S′ and δ′ phase precipitation in SiCp reinforced Al1.2wt.%Cu1wt.%MgχLi alloys

The microstructures obtained in reinforced and unreinforced Al-1.2Cu-1Mg-χLi-0.1Zr (in wt.%) alloys during ageing at 170 °C were studied by transmission electron microscopy and differential scanning calorimetry. A small volume fraction of inhomogeneously distributed S′ (Al2CuMg) is present in the monolithic 8090 alloy (Al-1.2Cu-1Mg-2.3Li-0.1Zr) aged up to 7 days. The addition of SiC particles reduces GPB-zone formation, but causes a denser and more homogeneous distribution of S′ which nucleates heterogeneously on dislocations. The addition of Li to an Al-Cu-Mg alloy reduces the Cu and/or Mg solubility, thus increasing the amount of S′ precipitates. The introduction of SiC particles enhances the formation during non-isothermal DSC heating of δ′ (Al3Li) precipitates and their coarsening rate during isothermal ageing. A model is presented which links the density of I (Al6Cu(Li, Mg)3) and δ (AlLi) grain boundary precipitates to the width of the precipitate free zones around grain boundaries and around SiC particles.