Microstructural evolution of Al–0.66Mg–0.85Si alloy during homogenization

The microstructural evolution of Al–0.66Mg–0.85Si alloy was investigated by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The as-cast microstructure is typical dendritical structure, consisting of α(Al), Al(FeMn)Si, Mg2Si, AlCuMgSi and Si phases. The electron diffraction analyses indicate that the Al(FeMn)Si phase is Al15(FeMn)3Si2 and the AlCuMgSi phase is Q(Al1.9CuMg4.1Si3.3). There are two kinds of Mg2Si phases in the as-cast microstructure. One is formed in the casting process, and the other is formed in the cooling process after casting process is finished. The phases have different crystal structures. After homogenization treatment at 545 °C for 20 h, Mg2Si, Si and Q intermetallic compounds are dissolved into matrix completely, and the remaining phases are α(Al) and Al15(FeMn)3Si2. The size of Al15(FeMn)3Si2 phase is decreased, and the phase is spheroidized and distributes along grain boundary discontinuously. The Zn-containing phases are not found during solidification and homogenization process.