The application of the analytic embedded atom method to bcc metals and alloys
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Johnson and Oh have recently developed Embedded Atom Method potentials for bcc metals (Na, Li, K, V, Nb, Ta, Mo, W, Fe). The predictive power of these potentials was first tested by calculating vacancy formation and migration energies. Due to the results of these calculations, some of the functions were slightly modified to improve their fit to vacancy properties. The modified potentials were then used to calculate phonon dispersion curves, surface relaxations, surface energies, and thermal expansion. In addition, Johnson's alloy model, which works well for fee metals, was applied to the bcc metals to predict dilute heats of solution. I. INTRODUCTION Atomic-scale computer modeling of materials has proven to be useful in the understanding of elusive areas in materials science. Quantum mechanical approaches are useful in studying small systems (
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FIG. 7. Calculated phonon dispersion curves for vanadium. Experimental data (set of points) are from Ref. 63. 646 http://journals.cambridge.org
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J. Mater. Res., Vol. 7, No. 3, Mar 1992 Downloaded: 19 Aug 2015
IP address: 131.111.164.128
A. M. Guellil and J. B. Adams: The application of the analytic embedded atom method to bcc metals and alloys
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FIG. 8. Calculated phonon dispersion curves for niobium. Experimental data (set of points) are from Ref. 64.
with a single impurity atom, while maintaining constant pressure. Experimental data71 could be found only for four different alloy systems. The results and comparison with experiment are shown in Table V. It should be noted that the experimental values given were for Fe-V and V-Fe at 1600 K, so that Fe is fee. Therefore, we carried out our calculations for the fee phase. It can be seen that all four alloys are off by as much as one order of magnitude, and that the V-Fe system predicts the wrong sign. Some of the discrepancy could be due to the difference between the theoretical concentrations (~1%) and the experimental concentrations (typically ~10%). Also, the calculations were carried out at 0 K, versus 1600 K for the Fe-V systems, and 384 K for the Na-K systems. Finally, the Na-K experimental data are for the liquid state, but the calculations are for the solid state. The results suggest that the application of Eqs. (6) and (7) may not be a reliable method for determining alloy properties of the bcc metals, even though it had proven
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to be a good method for the fee metals. However, more experimental data must be found to test the model more fully before any firm conclusions may be drawn. VIII. SUMMARY
(1) Models were created for nine common bcc metals using Johnson and Oh's functions, and some were slightly modified to improve their fit to vacancy formation and migration energies. (2) The functions were then used to calculate surface energies, which were found to be about 20% lower than experiment, typical of all many-body potentials. (3) Surface relaxations on the (100) surfaces were c
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