Morphology and Nucleation of Intermetallic Phases in Casting Al–Mg–Si Alloys

The changes in the structure of Al–Mg–Si casting alloys after additional alloying are observed. In order to predict and to explain with greater confidence, the phase transformations in the studied alloys equilibrium phase diagrams were calculated using Thermo-Calc software. The results of the Thermo-Calc are in good agreement with the microstructure analysis. Morphology and chemical composition of intermetallic phases were investigated by scanning electron microscopy, energy-dispersive x-ray analysis, and electron probe microanalysis on polished and deep etched microsections. Several intermetallic phases with Fe and Mn, phases that contain Zn and Cu, and Al3Ti crystals were described. It was found that the addition of Mn changes the morphology of Fe-containing intermetallics that improve strength and ductility of the alloys; crystals Al3Ti can act as nucleating particles of α-Al dendrites; Cu and Zn lead to the formation of several fusible eutectic phases. To better understand the influence of the different intermetallics, the mechanical properties (Brinell hardness, ultimate tensile strength, yield strength, elongation) were measured. Alloying with Cu and Zn exhibits the best values of hardness and strength (up to 85 HB and UTC = 251 MPa), while the highest ductility was achieved in the alloy with composition Al–5.5Mg–2.5Si–0.6Mn (6.7%). Alloys possess the lowest properties (both strength and ductility) with the highest concentrations of Fe and Si (UTS up to 173 MPa with elongation 3.4%).