In this work, we investigated a novel Al-Mg-Si alloy, which was developed from an AA 6082, in order to considerably improve the yield and tensile strengths whilst retain excellent ductility. The new alloy possesses a higher content of Si than specified by AA 6082, and, in addition, it contains copper and zirconium. The alloy was characterized in the as-cast condition, after homogenization, extrusion, and T6 heat treatment using light microscopy, scanning and transmission electron microscopy with energy dispersive spectrometry, X-ray diffraction, differential thermal analysis and tensile testing. After T6 temper, tensile strengths were around 490 MPa with more than 10% elongation at fracture. The microstructure consisted of small-grained Al-rich matrix with α-AlMnSi and Al3Zr dispersoids, and Q′-AlCuMgSi and β-Mg2Si-type precipitates.

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

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The aluminium alloy AA 6086 attains the highest room temperature strength among Al-Mg-Si alloys. This work studies the effect of Sc and Y on the formation of dispersoids in this alloy, especially L12-type ones, which can increase its high-temperature strength. A comprehensive investigation was carried out using light microscopy (LM), scanning (SEM), and transmission (TEM) electron microscopy, energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and dilatometry to obtain the information regarding the mechanisms and kinetics of dispersoid formation, particularly during isothermal treatments. Sc and Y caused the formation of L12 dispersoids during heating to homogenization temperature and homogenization of the alloys, and during isothermal heat treatments of the as-cast alloys (T5 temper). The highest hardness of Sc and (Sc + Y) modified alloys was attained by heat-treating alloys in the as-cast state in the temperature range between 350 °C and 450 °C (via T5 temper).