Compositional changes in
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INTRODUCTION
A L U M I N U M - L I T H I U M base alloys oxidize more rapidly than their Li-free counterparts by more than an order of magnitude. [~j This tendency arises because of the reactivity of Li and the nonprotective nature of the Li-containing oxides. At the solutionizing temperature, typically 530 ~ Li undergoes preferential oxidation, and consequently, the underlying alloy is depleted of Li. Due to the inability of X-ray microanalysis to detect Li, the extent of Li depletion is usually evaluated by alternative techniques. Many of the past studies on this topic inferred Li depletion by measuring microhardness profiles of the soft surface layer. 12-7] The effect of Li loss on physical and mechanical properties is well documented in the literature.18.91 In addition to microhardness measurements, the presence of grain growth, recrystallization, and dissolution of 3' precipitates 14] in the microstructure of the near surface were taken to imply loss of Li. These indirect tests for Li depletion have several limitations: (1) measurements have poor sensitivity, and the associated errors are rather large; (2) it is difficult to define the boundary compositions of the denuded zone, thus any attempt to quantify the microchemical phenomena in this region is precluded; and (3) these indicators lack specificity: low microhardness and changes in microstructure cannot be completely ascribed to Li depletion, because simultaneous changes in Mg and Cu concentrations and in microstructure may also contribute to the formation of the soft layer. Direct analytical measurements of Li distribution in AI-LiMg-Cu alloys were made only by Papazian eta/., I7'1~ using a nuclear-reaction analysis technique, and by Thorne et al.,IJlJ using secondary ion mass spectrometry K.K. SONI, Research Associate, is with the Enrico Fermi Institute, University of Chicago, Chicago, IL 60637. D.B. WILLIAMS, Harold Chambers Senior Professor and Chairman, is with the Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015. D.E. NEWBURY, Group Leader, G. GILLEN, Research Chemist, P. CHI, Physicist, and D.S. BRIGHT, Research Chemist, are with the Surface and Microanalysis Science Division, Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899. Manuscript submitted February 11, 1993. METALLURGICAL TRANSACTIONS A
(SIMS) and a nuclear microprobe. These researchers, however, did not describe the associated Mg and Cu profiles. An extensive study of the depth distribution of Li in binary A1-Li alloys was conducted by Soni et al.,[~21 using SIMS and neuron depth profiling. In this study, Li profiles were quantitatively measured both in the oxide layer and the underlying impoverished alloy. It was shown that the Li concentration at the oxide/alloy interface was nonzero, although it was small compared to the bulk concentration. Lithium depletion was detected at 530 ~ as well as at 200 ~ although it was much smaller in extent at the lower temperature. Many A1-Li-base
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