Dissolution of the second phases and transformation from Mg2Si to Q-Al5Cu2Mg8Si6 in solution heat treated quaternary Al-alloys

This work studies the formation of second phases and their dissolution for two alloys Al-6Si-7Cu-xMg (x = 5 and 7 wt%), focusing on Al5Cu2Mg8Si6 (Q-phase), which has been barely studied. The alloys were solution heat treated at 490 °C during times ranging from 1 to 24 h, and quenched in hot water. As-cast and solubilized alloys were characterized using Optical Microscopy (OM), X-Ray Diffraction (XRD), and Scanning Electron Microscopy (SEM). Results showed that second phases were eutectic Si, Al2Cu, Mg2Si, and Q, with total percentages of ∼18% and 22% for the alloys with 5% and 7% Mg, respectively. Al2Cu and Q-phase were the predominant second phases in each individual case. The solution heat treatments led to the dissolution of the second phases following conventional and well-reported mechanisms. Nevertheless, Mg2Si did not present these mechanisms because both Chinese script and primary Mg2Si transformed into Q-phase, keeping the original morphology of Mg2Si. The presence of Q with a Chinese script morphology has not been reported before in literature, which is discussed here. This transformation led to an unusual increase in the Q-phase percentage after the solution heat treatment.