The High Temperature Oxidation and Ignition Behaviour of Mg-X (X=Nd, Sr) Binary Alloys

The effects of Nd and Sr on the high temperature oxidation and the ignition temperature (T_i) of pure Mg have been investigated over a range of compositions. The oxide morphology on pure Mg was observed to change to a more protective structure with both Sr and Nd additions. The addition of alloying elements was also found to be effective on the T_i; while the pure Mg ignites at 640 °C, the T_i was delayed to 860 °C with 6 wt.% Sr addition. Different oxidation and ignition behavior was exhibited on Mg-Nd alloys. While 0.3 wt.% Nd additions have no significant effect on the ignition resistance of pure Mg, the T_i is raised up to 770 ^oC at 0.5 wt.% Nd additions. However, with further addition of Nd up to 6 wt.% Nd, no noteworthy change is detected. The initial alloy microstructure is shown to be crucial and determines the oxide stability. The oxidation of Mg-Nd dilute alloys having near single-phase structure (α-Mg) is dominated by the oxidation of the solid-solution phase where the formation of a composite Nd₂O₃/MgO scale improves the oxide stability. The oxide structure on two-phase alloys changes to a dual oxide-morphology with a Nd₂O₃ rich subscale. The intermetallic network forms an oxide layer rich in Nd₂O₃, which creates fast oxygen diffusion paths. Nd consumption due to the oxidation process leads to Nd depletion at the metal/oxide interface where the continuous Nd supply is restricted. The oxidation mechanism differs on Mg-Sr alloys; the T_i increases with increased alloy composition. The surface active nature of Sr leads to the formation of a protective SrO-rich layer and decreases the Mg vaporization on the alloy surface and inside the cracks delaying ignition. The amount of surface enrichment changes with Sr in the alloy despite of the negligible solubility.