Influence of Alloying Elements on the oxidation Characteristics of Ti-Al Intermetallic Compounds

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TiAl coupon specimens containing Hf of up to 5.2% were isothermally oxidised in a temperature range 1100 to 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 examinations using the conventional methods revealed that the initially formed Al_2O_3 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.There is almost no effect by the addition of 5.2% Hf at 1300 K.The addition of 0.2% Zr is similarly effective as the addition of 0.2% Hf. These additives also improve the resistance to cyclic oxidation with temperature varying between room temperature and 1300 K for at least 100 cycle (360 ks). However, the effect of the both additives becomes small at 1350 K and at 1400 K it is inverted. Ti_3Al and Ti_3Al-21%Nb coupon specimens were isothermally oxidised in a temperature range 1100 to 1300 K in a flow of purified O_2, Ar-21%O_2 or N_2-21%O_2 under atmospheric pressure. The N_2 in the oxidation atmosphere decreases the oxidation rate of Ti_3Al a little. This is attributable to the formation of TiN resulting in the enrichment of Al near the specimen surface. A thin TiN layr is formed near the scale/Ti_3Al-21Nb substrate, leading to the enrichment of Al_2O_3 beneath the TiN layr and thus to a considerable decrease in the oxidation rate.