Effect of Rare Earth Element Yttrium on the Isothermal Oxidation Behavior of Aluminide Coatings on Ti-Ni Shape Memory Alloys

Ti-Ni shape memory alloys with and without Y addition were aluminized at 650 °C using a pack-cementation method. The influences of various amounts of Y additions on the microstructure and the isothermal oxidation behaviors of aluminide coatings in air at 700 °C were investigated. The results show that aluminide coatings exhibit double layer structure, and the aluminization process is mainly controlled by the inner diffusion of Al. The Y addition promotes the growth of outer TiAl3 layer, but it retards the growth of inner NiAl3 layer when Y content is lower than 1at%. Isothermal oxidation results indicate that the scale growth rate is significantly reduced when the Y addition is 0.5at%. In contrast, the oxidation rate of Ti-Ni alloy is increased when 1at% Y and 5at% Y are added, especially 5at% Y addition. The effect of Y element on the coating formation and the coating oxidation behavior was discussed.