SOLUTE-ENRICHED SURFACE-LAYERS AND X-RAY-MICROANALYSIS OF THIN FOILS OF A COMMERCIAL ALUMINUM-ALLOY

Thin foils of the commercial Al alloy 7075 were subjected to X-ray microanalysis in a Jeol 100 CX TEMSCAN using a modified double-tilting stage. The ratio of the Cu K-alpha and Al K-alpha peaks, I(Cu)/I(Al), was found to decrease rapidly with increasing foil thickness, t, in regions thinner than about 200 nm. This was attributed to the formation of surface layers enriched in Cu during electropolishing, consistent with the findings of other investigators. An equation predicting the variation of I(B)/I(A) with t in an A-rich alloy in which B (or additional solute elements) is dilute is derived for a sample with surface layers enriched in B. The derivation predicts that I(B)/I(A) should vary linearly with t-1, and that the intercept of such a plot should be positive. It is suggested that consistency with this prediction serves as a criterion for true surface enrichment. Our data on Cu obey these predictions, and further analysis indicates that the layers on both surfaces are enriched by a factor of about 5.6. The ratio of the intensities of the Zn K-alpha and Al K-alpha X-ray peaks was found to be nearly independent of foil thickness to t almost-equal-to 400 nm. This result, combined with additional analysis, indicates that the apparent enrichment observed by other investigators is most likely due to the influence of instrumental factors.