Stability of tight-packed metals with the embedded-atom method
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Relationships between embedded-atom method parameters and the energies of fcc-hcp stability and intrinsic and extrinsic fee stacking-faults were studied for Cu, Ag, Au, Ni, Pd, and Pt. It was found that the relative magnitudes of these energies for different metals are determined primarily by the physical input data and are almost independent of the cutoff distance and the functions used in the model. These energies increase with increasing vacancy formation energy, decrease with increasing atomic volume and shear modulus, and are almost independent of variations in the cohesive energy and the bulk modulus. However, the shape of the energy versus cutoff distance curve is almost the same for all six metals and is determined primarily by the cutoff distance and the functions used in the model. The shape for a given model is almost independent of the physical input parameters used for fitting to specific metals, can yield either positive or negative values (determined primarily by the cutoff distance), and is similar for all three energies.
I. INTRODUCTION Numerous atomistic calculations based on two-body interactions and a many-body contribution accounting for local volume dependence have been reported since the introduction of such models by Daw and Baskes1 and Finnis and Sinclair.2 Both the name "Embedded-Atom Method" (EAM) and terminology associated with the EAM are used here to include all models with the same mathematical format. All such models can be normalized to consist of an effective two-body potential and an embedding function with zero slope at the equilibrium electron density.3 Equilibrium is attained independently with respect to the effective two-body potential and the normalized embedding function. The details of the EAM, which are well documented in the literature,1-4 will not be repeated here, and the notation used by Johnson5 will be followed. Empirical fitting is used to determine the parameters in EAM models, with the physical input parameters being, e.g., the lattice constant, elastic constants, cohesive energy, and vacancy formation energy. While local stability is maintained so that small perturbations will not lead to a spontaneous transformation to a different lattice structure, global stability is often not considered. A format for the EAM functions is chosen, either an analytic form or perhaps spline fitting, and the detailed shape of the functions is determined by forcing the model to match the physical input values. However, the range of the interactions in an EAM model, both two-body and electron density, are not normally fitted in this manner, and often end up being rather arbitrarily chosen. If the range is less than the ideal hep third neighbor distance, the energies of the hep and fee lattice structures are the same. The effect of range at greater values on fccJ. Mater. Res., Vol. 7, No. 4, Apr 1992
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hcp stability and the related stacking-fault energies is investigated in the present report. II. MODEL An EAM model is defined
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