Oxidation behavior of sulfidation processed TiAl–2 at.% X (X=V, Fe, Co, Cu, Nb, Mo, Ag and W) alloys at 1173 K in air

The oxidation behavior of sulfidation processed TiAl–2 at.% X (X=V, Fe, Co, Cu, Nb, Mo, Ag, and W) alloys was investigated at 1173 K in air for up to 630 ks under a heat-cycle condition between 1173 K and room temperature. Oxidation behavior was classified into two groups: group A with alloys containing Fe, Nb, Mo and W, and group B containing V, Co, Cu and Ag. The TiAl binary alloy belonged to group B. During the sulfidation processing, the alloying elements in group A formed aluminides on the TiAl3 layer, while the group B alloys formed a TiAl3 (TiAl2 included) layer including a small amount of the third element. The cross-sectional microstructures for group A show the sequence; oxide scale/TiAlX/TiAl2/alloy substrate, and for group B oxide scale/Ti3Al/alloy substrate. The alloys in group A (except Nb containing alloy) showed some scale exfoliation at the initial stage of oxidation, but only very little exfoliation after long oxidation times, whereas alloys in group B showed little exfoliation at the first several cycles, and then tended to exfoliate significantly, resulting in very rapid oxidation. The TiAlX/TiAl2 layers formed by the reaction between the X-aluminide and TiAl3 improve the oxidation properties of the group A alloys.