Studying the Effect of Small Additives of Sc and Zr on the Microstructure of Al–Mg–Si Alloy with Excess Silicon during Multi-Step Heat Treatment

The work is devoted to the study of the effect of multi-step heat treatment on the formation of the microstructure of the alloy of Al–Mg–Si series, alloyed with scandium and zirconium at the ratio of Mg/Si = 0.3. For this purpose, the base AlMgSi alloy, containing no scandium and zirconium, as well as its modification AlMgSiScZr with additions of these elements were cast. Multi-step heat treatment of AlMgSiScZr alloy consisted of four steps of annealing: at 550°C for 8 h + at 440°C for 8 h + at 500°C for 0.5 h + at 180°C for 5 h. For AlMgSi alloy, it included two steps: at 550°C for 8 h + at 180°C for 5 h. The microstructure of AlMgSiScZr alloy was studied in the cast state, as well as after each step of heat treatment by scanning and transmission electron microscopy, while AlMgSi alloy was studied in the cast state and after the final heat treatment. After each step of heat treatment, the microhardness of the studied alloys was measured. It was found that during cooling of the ingot, in the alloys under consideration coarse intermetallics of Fe₂Mg₇Si₁₀Al₁₈ type were formed, which partially were dissolved during subsequent heat treatment. At the same time, in the alloy with scandium and zirconium additives, particles that can be attributed either to (AlSi)₃ScZr or to the τ (phase) of AlSc₂Si₂ are present at the intergranular boundaries. No traces of intermittent decomposition of the scandium supersaturated solid solution during cooling of the ingot were found. Heat treatment at 550°C for 8 h + at 440°C for 8 h leads to the appearance of a phase that can be both AlSc₂Si₂ and Al₅SiZr₂, while no (AlSi)₃ScZr nanoparticles are formed. Heating at 500°C for 30 min enables complete dissolution of the magnesium-containing particles. At the final step, β" (Mg₅Si₆) particles are formed in the investigated alloys, while scandium has no significant effect on their formation.