Research Progress of Rare Earth Elements on High Temperature Oxidation Behavior of Alloys

The existing heat-resistant alloys are gradually unable to meet the performance requirements of advanced materials，and it is generally believed that adding a small amount of rare earth elements to the alloy is beneficial for improving its oxidation resistance. Although the influence of rare earth elements on the high-temperature oxidation behavior of alloys has been studied by many scholars and various mechanism models have been proposed，none of them have been widely recognized. This article systematically reviewed the effects of rare earth elements Y，Ce，La and the common addition of several rare earth elements on the high-temperature oxidation behavior of alloys，and analyzed the mechanism of rare earth elements. The results indicated that the addition of rare earth elements could refine the alloy grain size，promote the formation of a complete and dense oxide film，and improve the alloy's oxidation resistance. On the other hand，it could change the diffusion mechanism of anions and cations in the oxide film，reduce the voids between the oxide film and the alloy matrix，and improve the anti-peeling property of the oxide film. However，there was an optimal content of rare earth elements to improve the oxidation resistance of alloys，and excessive addition could lead to accelerated oxidation. This article also looked forward to future research directions on the influence of rare earth elements on the high-temperature oxidation behavior of alloys. Meanwhile，based on the discussion of the oxidation process，the oxidation mechanism of rare earth elements on the alloy oxidation behavior was analyzed. The results indicated that the oxidation of alloys could be roughly divided into two stages：the initial oxidation stage and the stable oxidation stage. In the initial oxidation stage，the oxide film was not completely formed，which was the growth process of the oxide film. The oxide film of the alloy without the addition of rare earth elements presented a discontinuous state on the surface of the alloy，which could not effectively isolate the alloy from oxygen. Due to short-circuit diffusion，the first generated oxide aggregated at grain boundaries，and at this time，the oxide was mainly formed by the outward diffusion of alloy elements and the combination with oxygen. Due to the migration of a large number of alloying elements，voids were formed at the interface of oxides and alloys. The presence of these voids weakened the adhesion between the oxide film and the substrate，reducing the anti-peeling property of the oxide film. The alloy with added rare earth elements could quickly form a uniform and dense oxide film on the surface due to the increase in the number of grain boundaries and the provision of sufficient nucleation points by rare earth elements. Meanwhile，due to the aggregation of rare earth elements in the oxide film，the diffusion of alloy elements was blocked，and the diffusion mechanism in the oxide film changed from alloy elements （large atoms）diffusing outward to oxygen（small atoms）diffusing inward. At this point，the number of voids in the oxide film significantly decreased. In the stable oxidation stage，the oxide film had basically formed，which had a good protective effect on the alloy matrix，and the thickness of the oxide film generally did not increase and remains within a certain range. At this point，the alloy without the addition of rare earth elements continuously diffused towards the outer side of the oxide film，and the number of voids at the interface between the oxide film and the alloy gradually increased，resulting in a decrease in the adhesion between the oxide film and the matrix，and a decrease in the anti-peeling property of the oxide film. At the same time，due to the increase of tensile stress inside the oxide film，small longitudinal cracks appeared inside the oxide film，which would also promote the oxidation of the alloy. Moreover，alloys with added rare earth elements would grow some needle like oxides in the gaps of the oxide film，which effectively filled the gaps in the oxide film and make it denser. Due to the extremely complex oxidation process of multi-component alloys，and the oxidation temperature was usually above 800 ℃，it was difficult to observe in situ during the alloy oxidation process，which greatly hindered the understanding of the mechanism of rare earth elements in the high-temperature oxidation process of alloys. The studies had shown that the critical concentration of active components in alloys transitioning from internal oxidation to protective external oxidation was highly sensitive to the addition of trace rare earth elements. However，the role of rare earth elements in the oxidation process was not yet clear. The simple binary alloy systems with significant differences in oxide stability should be choosen to study the effects of adding various rare earth elements with different contents on their high-temperature oxidation behavior，in order to clarify the fundamental reasons for the drastic changes in the oxidation mode of the alloys. © 2024 Editorial Office of Chinese Journal of Rare Metals. All rights reserved.