Effect of preheating treatment in Ar atmosphere on oxidation resistance of Mg-Y alloys and formation of oxide film

Magnesium and its alloys occupy a vital role in the aerospace, automotive and communications industries due to their low density, high specific strength and electromagnetic shielding capabilities. However, due to the strong affinity of Mg for O and the fact that MgO is generally not protective, application of Mg and its alloys at high temperature often results in catastrophic oxidation. One promising approach is based on the addition of Y in combination with a suitable pretreatment to achieve a protective oxide. Therefore, the oxidation resistance (OR) of Mg-xY (x = 0.5, 2.5 and 5.5 wt%) was investigated after using a preheating treatment in Ar atmosphere. The oxidation resistance of preheated Mg-xY alloys is greatly improved compared to unpreheated alloys and pure Mg, particularly for x = 2.5. This improvement is attributed to the enrichment of Y to the surface caused by outward diffusion of Y, and subsequent selective oxidation of Y under low oxygen partial pressure to generate a dense and compact protective film, composed of an outer thin MgO/Y₂O₃ composite layer and an inner thick Y₂O₃ layer. Some flocculent MgO/Y₂O₃ composites were observed on the surface of preheated Mg-5.5Y during the oxidation process, lowering the OR of this alloy.