Lithium diffusion in aluminum-lithium alloy 2090 clad with 7072
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I.
INTRODUCTION
THE addition of lithium to aluminum alloys is known to increase their rates of oxidation during typical solution heat treatments. [1-41This oxidation is of concern because the mechanical and electrical properties of the near-surface region of aluminum-lithium alloys change as oxidation proceeds, t3-1~ and the oxidation can result in measurably lower overall mechanical properties in thin (~1.5 mm) sheet.tlu Direct measurements of the lithium concentration profile after elevated-temperature heat treatments have shown that significant lithium depletion is associated with this oxidation and that the rate of lithium loss is relatively independent of the environment but limited primarily by the diffusivity of lithium in the alloy, t~21Several techniques have been explored to limit lithium loss during solution heat treatments; these include the use of protective atmospheres, I131 anodization before heat treatment, mj and the use of protective organic coatings. t~4~ A further possibility is to clad the aluminumlithium alloy with surface layers of less reactive aluminum alloys. Clad aluminum alloys are often used in order to minimize corrosion problems and to obtain attractive surface finishes, but with aluminum-lithium alloys, they may have the added benefit of suppressing lithium loss during solution heat treatment. A clad version of aluminum-lithium alloy 2090 is currently available. Therefore, it was of interest to determine to what extent cladding could reduce the lithium loss during solution heat treatment. In order to do this, samples of the clad alloy were heat-treated for various lengths of time at elevated temperatures, after which the lithium concentration profile in the near-surface region of the material was directly measured using a nuclear reaction analysis technique. This paper describes the results of these experiments. J.M. PAPAZIAN, Principal Staff Scientist, and R.L. SCHULTE, Laboratory Head, are with the Corporate Research Center, Grumman Corporation, Bethpage, NY 11714-3580. Manuscript submitted March 20, 1989. METALLURGICAL TRANSACTIONS A
II.
EXPERIMENTAL PROCEDURE
The experimental material was obtained from ALCOA and consisted of a 3.05-mm (0.120-in.)-thick sheet of 2090-T3E27 that had been clad on both sides with alloy 7072. The nominal thickness of the cladding was given as 2.5 pct of the overall thickness. Samples, 12.5 by 25 mm, of the as-received material were heat-treated for 4, 16, and 64 hours at 500 ~ in a vertical tube furnace and drop quenched into room-temperature water. The furnace atmosphere consisted of high-purity air (a 4:1 mixture of purified nitrogen and oxygen) that was water saturated by bubbling the feed gas through a water-filled Erlenmeyer flask at room temperature. After quenching, the samples were cut in half, yielding two 12.5- by 12.5by 3-mm coupons. The newly exposed cross section was mounted and polished using standard metallographic procedures. No water was used in the final stages of polishing in order to avoid reactions with the lithium. The mount
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