Lithium depletion during heat treatment of aluminum-lithium alloys

Abstract

The loss of lithium from the near surface region during heat treatment of two commercial aluminum-lithium alloys was studied using a nuclear reaction analysis technique. A finely collimated 2.5 MeV beam of3He ions was used to stimulate the7Li(3He,p)9Be reaction in samples of BAACo 8090 and 8091 alloys heat treated for 1, 4, and 16 hours at 500 °C. The emitted protons were detected as a cross section of the sample was traversed through the beam, thereby determining the lithium content as a function of distance from the external surface. Suitable calibration and control samples were used to validate the technique. The lithium concentration data were fit with assumed concentration profiles calculated from diffusion equations and modified for the particular experimental configuration employed. Extensive lithium depletion was found in both alloys, and the concentration profiles were found to be accurately predicted by the diffusion calculations. For heat treatment in either wet or dry air, the depth of lithium loss was the same, and can be approximately given as x = 1.5 √Dt. When heat treated in an argon atmosphere, the depth of lithium loss was reduced. The lithium loss appeared to be limited by the diffusive flux of lithium to the surface of the sample in wet and dry air, but was limited by other factors in argon. Porosity was observed in the lithium depleted region; this was ascribed to the accumulation of vacancies generated by the unequal fluxes of aluminum and lithium atoms.