Precipitation in stretched Al - Cu - Mg alloys with reduced alloying content studied by DSC , TEM and atom probe

The hardening and microstructural evolution during ageing of a Al-1.2Cu-0.5Mg and a Al-1.2Cu-1.2Mg (at.%) alloy has been investigated. Artificial ageing at 150°C of stretched and naturally aged samples initially (up to about 48 h) leads to very limited further strengthening, but ageing at 190°C results a quick increase in strength. Detailed microstructural investigation using differential scanning calorimetry, transmission electron microscopy and three-dimensional atom probe demonstrated that hardening at 150oC is mostly dominated by the formation of solute clusters with varying compositions and plate-like zones rich in copper at early stages of ageing (t<24h) and by the formation of S phase at the later stages of ageing. At higher 190oC no zones or clusters form and the ageing is dominated by the formation of S precipitates.