The high temperature oxidation behavior of Mg–Nd alloys. Part II: The effect of the two-phase microstructure on the on-set of oxidation and on oxide morphology

• Two-phase alloys show different oxidation behavior than the single-phase alloys.
• A non-uniform oxide film forms on two-phase Mg–Nd alloys.
• The intermetallics act as diffusion paths for O to the metal/oxide interface.
• The oxide scale consists of Nd2O3/MgO structure and a subscale rich in Nd2O3.

The oxidation behavior of two-phase Mg–Nd alloys was investigated via isothermal heating experiments conducted in dry air at 500 °C for 12 h. Thermodynamic assessment of the onset of oxidation suggested a possible oxidation sequence of Nd oxidation from the intermetallic phase, Mg oxidation from the intermetallic and the α-Mg phases, and then Nd oxidation from α-Mg. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) revealed a mixed MgO–Nd2O3 scale formation on the surfaces with a Nd2O3 rich subscale. Dual oxide morphology evolved during the oxidation process that mimics the two-phase microstructure of the alloys. This heterogeneous scale formation on the surface was attributed to the differing oxidation behavior of the two phases (α-Mg and Mg12Nd) in the alloys.