Characterization of precipitates in aluminium-based alloy AW 6016

The aluminium-based engineering alloy AW 6016 of nominal composition 98.44 Al-0.45 Mg-0.96 Si-0.11 Fe-0.034 Mn-0.0054 Zn-0.0038 Cr was investigated (all values are given in at.%). Heat treatment of solutionized and quenched alloys leads to an increase in hardness, which is due to the formation of small precipitates of nanometer scale size. Two complementary high-resolution methods, namely transmission electron microscopy (TEM) and three-dimensional atom probe (3-DAP), are used for investigations of the precipitate evolution after annealing at 185 °C and 235 °C. At very early stages, i.e. after 5 min at 185 °C, Mg-rich, Si-rich and Mg-Si co-clusters are observed. They are spherical and their diameter is between 1 and 3 nm. The number density of clusters as measured by 3-DAP is 9 x 10 23 /m 3 . After ageing for 25 min and 90 min at 185°C, GP zones and a needle-like β" phase are formed. Both are rich in Mg and Si. The concentration ratio of Mg to Si of the precipitates is ∼1. The number density of all precipitates after 90 min annealing time at 185 °C is 6 x 10 23 /m 3 . The needle-like β" precipitates at this ageing stage are about 4 nm in diameter and longer than 10 nm. The precipitates formed at 235°C after 10 min are much larger when compared to those formed at 185 °C. They are rod-shaped and have an average size as measured by TEM, about 50 nm in length and 3-4 nm in cross section. The concentration ratio between Mg and Si as measured by 3-DAP is about 1.