Ab-initio based modeling of diffusion in dilute bcc Fe-Ni and Fe-Cr alloys and implications for radiation induced segregation

A combination of ab-initio calculations and statistical mechanical models has been 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 both for vacancy and interstitial mediated diffusion. The key results are: (1) Cr is 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 (RIS) of Cr by vacancy (depletion) and interstitial (enrichment) diffusion mechanisms, perhaps explaining the lack of clear trends in experimentally determined Cr RIS profiles; (4) unlike the widely used Darken and Manning approaches, the calculated Lij can 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 can be significant in determining RIS in these alloys; and (6) solute drag is unlikely to occur for vacancy mediated diffusion. In addition, the LAB off-diagonal terms for interstitial transport, which are typically assumed to be small, are 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.