Characterization of complex surface oxide layers formed during the solidification of distinct Mg-RE alloys

Surface oxide layers play a significant role in forming secondary oxidation inclusions during the casting process. In this study, three typical Mg-RE alloys (Mg-3Nd (NZ30K), Mg-3Nd-3Gd (EV33) and Mg-3Nd-4Y (WE43A)) are selected. Their surface oxide layers formed during the solidification are characterized in detail, and the corresponding oxidation mechanisms are discussed. The results reveal that RE elements obviously influence the characteristics of surface oxide layers, which depends on their ability to purify the formed MgO in the melt via the reaction (2RE + 3MgO = 3Mg + RE₂O₃). On the one hand, as Nd and Gd do not easily displace MgO already formed in the melt, the loose oxide layers in NZ30K and EV33 alloys are mainly composed of MgO matrix with embedded RE-rich oxide particles. On the other hand, due to the strong ability of Y to purify MgO in the melt, the oxide layer of WE43A alloy becomes a denser and thinner Y₂O₃ oxide layer. Note that the differences in surface oxide layers well explain the different secondary inclusions that occur in three typical Mg-RE alloys during the casting process.