Mark Asta

Publications (3)2.17 Total impact

• Source

  Available from: Stefano Angioletti-Uberti

  Article: Ab initio simulations of molten Ni alloys

  Christopher Woodward, Mark Asta, Dallas R. Trinkle, James Lill, Stefano Angioletti-Uberti
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  ABSTRACT: Convective instabilities responsible for misoriented grains in directionally solidified turbine airfoils are produced by variations in liquid–metal density with composition and temperature across the solidification zone. Here, fundamental properties of molten Ni-based alloys, required for modeling these instabilities, are calculated using ab initio molecular dynamics simulations. Equations of state are derived from constant number-volume-temperature ensembles at 1830 and 1750 K for elemental, binary (Ni–X, X = Al , W, Re, and Ta) and ternary (Ni–Al–X, X = W , Re, and Ta) Ni alloys. Calculated molar volumes agree to within 0.6%–1.8% of available measurements. Predictions are used to investigate the range of accuracy of a parameterization of molar volumes with composition and temperature based on measurements of binary alloys. Structural analysis reveals a pronounced tendency for icosahedral short-range order for Ni–W and Ni–Re alloys and the calculations provide estimates of diffusion rates and their dependence on compositions and temperature.
  Journal of Applied Physics 07/2010; · 2.17 Impact Factor
• Article: Solid-liquid phase equilibria from free-energy perturbation calculations

  Stefano Angioletti-Uberti, Mark Asta, Mike W. Finnis, P. D. Lee
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  ABSTRACT: A method for calculating free-energy differences based on a free-energy perturbation (FEP) formalism in an alloy system described by two different Hamiltonians is reported. The intended application is the calculation of solid-liquid phase equilibria in alloys with the accuracy of first-principles electronic density-functional theory (DFT). For this purpose free energies are derived with a classical interatomic potential, and FEP calculations are used to compute corrections to these reference values. For practical applications of this approach, due to the relatively high computational cost of DFT calculations, it is critical that the FEP calculations converge rapidly in terms of the number of samples used to estimate relevant ensemble averages. This issue is investigated in the current study employing two classical interatomic-potential models for Ni-Cu. These models yield differences in predicted phase-boundary temperatures of approximately 100 K, comparable to those that might be expected between a DFT Hamiltonian and a well-fit classical potential. We show that for pure elements the FEP calculations converge rapidly with the number of samples, yielding free-energy differences converged to within a fraction of a meV/atom in a few dozen energy calculations. For a concentrated equiatomic alloy similar precision requires roughly a hundred samples. The results suggest that the proposed methodology could provide a computationally tractable framework for calculating solid-liquid phase equilibria in concentrated alloys with DFT accuracy.
  Phys. Rev. B. 10/2008; 78(13).
• Source

  Available from: arc.nucapt.northwestern.edu

  Article: Composition evolution of nanoscale Al3Sc precipitates in an Al–Mg–Sc alloy: Experiments and computations

  Emmanuelle A. Marquis, David N. Seidman, Mark Asta, Christopher Woodward
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  ABSTRACT: Controlling the distribution of chemical constituents within complex, structurally heterogeneous systems represents one of the fundamental challenges of alloy design. We demonstrate how the combination of recent developments in sophisticated experimental high resolution characterization techniques and ab initio theoretical methods provide the basis for a detailed level of understanding of the microscopic factors governing compositional distributions in metallic alloys. In a study of the partitioning of Mg in two-phase ternary Al–Sc–Mg alloys by atom-probe tomography, we identify a large Mg concentration enhancement at the coherent α-Al/Al3Sc heterophase interface with a relative Gibbsian interfacial excess of Mg with respect to Al and Sc, , equal to 1.9 ± 0.5 atom nm−2. The corresponding calculated value of is ∼1.2 atom nm−2. Theoretical ab initio investigations establish an equilibrium driving force for Mg interfacial segregation that is primarily chemical in nature and reflects the strength of the Mg–Sc interactions in an Al-rich alloy.
  Acta Materialia.