Ab Initio Based Thermokinetic Modeling of Point Defect Diffusion in Fe-Ni-Cr Alloys

   Julie Tucker^*, Dane Morgan ^*,** , Todd Allen^*  

* : Nuclear Engineering - Engineering Physics ; **:  Materials Science and Engineering

Under irradiation, the migration of point defects (including vacancies and interstitials) are crucial for materials degradation. To understand the diffusion mechanisms of radiation induced defects in multi-component alloys, we employ a multi-scale modeling approach. This approach combines ab initio, cluster expansion, Monte Carlo and kinetic Monte Carlo techniques to calculate concentration and temperature dependent diffusion coefficients in the technologically important Fe-Ni-Cr system. 

Initial results on vacancy mediated transport indicate that the Cr vacancy migration barrier in Ni-Cr alloys is significantly lower than previously believed. For equivalent local environments the Cr barrier is 100 meV or more lower than that for Ni. In addition, these barriers have a a strong dependence on local atomic configuration, not just overall concentration. The local environment effects indicate that simple models with constant or linearly varying barriers as a function of composition are unlikely to capture the relevant physics of this system.

Relaxed configurations of on lattice and activated states of a Cr atom migrating in a Ni-Cr alloy

We have also begun studies interstitial migration barriers in Ni-Cr and Ni-Fe alloys, and are in the process of constructing a kinetic model to determine the interstitials diffusion constants The ab initio thermokinetic approach will build a model that includes local environment and thermodynamic effect for both vacancies and interstitials, thereby provide insight into the fundamental mechanism of radiation effects.

We gratefully acknowledge financial support from the Department of Energy (DOE), Naval Nuclear Propulsion Fellowship Program sponsored by Naval Reactors Division, Nuclear Engineering Research Initiative (NERI), award number DE-PS07-05ID14713, and Nuclear Energy Research Initiative for Consortia (NERI-C), award number DE-PS07-07ID14812.   We gratefully acknowledge computing support from the National Science Foundation (NSF) National Center for Supercomputing Applications (NCSA), award number DMR060007

• Publication

1. Ab Initio Defect Properties for Modeling Radiation-Induced Segregation in Fe-Ni-Cr Alloys; Tucker, J D, Allen, T R, and Morgan, D, Proceedings of the 13th International Symposium on Environmental Degradation of Materials in Nuclear Power Systems, Whistler, BC Canada, August 19 - 23 (2007).