S' AND DELTA' PHASE PRECIPITATION IN SICP REINFORCED AL-1.2WT.PERCENT-CU-1WT.PERCENT-MG-XLI ALLOYS

The microstructures obtained in reinforced and unreinforced Al-1.2Cu-1 Mg-xLi-0.1Zr (in wt.%) alloys during ageing at 170 degrees C were stud ied by transmission electron microscopy and differential scanning calo rimetry. A small volume fraction of inhomogeneously distributed S' (Al 2CuMg) is present in the monolithic 8090 alloy (Al-1.2Cu-1Mg-2.3Li-0.1 Zr) 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 t o an Al-Cu-Mg alloy reduces the Cu and/or Mg solubility, thus increasi ng the amount of S' precipitates. The introduction of SiC particles en hances the formation during non-isothermal DSC heating of delta' (Al3L i) precipitates and their coarsening rate during isothermal ageing. A model is presented which links the density of I (Al6Cu(Li, Mg)(3)) and 6 (AlLi) grain boundary precipitates to the width of the precipitate free zones around grain boundaries and around SiC particles.