Microstructural evaluation of oxide layers in CaO-added Mg alloys

A transmission electron microscopy analysis was conducted on the oxide layers of Mg-5.5Al-0.25Mn (AM60) alloys with different levels of CaO content formed in a thermogravimetric analyzer (TGA) system at 450 °C for 7 h. It was found that the oxide layer in AM60 + CaO grew as a columnar structure and, resultantly, that the oxide layers had different thicknesses with different levels of CaO content. The columnar oxide grains that were formed on the Mg + CaO alloy grew on the [1 0 0]MgO, resulting in a more compact and thicker initial oxide layer compared to the normal oxide layer. The columnar growth of the oxide layer in AM60 + CaO formed due to the Ca, which dissolved from CaO.