Starink, M.J., Hobson, A.J., Sinclair, I. and Gregson, P.J.
Embrittlement of Al-Li-Cu-Mg alloys at slightly elevated temperatures: microstructural mechanisms of hardening.
Materials Science and Engineering: A, 289, (1-2), . (doi:10.1016/S0921-5093(00)00912-6).
When underaged, damage tolerant Al-Li based alloys are exposed to slightly elevated temperatures (60 to 100ºC), their strength increases and toughness decreases. To study the first stages of this process, 8090 sheet in three underaged heat treatment conditions, including T3, T81 and a multi-stage temper, was exposed for 1000 h at 70°C. Using a model for strengthening in combination with a quantitative analysis of differential scanning calorimetry data, it is shown that strengthening can be explained by an increase in the volume fraction of the delta' phase and, to a lesser extent, to the formation of Guinier-Preston (GPB) zones. For the T3 temper, coarsening of delta' precipitates is the main strengthening mechanism during exposure at 70ºC. The solvus of delta' and GPB, which to a large extent defines this embrittlement, is discussed. Toughness reduction during exposure at slightly elevated temperatures can be minimised via a multi stage temper or via a reduction of the Li content of the alloy.
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