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The kinetic pathways resulting from the formation of coherent L1 2 -ordered γ'-precipitates in the γmatrix (f.c.c.) of 2 Cr at.% alloys, aged at 873 K, are investigated by atom-probe tomography (APT) over a range of aging times from 1/6 to 1024 hours; these alloys have approximately the same volume fraction of the γ'-precipitate phase. Quantification of the phase decomposition within the framework of classical nucleation theory reveals that the γmatrix solid-solution solute supersaturations of both alloys provide the chemical driving force, which acts as the primary determinant of the nucleation behavior. In the coarsening regime, the temporal evolution of the γ'-precipitate average radii and the γ-matrix supersaturations follow the predictions of classical coarsening models, while the temporal evolution of the γ'-precipitate number densities of both alloys do not. APT results are compared to equilibrium calculations of the pertinent solvus lines determined by employing both Thermo-Calc and Grand-Canonical Monte Carlo simulation. 6/26/2007

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The site substitution behavior of Cr and Ta in the Ni 3 Al ͑L1 2 ͒-type ␥Ј-precipitates of a Ni-Al-Cr-Ta alloy is investigated by atom-probe tomography ͑APT͒ and first-principles calculations. Measurements of the ␥Ј-phase composition by APT suggest that Al, Cr, and Ta share the Al sublattice sites of the ␥Ј-precipitates. The calculated substitutional energies of the solute atoms at the Ni and Al sublattice sites indicate that Ta has a strong preference for the Al sites, while Cr has a weak Al site preference. Furthermore, Ta is shown to replace Cr at the Al sublattice sites of the ␥Ј-precipitates, altering the elemental phase partitioning behavior of the Ni-Al-Cr-Ta alloy.