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. [Display omitted] •The effect of the alloying elements on the formation of second phases for quaternary Al-Si-Cu-Mg alloys.•The formation of high percentage of second phases with different morphologies including Chinese script.•The transformation from Mg2Si to Al5Cu2Mg8Si6 due to the heat treatment.•The Chinese script morphology for the Al5Cu2Mg8Si6 phase obtained from the solid state transformation.•SEM studies and shape descriptors allowed the study of the dissolution process of the second phases,