Study on the Microstructure of Mg-4Zn-4Sn-1Mn-xAl As-Cast Alloys

In this study, the microstructure of the Mg-4Zn-4Sn-1Mn-xAl (x = 0, 0.3 wt.%, denoted as ZTM441 and ZTM441-0.3Al) as-cast alloys was investigated using scanning electron microscopy (SEM), focused-ion/electron-beam (FIB) micromachining, transmission electron microscopy (TEM), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The analysis results revealed that the microstructure of the ZTM441 and ZTM441-0.3Al as-cast alloys both mainly consist of the α-Mg matrix, skeleton-shaped MgZn₂ eutectic texture, block-shaped Mg₂Sn, and Zn/Sn-rich nanoscale precipitate bands along the grain boundary and the interdendrite. Nanoscale α-Mn dispersoids formed in the grain in the ZTM441 alloy, while no α-Mn formed in the ZTM441-0.3Al alloy instead of nanoscale Al₃Mn₂ particles. In the ZTM441 as-cast alloy, part of the Zn element is dissolved into the α-Mn phase, and part of the Mn element is dissolved into the MgZn₂ phase, but in the ZTM441-0.3Al alloy, there are no such characteristics of mutual solubility. Zn and Mn elements are easy to combine in ZTM441 as-cast alloy, while Al and Mn are easy to combine in ZTM441-0.3Al as-cast alloy. The Mg-Zn phases have not only MgZn₂-type crystal structure but also Mg₄Zn₇- and Mg₁₄₉Zn-type crystal structure in the ZTM441-0.3Al as-cast alloy. The addition of Al changes the combination of Mn and Zn, promotes the formation of Al₃Mn₂, and the growth of the grain.