Insights into the design of oxidation-resistant Mg alloy by alloying with rare-earth elements

The strategy of rare-earth elements addition into Mg alloys has been successfully developed and applied to enhance the mechanical performance and corrosion resistance of Mg alloys. Although this strategy has also been applied to enhance their high-temperature oxidation resistance, the mechanistic understanding of the beneficial effects remains elusive. Here, the oxidation of Mg-4Nd and Mg-4Nd-1Y alloys in Ar-20%O2 at 500 degrees C was studied and compared. It was found that even though a continuous layer of Nd2O3 did not form, the Nd addition could still enhance the oxidation tolerance of Mg-4Nd alloy by facilitating the generation of a more continuous and intact oxide scale and working as oxygen sinks to delay the violent oxidation of Mg. The formation of a continuous Y2O3 layer on Mg-4Nd-1Y alloy suggests that Y was more capable of facilitating the external oxidation due to its much faster diffusion rate in Mg matrix than that of Nd. However, the Nd addition could decrease the critical content of Y necessary for the oxidation transition from internal to external because of the synergistic effect of the Nd and Y addition. The dissolution of the thermal unstable Mg12Nd precipitates resulted in a localized increase of Nd content, accelerating the oxidation by increasing the preferential oxidation of Nd. Hence, in the design of oxidation-resistant Mg alloys, the addition of RE elements with faster diffusion rate and the addition of multiple alloy elements are preferred. In addition, the number of thermal unstable precipitates needs to be strictly controlled.