Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session T39: Metals Alloys and Metallic Structures |
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Sponsoring Units: DCMP Chair: David Parker, Oak Ridge National Laboratory Room: 348 |
Thursday, March 21, 2013 8:00AM - 8:12AM |
T39.00001: Spin-lattice coupling in BCC iron Junqi Yin, Markus Eisenbach, Don Nicholson For empirical iron potentials, the magnetic contribution is usually implicitly considered, and the spin-lattice coupling is simply neglected. From first principle calculations, we proposed a Heisenberg type of exchange for BCC iron that couples the spin and lattice degrees of freedom. The parameterization is based on quantities already employed in embedded-atom potentials. Therefore, the model is a natural augmentation of the existing iron potentials, and is applicable to molecular dynamics simulations. Our model built on Dudarev potential can reproduce iron's specific heat from the Curie temperature down to about 400K, and the estimate of the spin-lattice contribution indicates that it is significant near the transition. We applied our model to studying a $<111>$ screw dislocation in BCC iron, and found evidences that the dislocation core has a local transition temperature different from the bulk one. Work is sponsored by the U.S. DOE, Office of Basic Energy Sciences, Materials Sciences and Engineering Division (M. E., D. M. N.), and by Office of Advanced Scientific Computing Research (J. Y.). This research used resources of the Oak Ridge Leadership Computing Facility at the ORNL, which is supported by the Office of Science of the U.S. DOE under Contract No. DE-AC05-00OR22725. [Preview Abstract] |
Thursday, March 21, 2013 8:12AM - 8:24AM |
T39.00002: The modification of core structure and Peierls barrier of 1/2$<111>$ screw dislocation in bcc Fe in presence of Cr solute atoms German Samolyuk, Yuri Osetsky, Roger Stoller, Don Nicholson, George Malcolm Stocks Mobility of screw dislocations controls low temperature plasticity in bcc metals including ferritic alloys. Density functional theory (DFT) is an effective tool in providing parameter-free information on the energetic and magnetic properties of defects including screw dislocations. We summarize DFT calculations on atomic properties of 1/2$<111>$ screw dislocations in Fe-Cr system. The periodic quadrupole approach was applied to model the core dislocation structure, core interaction with Cr solute atoms and to estimate their effect on Peierls stress and barrier. The binding energy of Cr impurity atoms with a screw dislocation and its effect on the dislocation core structure are discussed and the importance of magnetism in the effects of Cr on screw dislocation mobility is demonstrated. This work was supported by the Center for Defect Physics, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences. [Preview Abstract] |
Thursday, March 21, 2013 8:24AM - 8:36AM |
T39.00003: Formation Energies and Electronic Properties of Vanadium Carbides Found in High Strength Steel Alloys Krista Limmer, Julia Medvedeva Carbide formation and stabilization in steels is of great interest owing to its effect on the microstructure and properties of the Fe-based alloys. The appearance of carbides with different metal/C ratios strongly depends on the carbon concentration, alloy composition as well as the heat treatment. Strong carbide-forming elements such as Ti, V, and Nb have been used in microalloyed steels; with VC showing an increased solubility in the iron matrix as compared with TiC and NbC. This allows for dissolution of the VC into the steel during heating and fine precipitation during cooling. In addition to VC, the primary vanadium carbide with cubic structure, a wide range of non-stoichiometric compositions VCy with y varying from 0.72 to 0.88, has been observed. This range includes two ordered compounds, V8C7 and V6C5. In this study, first-principles density functional theory (DFT) is employed to examine the stability of the binary carbides by calculating their formation energies. We compare the local structures (atomic coordination, bond distances and angles) and the density of states in optimized geometries of the carbides. Further, the effect of alloying additions, such as niobium and titanium, on the carbide stabilization is investigated. We determine the energetically preferable substitutional atom location in each carbide and study the impurity distribution as well as its role in the carbide formation energy and electronic structure. [Preview Abstract] |
Thursday, March 21, 2013 8:36AM - 8:48AM |
T39.00004: A computational study of high entropy alloys Yang Wang, Michael Gao, Michael Widom, Jeff Hawk As a new class of advanced materials, high-entropy alloys (HEAs) exhibit a wide variety of excellent materials properties, including high strength, reasonable ductility with appreciable work-hardening, corrosion and oxidation resistance, wear resistance, and outstanding diffusion-barrier performance, especially at elevated and high temperatures. In this talk, we will explain our computational approach to the study of HEAs that employs the Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) method. The KKR-CPA method uses Green's function technique within the framework of multiple scattering theory and is uniquely designed for the theoretical investigation of random alloys from the first principles. The application of the KKR-CPA method will be discussed as it pertains to the study of structural and mechanical properties of HEAs. In particular, computational results will be presented for Al$_x$CoCrCuFeNi ($x$ = 0, 0.3, 0.5, 0.8, 1.0, 1.3, 2.0, 2.8, and 3.0), and these results will be compared with experimental information from the literature. [Preview Abstract] |
Thursday, March 21, 2013 8:48AM - 9:00AM |
T39.00005: First-principles Calculations on the Stability of High Entropy Alloy M. Claudia Troparevsky, Paul Kent, James R. Morris, G. Malcolm Stocks High entropy alloys (HEAs) constitute a new class of materials comprised of four or more elements in equimolar or near equimolar ratio, which tend to form simple solid solutions, mainly FCC or BCC. Despite extensive attention due to their potential applications as structural materials little is known about why these compounds are stable with respect to phase separation. We study the structural and thermodynamic properties of HEAs composed of Cr, Pd, Mn, Fe, Co, and Ni using density functional theory. We investigate the minimum energy structures of several alloys as well as the competing intermetallic compounds in an effort to assess the stability of the HEAs with respect to phase separation. We find that the enthalpy of formation of the alloys is frequently insufficient to explain their stability and that the entropy of mixing can in some cases account for the stability of these compounds. However, for some five-component alloys this does not appear to be sufficient. In this presentation, we will discuss the degree to which the entropy of mixing can stabilize these alloys. [Preview Abstract] |
Thursday, March 21, 2013 9:00AM - 9:12AM |
T39.00006: Non-local first-principles calculations in Cu-Au, Ag-Au and Cu-Ag Yongsheng Zhang, Georg Kresse, Christopher Wolverton Cu-Au is the prototypical alloy system used to exemplify ordering and compound formation, and serves as a testbed for all new alloy theory methods. Yet, despite the importance of this system, conventional density functional theory (DFT) calculations with semi-local approximations (GGA) have two dramatic failures in describing the energies of this system: 1) DFT predicts incorrect ordered ground states for Au-rich compositions, and 2) DFT formation energies of the observed Cu$_3$Au and CuAu compounds are nearly a factor of two smaller in magnitude than experimental values. Here, we show how modern extensions of DFT based on non-local interactions can rectify both of these failures. Using the self-consistent non-local HSE06 functional, the formation energies of Cu$_3$Au and CuAu are -71 and -91 meV/atom, respectively, which are in excellent agreement with the experimental measurements. The semi-local GGA predicted CuAu$_2$ is not a stable phase in the HSE06 calculations, and CuAu$_3$ with the L1$_2$ structure is theoretically predicted as a stable phase. For Ag-Au, both semi-local GGA and non-local HSE06 functionals give similar formation energies. The electronic structures are used to explain these different phenomena in Cu-Au and Ag-Au. [Preview Abstract] |
Thursday, March 21, 2013 9:12AM - 9:24AM |
T39.00007: Stress dependent defect energetics in Tungsten from first-principles Md. Hossain, Jaime Marian Tungsten (W) is an important material for high temperature applications due to its refractory nature. However, like all transition metals from the VI-A group, W suffers from low-temperature brittleness and lack of ductility, which poses serious questions for its use as a structural material. Tungsten's mechanical properties can be enhanced by alloying with elements with d-electrons, such as Re, which has resulted in successful commercial alloys. In this work, we obtain the formation and migration energetics of Re solute atoms in terms of their interaction with vacancies and dislocations. To explore the influence of external stresses on Re transport properties, we examine the role of hydrostatic and shear deformation on the vacancy formation energy (VFE) and migration energy barrier (Em) in BCC W from first-principles calculations by developing a pseudopotential with 6s2, 6p0, 5d4, and 5f0 electronic states for the valence electrons. We find that under hydrostatic deformation, increase or decrease of vacancy formation energy depends on the type of deformation -- tensile or compressive, while for shear deformation it decreases irrespective of the magnitude of applied deformation. On the other hand, migration energy barrier always decreases under hydrostatic deformation, but shows path-length dependent behavior under shear deformation. This talk will discuss the underlying principles and possible routes for enhancing mechanical strength from a physics perspective. [Preview Abstract] |
Thursday, March 21, 2013 9:24AM - 9:36AM |
T39.00008: ABSTRACT WITHDRAWN |
Thursday, March 21, 2013 9:36AM - 9:48AM |
T39.00009: First Principles Calculation of Elastic Properties of Early-Late Transition Metal Alloys William Huhn, Michael Widom Amorphous metals are of practical interest in applications requiring high strength materials. We choose to examine the elastic properties of crystalline phases to understand the elastic properties of amorphous solids. In this talk, we discuss our work using first principles methods to calculate elastic properties for crystalline alloys in various chemical families containing transition metals, specifically early (Ta,W) and late (Fe,Co,Rh,Ni,Cu,Zn) due to their good glass forming ability, as well as select borides. Certain Laves phases, which are known to have local chemical ordering similar to amorphous solids, are focused on. We analyze trends in the elastic properties of chemical families based on computed enthalpies of formation, elastic properties of pure elemental phases, and electronic and structural information. In particular, we use effective medium theories and enthalpies of formation to predict trends in bulk moduli. This information can be used to predict future candidate systems for high-strength amorphous metals. [Preview Abstract] |
Thursday, March 21, 2013 9:48AM - 10:00AM |
T39.00010: Structural transformations in the physical mixture of Pd and Cu nanoparticles Vineetha Mukundan, Jun Yin, Chuan-Jian Zhong, Oana Malis Pd-Cu bimetallic nanoparticles have the potential to replace palladium, the second most active metal having important applications as a catalyst in fuel cell and hydrogen storage reactions. We investigated the temperature-induced transformations in physical mixtures of Pd and Cu nanoparticles, using in-situ real-time synchrotron based x-ray diffraction. These nanoparticle mixtures undergo coalescence and structural phase transformations at relatively low temperature, and sinter at higher temperature. They form alloys with ordered bcc (B2) structure at low temperature (300C). At higher temperature (450C), it transforms into a disordered fcc (alloy) structure. The structural parameters probed are size, phase, composition and morphology. Grain growth was modeled with growth laws proposed for nanocrystalline materials and the diffusion mechanism driving sintering was explored. The effect of elemental compositions, different substrates and annealing atmospheres on the evolution of the PdCu alloy nanoparticles was also explored. [Preview Abstract] |
Thursday, March 21, 2013 10:00AM - 10:12AM |
T39.00011: Twinning in nanocrystalline fcc and bcc metals Vladimir S. Boyko, Roman Ya. Kezerashvili The deformation twinning in nanocrystalline (nc) face-centered cubic (fcc) metals, body-centered cubic (bcc) metals, and in nc Si is analyzed. The phenomenological approach is used to make a bridge between microscopical mechanisms of twin nucleation and macroscopical characteristics of twinning with different crystal structures and to calculate the grain size range of the twinning propensity, the requisite external stress for twinning propagation in nc polycrystals, and the grain size at which the slip begins to prevail over the twinning. The developed approach allows to derive analytical expressions and estimate lower and and upper limits of grain sizes at which a twinning propensity is occurred. Results of calculations for the nc fcc metals Al, Cu, Ni, Pd, Au, nc bcc metals Ta, Fe, Mo, W, Nb, and nc diamond-cubic Si are compared with the experimental data, otherwise predictions are made. [Preview Abstract] |
Thursday, March 21, 2013 10:12AM - 10:24AM |
T39.00012: Irradiation-induced formation of nano-crystallites with C15 Laves phase structure in bcc iron Mihai-Cosmin Marinica, Fran\c{c}ois Willaime, Jean-Paul Crocombette The thermal diffusion of defects as vacancies or interstitials is the main process which drives the material towards equilibrium after or in parallel to the damage production. A three dimensional periodic structure is proposed for self-interstitial clusters in body-centered-cubic metals, as opposed to the conventional two dimensional loop morphology [1]. The underlying crystal structure corresponds to the C15 Laves phase. The new three dimensional structures generalize previous observations [1, 2]. By systematic exploration of the energy landscape performed using an Eigenvector Following method [3] and Density Functional Theory calculations, we demonstrate that in $\alpha $--iron these C15 aggregates are highly stable and immobile and that they exhibit large antiferromagnetic moments. These clusters form directly in displacement cascades and they can grow by capturing self-interstitials. This new morphology of self-interstitial clusters thus constitutes an important element to account for when predicting the microstructural evolution of iron base materials under irradiation. \\[4pt] [1] M.-C. Marinica et al. , Phys. Rev. Lett. 108, 025501 (2012).\\[0pt] [2] D. J. Bacon et al., J. Nucl. Mater. 276, 1 (2000); D. Terentyev et al,. Phys. Rev. Lett. 14, 145503 (2008)\\[0pt] [3] G.T. Barkema and N. Mousseau, Phys. Rev. Lett. 77, 4358 (1995); M.-C Marinica et al., Phys. Rev. B 83, 094119 (2011). [Preview Abstract] |
Thursday, March 21, 2013 10:24AM - 10:36AM |
T39.00013: Composition fluctuation, local clustering, and crystallization in multi-component systems Minglei Wang, Kai Zhang, Stefanos Papanikolaou, Jan Schroers, Corey S. O'Hern We perform molecular dynamics simulations of model multi-component metallic liquids to study mechanisms for non-polymorphic crystallization. We measure local concentration fluctuations, nucleation rates, and clustering as a function of the cooling rate for different size ratios, stoichiometries, and attraction strengths. In preliminary studies, we find that over a wide range of particle size ratios and cooling rates, small particles cluster in the interstices of contact networks formed by the large particles. These studies are important for understanding which systems are prone to crystallization and which are good glass-formers. [Preview Abstract] |
Thursday, March 21, 2013 10:36AM - 10:48AM |
T39.00014: Ab-initio study of the structure and dynamics of bulk liquid Cadmium and its liquid-vapour interface David J. Gonzalez, Lazaro Calderin, Luis E. Gonzalez Several static and dynamic properties of bulk liquid cadmium at a thermodynamic state near its triple point have been calculated by {\em ab-initio} molecular dynamics simulations. The calculated static structure shows a very good agreement with the available experimental data. The dynamical structure reveals collective density excitations with an associated dispersion relation which points to a small positive dispersion. Results are also reported for several transport coefficients. Additional simulations have also been performed in order to study the structure of the free liquid surface. The ionic density profile shows an oscillatory behavior with two different wavelengths as the spacing between the outer and first inner layer is different from that between the other inner layers. The calculated reflectivity shows a marked maximum whose origin is related to the surface layering along with a shoulder located at a much smaller wave-vector transfer. [Preview Abstract] |
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