Synergistic sputtering properties of binary alloys

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Abstract
We have found that dilute concentrations of lithium in copper produce surfaces which are nearly pure lithium when heated and subject to irradiation. In order to better understand the experimental results, we have modeled the Cu-Li system using a modified version of the TRIM computer code and an alloy segregation program developed by N. Q. Lam. The TRIM code calculates the sputtering yield and depth of origin of the sputtered atoms for materials in which the composition varies from one atomic layer to the next and the segregation program uses these sputtering yields to trace the evolution of the concentration profile. The initial result of sputtering is to preferentially deplete the surface species. Continued irradiation, however, creates a subsurface region of high displacement damage. In the Cu-Li alloy, lithium moves very rapidly through this region, resulting in subsurface lithium enrichment. The enriched region broadens and eventually reaches the surface. The exact effect on the lithium concentration in the first two atomic layers depends on the temperature, damage profile, and particle flux. Results of the calculation are presented, along with a discussion of their implications for fusion reactor materials. It is often assumed, when dealing with the sputtering of compounds and alloys, that each component has the same sputtering properties as the pure element, and that the sputtering rate of a given atomic species is, therefore, linear with atomic concentration. We have investigated the validity of this assumption in the context of dilute, highly segregating alloys proposed for fusion applications. It is found that as the concentration of a given element changes with time and from one atomic layer to the next, the sputtering yield also changes significantly.