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 MgSi 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 1023 m3. After ageing for 25 min and 90 min at 185 C, GP zones and a needle like 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 1023 m3. The needle like 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