Lithium depletion during heat treatment of aluminum-lithium alloys
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INTRODUCTION
ALUMINUM-lithium alloys are being developed as potential replacements for the high strength aluminum alloys currently used for aerospace applications. ~ The new alloys are expected to provide properties equivalent to those of current materials but with significantly reduced density. An attractive feature of the aluminum-lithium alloys is their ability to be substituted for current alloys with little change in manufacturing procedures and facilities because most of their properties are expected to be similar to those of the alloys they will replace. One aspect of the behavior of the aluminum-lithium alloy family that may require attention in this regard is their tendency to oxidize at elevated temperature. 2'3'4 Most current aluminum alloys can be readily heat treated in air because they tend to form protective oxide coatings. However, the aluminum lithium alloys oxidize at a far greater rate, and apparently will continue to oxidize until all of the lithium in the material has been exhausted. Previous work on aluminum-lithium has shown that the rapid oxidation rate is little affected by alloying elements and has identified some of the oxidation products. This work mainly involved weight gain measurements and transmission electron microscopy. 2'3'4 Microhardness measurements were also made, which showed a softer surface layer that was assumed to be due to lithium depletion. 5 More recently, Wert and Ward used a combination of eddy current conductivity, microhardness measurements, and metallography to demonstrate lithium depletion during solution treatment in air. 6 However, to date there seems to have been no direct measurement of lithium loss during heat treatment nor any measurement of the resultant lithium distribution profile. Such information would be useful in establishing J.M. PAPAZIAN, Senior Staff Scientist, R.L. SCHULTE, Staff Scientist, and P. N. ADLER, Laboratory Head, are with Corporate Research Center, A01-26, Grumman Corporation, Bethpage, NY 11714. Manuscript submitted May 28, 1985.
METALLURGICALTRANSACTIONS A
proper heat treatment procedures and assessing the potential impact of oxidation during solution treatment on material properties. One reason for a lack of data in this area is the difficulty of detecting lithium using common microanalysis techniques. Fortunately, nuclear reaction analysis techniques have become available that are particularly well suited to detection of light elements because they do not rely on X-ray or electron emission, but instead stimulate the lithium nucleus with a suitable particle causing it to transmute with the attendant emission of detectable radiation. These techniques have been profitably employed in a variety of investigations] Thus, our objective in this work was to attempt to apply nuclear reaction analysis to the measurement of lithium in the near surface region of candidate aluminum-lithium alloys and to establish the concentration profiles of lithium in these alloys after heat treatment at elevated temperatures in various atmospheres. II.
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