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The temperature-dependent diffusion coefficients of interstitial hydrogen, deuterium, and tritium in nickel are computed using transition state theory. The coefficient of thermal expansion, the enthalpy and entropy of activation, and the pre-exponential factor of the diffusion coefficient are obtained from ab initio total energy and phonon calculations including the vibrations of all atoms. Numerical results reveal that diffusion between octahedral interstitial sites occurs along an indirect path via the metastable tetrahedral site and that both the migration enthalpy and entropy are strongly temperature dependent. However, the migration enthalpy and entropy are coupled so that the diffusion coefficient is well described by a constant activation energy, i.e., D = D 0 exp͓−Q / ͑RT͔͒, with Q = 45.72, 44.09, and 43.04 kJ/ mol and D 0 = 3.84ϫ 10 −6 , 2.40ϫ 10 −6 , 1.77 ϫ 10 −6 m 2 s −1 for H, D, and T, respectively. The diffusion of deuterium and tritium is computed to be slower than that of hydrogen only at temperatures above 400 K. At lower temperatures, the order is reversed in excellent agreement with experiment. The present approach is applicable to atoms of any mass as it includes the full coupling between the vibrational modes of the diffusing atom with the host lattice.

The temperature-dependent mass diffusion coefficient is computed using transition state theory. Ab initio supercell phonon calculations of the entire system provide the attempt frequency, the activation enthalpy, and the activation entropy as a function of temperature. Effects due to thermal lattice expansion are included and found to be significant. Numerical results for the case of hydrogen in nickel demonstrate a strong temperature dependence of the migration enthalpy and entropy. Trapping in local minima along the diffusion path has a pronounced effect especially at low temperatures. The computed diffusion coefficients with and without trapping bracket the available experimental values over the entire temperature range between 0 and 1400 K.

The enthalpies of formation of metastable fee Ag-Cu solid solutions, produced by ball milling of elemental powders, were determined by differential scanning calorimetry. Experimental thermodynamic data for these metastable alloys and for the equilibrium phases are compared with both calculation of phase diagrams (CALPHAD) and atomistic simulation predictions. The atomistic simulations were performed using the free-energy minimization method (FEMM). The FEMM determination of the equilibrium Ag-Cu phase diagram and the enthalpy of formation and lattice parameters of the metastable solid solutions are in good agreement with the experimental measurements. CALPHAD calculations made in the same metastable regime, however, significantly overestimate the enthalpy of formation. Thus, the FEMM is a viable alternative approach for the calculation of thermodynamic properties of equilibrium and metastable phases, provided reliable interatomic potentials are available. The FEMM is also capable of determining such properties as the lattice parameter which are not available from CALPHAD calculations.

The structural and energetic properties of [loo] and [llO] steps on the (001) surface of fee metal have been determined by T = 0 atomistic simulations. The interactions between [NO] steps and between [llO] steps on the (001) surface are determined from the surface energy of a series of (Oln) and (ilm) surfaces, respectively. For step spacings larger than three fee lattice parameters (R > 3a,), we find that the interaction energy between two similar steps on the (001) surface can be reasonably represented by the functional form R-', in agreement with the prediction of a simple linear elastic analysis based upon a line dipole force model of a step. However, we observe qualitative differences between the displacement fields determined by the two methods. For R < 3a,, on the other hand, we find that the interaction between steps deviates significantly from the form R-'. These deviations demonstrate that both dipole and quadrupole force distributions are necessary to account for step-step interactions for spacings as small as a fraction of a lattice parameter up to infinite step spacings. We show that a [lOO] step on the (001) surface in Au and Pt (but not in Ag, Au, Cu, or Pd) may lower the surface energy by transforming into a zigzagged [llO] step.

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The vacancy formation thermodynamics in six FCC metals A& Au, Cu, Ni, Pd and Pf are determined from atomistic simulations as a function of temperature. This investigation is performed using the embedded atom method interatomic potentials and the finite temperature propenies are determined within the local harmonic and the quasiharmonic frameworks. The temperature dependence of the vacancy formation free energy, entropy. enthalpy and vacancy formation volume are determined. We find that the temperature dependence of the vacancy formation energy can make a significant wnlribution to the vacancy c o n c e n~o n at high temperatures. An additional goal of the present study is to evaluate the accuracy of the local harmonic method under circumstances in which the excess entropy associated with the fomation of a defect is very small. Ow data demonstrate that while the ermrs associated with determining the vacancy formation entropy in the local harmonic model are large, a simple extension to the local harmonic method yields thermodynamic properties mmparable to that obtained in the quasiharmonic model, but with much higher wmpurational efficiency. MI 48109, USA Recently, Gillan [I] has studied vacancy formation energies by applying the pseudopotential method and local-density approximation for exchange and correlation in a periodically repeating supercell geometq. While this is a first-principle approach, it is limited to zero Kelvin. Finite-temperature studies, based upon empirical descriptions of atomic bonding and the molecular dynamics [21 and Monte Carlo [3.4] methods, have been able to overcome these limitations. In the present study, we employ embedded atom method (EAM) potentials [5] to describe atomic interactions, classical lattice dynamics methods ana a novel finite temperature simulation approach to examine vacancy formation thermodynamics in six face centred cubic (Zc) metals as a function of temperature.