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°C), 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 δ′ phase and, to a lesser extent, to the formation of Guinier–Preston (GPB) zones. For the T3 temper, coarsening of δ′ precipitates is the main strengthening mechanism during exposure at 70°C. The solvus of δ′ 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.