Embrittlement of Al-Li-Cu-Mg alloys at slightly elevated temperatures: microstructural mechanisms of hardening

When underaged, damage tolerant Al-Li based alloys are exposed to slightly elevated temperatures (60-100 degrees C), their strength increases and toug hness decreases. To study the first stages of this process, 8090 sheet in t hree underaged heat treatment conditions. including T3, T81 and a multi-sta ge temper, was exposed for 1000 h at 70 degrees C. Using a model for streng thening in combination with a quantitative analysis of differential scannin g calorimetry data, it is shown that strengthening can be explained by an i ncrease 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, coarse ning of delta' precipitates is the main strengthening mechanism during expo sure at 70 degrees C. The solvus of delta' and GPB, which to a large extent defines this embrittlement, is discussed. Toughness reduction during expos ure at slightly elevated temperatures can be minimised via a multi stage te mper or via a reduction of the Li content of the alloy. (C) 2000 Elsevier S cience S.A. All rights reserved.