $A_{f}$ temperature increase with aging time in martensite and the $M_{s}$ temperature decrease with aging time in parent phase, which keep good agreement with the experimental observations [2]. In addition, quantitative analysis of the atomic configurations during aging reveals that the aging effects are not associated with a change in the average structure. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L35.00005: Enhanced High Temperature Mechanical Behavior of FeCo-Based Alloys Robert Cammarata, Dezhi Zhang, Chia-Ling Chien FeCo alloys have been used for a variety of soft magnetic material applications, including for use in high temperature engine applications. However, inferior mechanical properties, in particular relatively low creep resistance, can limit their use at elevated temperatures. We have investigated a variety of microstructural engineering approaches to improve the creep resistance without significantly degrading the magnetic properties. Two such approached will be discussed: oxide dispersion strengthening and annealing treatments leading to grain growth and precipitation hardening. We have shown that both of these methods allow for sensitive control of the resulting microstructural evolution. This control in turn allows for substantial improvement in both the room temperature yield strength as well as the high temperature creep resistance. Detailed microstructural characterization as well as tensile and testing results will be presented. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L35.00006: Fermi surface of an important nano-sized metastable phase: Al$_{3}$Li Stephen Dugdale, Jude Laverock, Ashraf Alam, Mina Roussenova, Joanne Wensley, Jadwiga Kwiatkowska, Nobu Shiotani Nanoscale particles embedded in a metallic matrix are of considerable interest as a route towards identifying and tailoring material properties. In particular, Al-Li alloys, which form ordered nanoscale precipitates of $L1_2$ Al$_3$Li for a range of Li concentrations, have been deployed successfully in the aerospace industry owing principally to their superior strength-to-weight ratio. These precipitates, however, are metastable and only form within the surrounding Al matrix, meaning their electronic structure, thought to be important in contributing to the enhanced material properties through its Young's modulus, has so far been inaccessible through conventional techniques. Here, we take advantage of the strong positron affinity of Li to directly probe the Fermi surface of metastable Al$_3$Li nanoscale precipitates of Al-Li. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L35.00007: Metastable states along the Bain path in AgZr with AFLOW Michael Mehl, Gus Hart, Michal Jahnatek, Stefano Curtarolo AgZr crystallizes in the B11 structure, which is bcc-like with stacking AABB along [001]. Using AFLOW\footnote{S. Curtarolo {\em et al.}, http://materials.duke.edu/aflow.html} we find another low energy structure, ``Z2'' ,\footnote{Z.W. Lu {\em et al.}, {\em Phys. Rev. B} {\bf 44}, 512 (1991)} an fcc-like variant of B11. The B11 to Z2 transition follows the Bain path, with c/a changing from 1.9 (B11) to 2.6 (Z2). This seems similar to results for elemental bcc solids,\footnote{M. J. Mehl {\em et al.}, {\em Phys. Rev. B} {\bf 70}, 014105 (2004)} where we find a secondary Bain path minimum which is elastically unstable. Here there is no simple path from the Z2 structure back to the B11 structure, and the Z2 structure is metastable. Using first- principles DFT we demonstrate the possibility of a pressure induced phase transition from B11 to Z2 at 35~GPa. We also examine the L1$_0$ structure, which is higher in energy than Z2 at zero pressure. We find that a transition from B11 to L1$_0$ at 32~GPa, so that L1$_0$ is the true high-pressure phase of AgZr. We discuss the stability of all three of these phases at both zero and high pressure, and the possibility of similar transitions in more useful materials. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L35.00008: Localized Rattling of Al atoms in VAl$_{10+\delta}$ Douglas Safarik, Tomasz Klimczuk, Anna Llobet, Darrin Byler, Ekhard Salje We have studied the localized rattling mode in the \lq{Einstein solid}\rq~VAl$_{10+\delta}$ using a suite of thermodynamic, transport, and neutron diffraction measurements. The rattling originates from Al atoms that occupy the large void within Z$_{16}$ Friaf polyhedra, of which there are eight per unit cell in the VAl$_{10+\delta}$ structure. Our heat capacity, thermal expansion, and electrical resistivity data are all qualitatively consistent with a low-frequency harmonic vibration of the atom. However, our neutron diffraction data show that the rattling atom potential is better described as sixth-order, rather than harmonic. Using a single-site, sixth-order effective potential for the rattling atom, we can explain our thermodynamic, transport, and structural data, including the unusual temperature dependence of the elastic constants. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L35.00009: High-energy diffraction measurements of deeply undercooled Co-Pd liquids using electrostatic levitation G.E. Rustan, N.A. Mauro, J.C. Bendert, K.F. Kelton, A. Kreyssig, A.I. Goldman Co-Pd liquids in their deeply undercooled state have attracted a great deal of interest because of the potential for magnetically triggered nucleation of the solid phase. We report on the results of high-energy x-ray diffraction measurements, using 129 keV x-rays at the Advanced Photon source, on a series of liquid Co-Pd alloys in a containerless environment employing electrostatic levitation. Diffraction data were collected using a flat-plate two-dimensional detector during free cooling from temperatures well above the liquidus, to as much as 200 degrees C of undercooling for compositions ranging from 50:50 to 80:20 (Co:Pd). The composition dependence and temperature dependence of differences in structure will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L35.00010: Ab initio studies of the effect of nanoclusters on magnetostriction of Fe1-xGax alloys Hui Wang, Yanning Zhang, Teng Yang, Z.D. Zhang, Lizhi Sun, Ruqian Wu We investigated the effects of B2-like and D0-like nanoclusters on the magnetostriction of Fe-xGax alloys, through systematic density functional calculations. While the B2-like Fe-Ga clusters still undergo slightly tetragonal distortion, D03-like Fe-Ga clusters remain to be cubic in the Fe matrix. Moreover, we found that B2-like nanostructures produce negative magnetostriction whereas D03- like nanostructures give small positive magnetostriction in Fe-xGax alloys. The formation of nanoscale precipitates cannot be the reason for the extraordinary enhancement of magnetostriction of Fe1-xGax alloys. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L35.00011: The lost heat capacity and entropy in the helical magnet MnSi Sergei Stishov, Alla Petrova, Anatoly Shikov, Thomas Lograsso, Eyvaz Isaev, Borie Johansson, Luke Daemen We report results of measurements and analysis of the heat capacity of MnSi. The measurements included data collection at a magnetic field of 4T, which suppresses strongly the longitudinal spin fluctuations and the phase transition. To analyze the experimental data, calculations of the phonon spectrum and phonon density of states in MnSi were performed. Inelastic neutron scattering with a polycrystalline sample of MnSi was used to validate the computational results. The combination of the experimental and theoretical data turned out to be decisive in revealing some hidden features of the thermal excitations in MnSi. In particular, the analysis of the available data led conclusively to the existence of a negative contribution to the heat capacity and entropy in MnSi at $T>T_c$, implying that a specific spin ordering process did occur in the paramagnet phase of MnSi. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L35.00012: A common magnetic origin for the Invar effects in fcc iron-based ferromagnets Chris Hooley, Francois Liot Using first-principles calculations, in conjunction with Ising magnetism, we undertake a theoretical study to elucidate the origin of the experimentally observed Invar effects in disordered fcc iron-based ferromagnets. First, we show that our theory can account for the Invar effects in iron-nickel alloys, the anomalies being driven by the magnetic contributions to the average free energies. Second, we present evidence indicating that the relationship between thermal expansion and magnetism is essentially the same in all the studied alloys, including those which display the Invar effect and those which do not. Hence we propose that magnetism plays a crucial role in determining whether a system exhibits normal thermal expansion, the Invar effect, or something else. The crucial determining factor is the rate at which the relative orientation of the local magnetic moments of nearest-neighbor iron atoms fluctuates as the system is heated. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L35.00013: Electronic Origin of Fast Sulfur Diffusion in 3d Transition Metals Dmitri Novikov, Alan Cetel, Michael Maloney, Kevin Schlichting, Brad Cowles, Sergey Okatov, Iliya Lomayev, Yuri Gornostyrev, Segei Burlatsky The microscopic origins of abnormally fast diffusion of sulfur in nickel have been investigated. Transition state theory of vacancy mediated diffusion of substitutional impurities with parameters calculated from first-principle density-functional theory (DFT) was used to determinate of the diffusion coefficients of S and Al impurities in fcc Ni. Sulfur diffusion coefficient was found to be two orders of magnitude higher than for aluminum in good agreement with experimental data. We found that sulfur has a very low barrier for jump toward vacancy and also significantly decreases migration barriers for neighboring nickel atoms. We discuss the microscopic factors contributing to the dramatic difference in S and Al diffusion coefficients and show that electronic structure and chemical bonding play crucial role in enhanced diffusion of S. We also found that S considerably increases Ni self-diffusion rate. The implications of S effect on the stability of thermally grown oxides in superalloys are discussed. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L35.00014: Nitrogen Adsorption, Solubility and Transport within Group V Metals Panithita Rochana, Ekin Ozdogan, Jennifer Wilcox It is well known that Group V metals have strong-binding characteristics to diatomic molecule, e.g. N$_{2}$, O$_{2}$, H$_{2}$, and CO. Within this study, N$_{2}$ has been investigated to determine the mechanism of surface adsorption, dissociation and subsequent atomic diffusion into the bulk crystal structure of vanadium(V). Alloys of ruthenium(Ru)-V have been examined indicating that Ru can be used to tune the electronic structure of the bulk to enhance atomic diffusion. Electronic structure calculations based on density functional theory have been studied on the investigation of N$_{2}$ adsorption on 3 low-index surfaces, (110), (100) and (111). Preliminary investigations indicate that the V(111) surface binds N$_{2}$ the strongest at fcc site (E$_{ads}$ = 1.4eV). To determine bulk solubility, binding energy calculations are carried out as a function of N concentration. N was found to be stable primarily at O-sites within the bulk V lattice. Bader charge and density of states analyses are analyzed to investigate the mechanism of bulk absorption and solubility phenomena. Results will be presented on the adsorption, bulk solubility, and transport of N in V and V-based alloys. The application to this study is toward the design of an N$_{2}$-selective dense membrane in which atomic N may be produced on the permeate side with hydrogen as a sweep gas for the ammonia synthesis process. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L35.00015: The wake of H in V, Nb and Ta at elevated temperatures: Irreversibility and non-central forces revisited Franz Reidinger At elevated temperatures U and Do of the Arrhenius equation for diffusion describe the amplitude and relaxation rate, respectively, of the stern wave wake of H in V, Nb and Ta. The key evidence for this hypothesis is the close correlation between the isotope dependence of U derived from the Gorsky measurements$^{1}$ and the shear distortion of the orthorhombic phases of NbH(D) and TaH(D). The isotope dependence of U can be expressed in closed form: U=a$\sqrt{M}$ + b$\sqrt{m}$ where M and m are the atomic numbers of the host metal and H isotope and a and b are 7.4 and 37 for Nb and Ta, and 0 and 55 for V, respectively, in units of meV. I explain this correlation in two steps: a) the cubic symmetry of the nearest neighbor strain field$^{2}$ of the interstitial H is the result of a dynamic superposition, possibly caused by a JT resonance$^{3}$, of the two orthorhombic variants of $\beta $-NbH0.75 and b) the successful characterization of the diffusion process as jump diffusion$^{4}$ eliminates the transition state from consideration. Instead it is the relaxation of the just emptied site from its residual orthorhombic distortion towards the cubic symmetry of the bcc metal which is being measured. 1)Z Qi, J Voelkl, R Laesser and H Wenzl: J. Phys. F 13, 2053 (1983) 2)G Bauer, E Seitz, W Schmatz and H Horner: Sol. State Comm. 17, 161 (1975) 3)G C Abell: J. Phys. F 12, 1143 (1982) 4) V Lottner, A Heim and T Springer: Z. Physik B 32, 157 (1979). [Preview Abstract] |
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