Modeling of enhanced diffusion under ion irradiation

Abstract

The differential equations which govern enhanced diffusion under ion irradiation are solved using recently developed numerical techniques. Accurate time‐dependent profiles for vacancies, interstitials, and diffusing atoms are generated thereby with minimal computer time. The theoretical description is improved further by incorporating refined calculations of the atomic displacement rate by energetic ions. Three representative cases of enhanced diffusion are treated in detail: Al diffusion in Al under 100‐keV Al irradiation, Al diffusion in Al under uniform irradiation, and W diffusion in W under 100‐keV proton irradiation. It is shown that more information may be obtained from the diffused atomic profiles if the irradiation damage rate varies with depth in a known way over the diffusion region. Under this condition, both the shape and the time dependence of the atomic profile are sensitive to the rate coefficient for point‐defect annihilation. When the annihilation coefficient is so determined, the point‐defect creation rate can be uniquely related to the enhanced atomic diffusion coefficient. Under irradiation with a limited range, the atomic profiles typically pass through a complicated form, but ultimately reach a flat shape with a relatively abrupt drop‐off at greater depths. This end condition is qualitatively independent of the detailed structure of the irradiation profile, but the sharpness of the drop‐off is a function of the rate coefficient for point‐defect annihilation.