Atomic-scale oxidation mechanisms of single-crystal magnesium

Understanding the oxidation process of active metals plays a crucial role in improving their mechanical/oxidation properties. Using in situ environmental transmission electron microscopy and density-functional theory, we firstly clarify the oxidation process of single-crystal Mg at the atomic scale by using a new double-hole technique. A unique incipient interval-layered oxidation mechanism of single-crystal Mg has been confirmed, in which O atoms intercalate through the clean (21[combining macron]1[combining macron]0) surface into the alternate-layered tetrahedral sites, forming a metastable HCP-type MgO_0.5 structure. Upon the increased incorporation of oxygen at the neighboring interstitial sites, the HCP-type Mg-O tetrahedron structure sharply transforms into the FCC-type MgO oxide_. In addition, a typical anisotropic growth mechanism of oxides has been identified, wherein it involves two routes: the epitaxial growth of the MgO layer and the inward migration of the MgO/Mg interface. The whole oxidation rate of single-crystal Mg is mostly determined by the inward migration rate of the MgO/Mg interface, which is about six times higher than that of the epitaxial growth rate of the MgO layer along the same orientation planes. Moreover, the inward migration rate of the (020)_MgO‖(011[combining macron]0)_Mg interface is about twice as large as that of the (200)_MgO‖(0002)_Mg interface. This continuous oxide growth is mainly related to the defects in the MgO layer, which builds effective channels for the diffusion of O and Mg atoms. The in situ double-hole observations together with theoretical calculations provide a potential trajectory to probe the oxidation fundamentals of other active metals.