Study of minor Sc and Zr additions effect on silicon rich Al–Mg–Si aluminum alloy microstructure during Sc multistage thermal treatment

The study addresses the effect of multistage thermal treatment on microstructure formation of Al–Mg–Si series alloy with scandium and zirconium additions in Mg/Si = 0.3 proportion. Basic alloy AlM–Si without Sc and Zr and them modification AlMg1SiZrSc were cast. AlMgSiScZr alloy multi-stage thermal treatment included the following 4 annealing stages –550 °C 8 h + 440 °C 8 h + 500 °C 0.5 h + 180 °C 5 h. For AlMg1Si alloy it was performed following 550 °C 8 h + 180 °C 5 h practice. AlMgSiScZr microstructure was examined using scanning (SEM) and transmission (TEM) microscopy both in as-cast state and after each stage of thermal treatment, for AlMg1Si – both in as-cast state and after final thermal treatment. Coarse intermetallic particles were examined using SEM, while fine particles were examined using TEM. Besides, microhardness values were measured after various thermal treatment stages. It was found out, that coarse intermetallic compounds of Fe2Mg7Si10Al18 type are formed in both alloys during as-cast structure cooling down with such compounds partial dissolution during further thermal treatment. At the same time particles, that can be attributed either to (AlSi)3ScZr or to AlSc2Si2τ-phase, occur in inter-grain boundary of alloys with scandium-zirconium additions. No traces of scandium solid solution discontinuous decomposition have been identified during ingot cooling down. Thermal treatment during 8 h at 550 °C 8 and 8 h at 440 °C results in formation of the phase, which can represent both AlSc2Si2 and Al5SiZr2. At the same time nanoparticles (AlSi)3ScZr do not form during this thermal treatment stage. Heating to 500 °C during 30 min interval allows complete dissolution of magnesium in supersaturated aluminum solid solution. During the final stage β’’ particles (Mg5Si6), representing the major type, form in both alloys; in these conditions scandium does not produce significant effect on their formation.