Effect of Hf additions on the isothermal-oxidation behavior of TiAl at high temperatures

The isothermal-oxidation behavior of TiAl coupons containing Hf of up to 5.2 mass % has been studied in the temperature range 1100-1400 K in a flow of purified oxygen under atmospheric pressure. The addition;of 0.2% Hf is very effective to decrease the oxidation rate at 1200 and 1300 K. Metallographic examination using conventional methods revealed that the initially-formed Al2O3 scale is maintained very sound by the addition. However, further additions of Hf result in a slight enhancement of oxidation at 1200 K and a gradual decrease of the effect at 1300 K. Finally, there is almost no effect by the addition of 5.2% Hf at 1300 K. Excess amounts of Hf lead to the formation of oxide mounds on the Al2O3 scale. They grow in size and number during subsequent oxidation until the whale specimen surface is covered with a thick scale. Such a scale is not protective having a structure often, reported in the literature. The effect of the addition of 0.2% Hf becomes small at 1350 K and at 1400 K it is inverted. Possible mechanisms for the improvement attained by the suitable addition are discussed.