Effect of Oxidation Resistant Coatings on the Deformation Behavior of TiAl Alloys at 1173 K in Air

The effects of coatings on the deformation and oxidation behavior of Ti-50Al alloy were investigated at 1173 K in air at a constant loading of 30 MPa. The coating was formed by a two-step Cr/Al diffusion treatment and consisted of an outermost TiAl2 layer, an outer Al-rich γ layer, an intermediate γ, Laves and β mixed layer, and a Cr diffusion zone. Tensile tests were also carried out with sole Cr or Al coated TiAl and uncoated TiAl at 1173 K for comparison purposes. The oxide scales formed on the uncoated TiAl and the sole Cr coated specimens were a mixture of TiO2 and Al2O3, which displayed extensive exfoliation. Both the two-step Cr/Al coated TiAl and the sole Al coated specimens formed a protective Al2O3 layer and little oxide exfoliation was observed here. Significant cracks were observed in the sole Al coated TiAl, while no cracks were observed in the sole Cr coated TiAl; the two-step Cr/Al coated TiAl showed a number of cracks in the coatings. Low creep rates in the two-step Cr/Al coated TiAl could be due to the Laves phase with a hexagonal C14 structure in the intermediate, γ, β and Laves phase mixture, and the high creep rates of the sole Cr coated TiAl may originate in the major β phase component with a B2 structure in the γ, β, and Laves phase mixture.