EVALUATION OF THE OXYGEN DIFFUSION COEFFICIENT IN NICKEL-BASE ALLOYS

Nickel-base alloys such as alloy 600 (Ni-16Cr-9Fe) are known to exhibit intergranular stress corrosion cracking (IGSCC) at pressurized water reactor (PWR) primary water environments. From the microscopic observations, it was found that oxygen plays a role in primary water stress corrosion cracking (PWSCC) of nickel-base alloys and Scott suggests an internal oxidation model. However, it was found that needed oxygen diffusivity to explain the internal oxidation model should be several orders greater than the measured oxygen diffusivity. In this study, oxygen diffusion coefficients in the nickel-base alloys were evaluated by atomistic modeling of oxygen diffusion process based on the proposed vacancy-mediated diffusion model. Density functional theory is used to calculate the energy of a system. Activation barrier energy of diffusion of atomic oxygen is quantified by finding minimum energy path through the most favorable path. Phonon analysis is performed using the direct force-constant method