The Effect of Scandium on the Microstructure of the Aluminium Alloy AA 6086

The investigation studied the effects of 0.2 wt.% and 1 wt.% scandium (Sc) additions on themicrostructure of the aluminium alloy AA 6086 in different conditions. The alloys were produced bycasting into a metallic mould, followed by various heat treatments. The alloys were examined usinglight microscopy, scanning and transmission electron microscopy, microchemical analysis, differentialscanning calorimetry and X-ray diffraction. The phase compositions and solidification sequenceswere modelled using the CALPHAD approach, which reasonably agreed with the experimentalresults. The addition of Sc to AA 6086 strongly reduced the grain size of the Al-rich solid solutionand induced the appearance of Sc-rich phases AlSc2Si2 and L12-Al3X. Other phases identified inthe Sc-free alloy were also found in the Sc-modified alloys. Homogenisation caused the dissolutionof most phases and the formation of different types of dispersoids. In the alloy with 0.2% Sc, thedistribution of dispersoids was not uniform. The plate-like AlMnCrSi dispersoids formed mainly atthe dendrite centres, together with spherical L12 precipitates, while smaller -AlMnSi and tetragonalt-Al3Zr dispersoids were created elsewhere. The addition of 0.2% Sc did not considerably affect thestrengthening of AA 6086. The precipitation during isothermal ageing was slightly delayed andshifted to higher temperatures during continuous heating.