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Now showing items 1 - 16 of 74

  • High-throughput calculations in the context of alloy design

    van de Walle, Axel   Asta, Mark  

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  • Summary report of CALPHAD XLI – Berkeley, California, USA, 2012

    Asta, Mark   Turchi, Patrice E.A.  

    156 scientists from 24 countries participated in CALPHAD XLI, which was held in Berkeley, California, USA, June 3-8, 2012, with 74 morning and afternoon presentations, and 72 evening presentations. The topics covered during the conference were gathered in five categories: Modeling - Software; Kinetics Microstructure; CALPHAD Assessments - Experiments; Ab initio; and Nuclear Fuel Materials. In this brief summary, highlights of the conference are presented with titles and abstracts of all presentations.
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  • Effect of non-Schmid stresses on 〈a〉-type screw dislocation core structure and mobility in titanium

    Poschmann, Max   Asta, Mark   Chrzan, D.C.  

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  • Solid–Liquid Interfaces: Molecular Structure, Thermodynamics, and Crystallization

    Asta, Mark   Spaepen, Frans   Veen, J. Friso van der  

    Materials phenomena ranging from the melting or freezing of ice to biomineralization in living organisms, to lubrication and the commercial casting of superalloys, are known to be critically influenced by molecular-scale structure and processes occurring at the interfaces between the crystalline solid and liquid phases. The properties of solid-liquid interfaces have long been a topic of intense interest in materials science, primarily because of their role in governing nucleation, growth, and morphological evolution in crystal growth from the melt or solutions. This issue of the MRS Bulletin provides an overview, highlighting new developments in experiment, theory, and modeling techniques that have led to substantial recent progress in the characterization of the molecular-level structural and thermodynamic properties of solid-liquid interfaces and their consequences for a variety of crystallization phenomena.
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  • Thermodynamic properties of FCC-based Al-Ag alloys

    Asta, Mark   Johnson, D.D.  

    The energetics and thermodynamics of fcc-based Al-Ag alloys have important consequences for the formation of Guinier-Preston (GP) zones in Al-rich alloys. Bulk thermodynamic properties of stable and metastable fcc-based phases in the Ag-Al system are studied from first principles using quantum-mechanical, total-energy methods combined with the cluster-variation method. From the results of these calculations the metastable fcc-based Ag-Al phase diagram has been calculated. Our calculated phase diagram features a monotectoid reaction. A miscibility gap in the fcc solid solution phase is present above the monotectoid temperature and a two-phase field between the Al-rich solid solution and an ordered fcc-based Ag2Al phase is predicted for low temperatures. Therefore, our calculations provide clear evidence that the structure of ? (high temperature) and η (low temperature) GP-zone precipitates should be disordered and long-range ordered, respectively. Our preliminary findings using a full-potential electronic-structure technique indicate that lattice relaxations in the alloys can significantly alter the formation energies.
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  • Theoretical study of alloy phase stability in the Cd-Mg system

    Asta, Mark   McCormack, Ryan   de Fontaine, Didier  

    A theoretical study of thermodynamic properties and of the phase diagram for crystalline hcp-based Cd-Mg alloys is presented. Many first-principles studies of phase diagrams for metallic alloys have considered only configurational contributions to the free energy, which arise from the effects of substitutional disorder on the entropy and enthalpy. In this paper, the additional effects of the vibrational free energy, the electronic entropy, and the energy associated with structural relaxations on the thermodynamic properties and calculated phase equilibria for Cd-Mg alloys are studied. Ground-state properties and the densities of states of stable and metastable Cd-Mg compounds with hcp-based structures have been calculated with the linear muffin-tin orbital (LMTO) method. The results of these LMTO calculations are combined with the cluster variation method to calculate the configurational free energy and electronic entropy of ordered and disordered alloys from first principles. The vibrational free energy is treated using the Debye model; the configurational dependence of the Debye temperature is obtained semiempirically from experimentally measured entropies of formation in combination with the results of the LMTO calculations. The energy associated with structural relaxations is estimated from experimentally measured lattice parameters and elastic constants. We find that, although the configurational free energy is the largest component of the total alloy free energy, nonconfigurational effects contribute significantly to thermodynamic properties, and hence appreciably affect the calculated Cd-Mg phase diagram.
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  • First-principles study of phase stability of Ti–Al intermetallic compounds

    Asta, Mark   de Fontaine, Didier   van Schilfgaarde, Mark  

    Thermodynamic and structural properties of FCC- and HCP-based Ti-Al alloys are calculated from first-principles and are used to perform an ab initio study of phase stability for the intermetallic compounds in this system. The full potential linear muffin tin orbital method is used to determine heats of formation and other zero-temperature properties of 9 FCC- and 7 HCP-based intermetallic compounds, as well as of elemental FCC and HCP Ti and Al. From the results of these calculations, sets of effective cluster interactions are derived and are used in a cluster variation method calculation of the solid-state portion of the composition-temperature phase diagram for FCC- and HCP-based alloy phases. The results of our calculations are compared with those of experimental studies of stable and metastable phases in the Ti-Al system
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  • A First-Principles Study of the Phase Stability of FCC-Based Ti-Al Alloys

    Asta, Mark   Fontaine, Didier De   Schilfgaarde, Mark Van   Sluiter, Marcel   Methfessel, Michael  

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  • First-principles phase-stability study of fcc alloys in the Ti-Al system

    Asta, Mark   de Fontaine, Didier   van Schilfgaarde, Mark   Sluiter, Marcel   Methfessel, Michael  

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  • A First-Principles Study of the Phase Stability of Fcc-and Hcp-Based Ti-Al Alloys

    Asta, Mark   van Schilfgaarde, Mark   de Fontaine, Didier  

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  • First-Principles Study of Intermetallic Phase Stability in the Ternary Ti-Al-Nb Alloy System

    Asta, Mark   Ormeci, Alim   Wills, John M.   Albers, Robert C.  

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  • First-principles calculations of bulk and interfacial thermodynamic properties for fcc-based Al-Sc alloys

    Asta, Mark   Foiles, S. M.   Quong, A. A.  

    The configurational thermodynamic properties of fee-based AI-Se alloys and coherent Al/Al(3)Se interphase-boundary interfaces have been calculated from first principles. The computational approach used in this study combines the results of pseudopotential total-energy calculations with a cluster-expansion description of the alloy energetics. Bulk and interface configurational-thermodynamic properties are computed using a low-temperature-expansion technique. Calculated values of the {100} and {111} Al/Al(3)Se interfacial energies at zero temperature are, respectively, 192 and 226 mJ/m(2). The temperature dependence of the calculated interfacial free energies is found to be very weak for {100} and more appreciable for {111} orientations; the primary effect of configurational disordering at finite temperature is to reduce the degree of crystallographic anisotropy associated with calculated interfacial free energies. The first-principles-computed solid-solubility limits for Sc in bulk fee Al are found to be underestimated significantly in comparison with experimental measurements. It is argued that this discrepancy can be largely attributed to nonconfigurational contributions to the entropy which have been neglected in the present thermodynamic calculations.
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  • Embedded-atom-method study of structural, thermodynamic, and atomic-transport properties of liquid Ni-Al alloys

    Asta, Mark   Morgan, Dane   Hoyt, J. J.   Sadigh, Babak   Althoff, J. D.   de Fontaine, D.   Foiles, S. M.  

    Structural, thermodynamic, and atomic-transport properties of liquid Ni-Al alloys have been studied by Monte Carlo and molecular-dynamics simulations based upon three different embedded-atom method (EAM) interatomic potentials, namely those due to Foiles and Daw (FD) [J. Mater. Res. 2, 5 (1987)], Voter and Chen (VC) [in Characterization of Defects in Materials, edited by R. W. Siegel et al. MRS Symposia Proceedings. No. 82 (Materials Research Society, Pittsburgh, 1987), p.175] and Ludwig and Gumbsch (LG) [Model. Simul. Mater. Sci. Eng. 3, 533 (1995)]. We present detailed comparisons between calculated results and experimental data for structure factors, atomic volumes, enthalpies of mixing, activities, and viscosities. Calculated partial structure factors are found to be in semiquantitative agreement with published neutron scattering measurements for Ni20Al80 alloys, indicating that short-range order in the liquid phase is qualitatively well described. Calculated thermodynamic properties of mixing are found to agree very well with experimental data for Ni compositions greater than 75 atomic %, while for alloys richer in Al the magnitudes of the enthalpies and entropies of mixing are significantly underestimated. The VC and LG potentials give atomic densities and viscosities in good agreement with experiment for Ni-rich compositions, while FD potentials consistently underestimate both properties at all concentrations. The results of this study demonstrate that VC and LG potentials provide a realistic description of the thermodynamic and atomic transport properties for NixAl1-x liquid alloys with x greater than or equal to 0.75, and point to the limitations of EAM potentials for alloys richer in Al.
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  • The concentration and temperature dependences of antiphase-boundary energies in gamma -TiAl: A first-principles study

    Asta, Mark   Quong, Andrew A.  

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  • Embedded-atom-method effective-pair-interaction study of the structural and thermodynamic properties of Cu-Ni, Cu-Ag, and Au-Ni solid solutions

    Asta, Mark   Foiles, Stephen M.  

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  • NMR Crystallography: Evaluation of Hydrogen Positions in Hydromagnesite by ¹³C{¹H} REDOR Solid-State NMR and Density Functional Theory Calculation of Chemical Shielding Tensors

    Cui, Jinlei   Olmsted, David L.   Mehta, Anil K.   Asta, Mark   Hayes, Sophia  

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