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Abstract
The phase separation of a Ni–6.5 Al–9.5 Cr at. % alloy aged at 873 K was studied by atom-probe tomography and compared to the predictions of classical precipitation models. Phase separation in this alloy occurs in four distinct regimes: (i) quasi-stationary-state γ′(L12)-precipitate nucleation; (ii) concomitant precipitate nucleation, growth and coagulation and coalescence; (iii) concurrent growth and coarsening, wherein coarsening occurs via both γ′-precipitate coagulation and coalescence and by the classical evaporation–condensation mechanism; and (iv) quasi-stationary-state coarsening of γ′-precipitates, once the equilibrium volume fraction of precipitates is achieved. The predictions of classical nucleation and growth models are not validated experimentally, likely due to the complexity of the atomistic kinetic pathways involved in precipitation. During coarsening, the temporal evolution of the γ′-precipitate average radius, number density and the γ(fcc)-matrix and γ′-precipitate supersaturations follow the predictions of classical models.
Acknowledgements
This research was sponsored by the National Science Foundation under grant DMR-080461, Dr. H. Chopra, grant officer. APT measurements were performed at the Northwestern University Center for Atom Probe Tomography (NUCAPT). The LEAP tomograph was purchased and upgraded with funding from the NSF-MRI (DMR 0420532, Dr. C. Bouldin, grant officer) and ONR-DURIP (N00014-0400798 and N00014-0610539, Dr. J. Christodoulou, grant officer). We extend our gratitude to Dr. C. Campbell of the National Institute of Standards and Technology, Metallurgy Division, (Gaithersburg, MD) for diffusivity calculations and to Dr. D. Isheim for managing NUCAPT, and for discussions of our results. We thank Prof G. B. Olson and Dr. H.-J. Jou of QuesTek LLC (Evanston, IL) for use of PrecipiCalc.