SEGREGATION STUDIES OF AN AL-LI ALLOY

The segregation of Li to the surface of an Al-6.5at%Li single crystal was investigated, using Auger electron spectroscopy, low energy electron diffraction, secondary ion mass spectrometry, and work function measurements (DELTAphi). A rapid enhancement of Li to the (111) facets on the surface is observed at approximately 500 K, reaching a maximum surface concentration of 0.17; this enhancement is associated with a fast decrease in DELTAphi in the same temperature region. The formation of a (square-root 3 x square-root 3)R30-degrees structure is observed at temperatures greater-than-or-equal-to 500 K. Upon cooling the sample, the bulk of the segregated Li remains on the surface resulting in a final average surface concentration of 0.16. The slight decrease in Li concentration and an increase in DELTAphi when the sample is cooled below 450 K suggests that some of the Li redissolves into the bulk. Our calculated value of the Li diffusion coefficient is much lower than expected and is associated with a region of enhanced Li concentration extending approximately 6 nm from the surface, as observed by dynamic secondary ion mass spectrometry.