A combination of ab initio calculations and statistical mechanical models was used to study diffusion behavior in dilute ferritic Fe–Cr and Fe–Ni alloys. A full set of Onsager matrix coefficients (Lij) and tracer diffusion coefficients (D*) were calculated for both vacancy and interstitial mediated diffusion. The key results are: (1) Cr was the fastest diffusing species by both vacancy and interstitial mediated transport followed by Ni for vacancy and Fe for interstitial mediated diffusion, respectively; (2) weak interactions exist between Ni and Cr with vacancies as first nearest neighbors; (3) the calculated D* predict opposite trends of radiation-induced segregation of Cr by vacancy (depletion) and interstitial (enrichment) diffusion mechanisms, perhaps explaining the lack of clear trends in experimentally determined Cr radiation-induced segregation profiles; (4) unlike the widely used Darken and Manning approaches, the calculated Lij could reliably predict the direction of the vacancy defect flux, particularly at low temperatures; (5) the Lij calculated for interstitial mediated transport indicate that the contribution of interstitial flux could be significant in determining radiation-induced segregation in these alloys; and (6) solute drag was unlikely to occur for vacancy mediated diffusion. In addition, the LAB off-diagonal terms for interstitial transport, which were typically assumed to be small, were shown to be as large as the diagonal elements LBB. These results provide a basis for understanding the complex radiation induced segregation behavior of Cr and Ni in ferritic/martensitic alloys.

Ab-initio Based Modeling of Diffusion in Dilute BCC Fe–Ni and Fe–Cr Alloys and Implications for Radiation Induced Segregation. S.Choudhury, L.Barnard, J.D.Tucker, T.R.Allen, B.D.Wirth, M.Asta, D.Morgan: Journal of Nuclear Materials, 2011, 411[1-3], 1-14