This work studies the solution heat treatment of two quaternary Al-alloys with high content of alloying elements (Al-6Si-7Cu-5Mg and Al-6Si-7Cu-7Mg, in wt.%), focusing on the dissolution behavior second phases, specifically of Q-phase (Al5Cu2Mg8Si6), which has been barely studied. These alloys present percentages of Mg and Cu considerably higher than the used in conventional Al-Si-Cu-Mg alloys, leading to obtain a significant content of second phases. Second phases observed in the as-cast alloys were Si, Mg2Si, Al2Cu and Q, reaching a total percentage of ~22% for the alloy with 7%Mg. This was corroborated through the characterization of the alloys using Optical Microscopy (OM), X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The most important microstructural modification originated due to the high content of alloying elements was the presence of Q as the predominant second phase, which commonly is a minority phase for Al-Si-Cu-Mg alloys. The experimental alloys were solution heat treated at 490 °C during times ranging from 1 to 24 h, resulting in the dissolution of the second phases following conventional and well reported mechanisms including dissolution, fragmentation, spheroidization and/or coarsening. Nevertheless, Mg2Si did not present these mechanisms because both Chinese script and primary Mg2Si transformed into Al5Cu2Mg8Si6 during the solution heat treatment. Q phase formed through this transformation kept 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 process starts with the transformation at the interface Mg2Si/a-Al matrix, followed by the complete formation of Q. This transformation led to an unusual increase in the Q phase percentage after the solution heat treatment.
