The influence of alloying and microstructure on oxidation resistance of β-solidifying γ-TiAl alloys

One of the critical issues of γ-TiAl intermetallic alloys preventing long-term use at temperatures above 700 – 750°C is insufficient oxidation resistance. The oxidation resistance has been studied for three β-solidifying γ-TiAl alloys: Ti-43.5Al-4Nb-1Mo-0.1B (TNM) and Ti-(43.5 – 44)Al-6(Nb, Zr, Hf)-(0.1– 0.15)B (TNZ / TNZ+) (at.%). Before oxidation, the as-cast alloys were subjected to the isothermal forging and different heat treatment, which resulted in near duplex and near lamellar microstructural conditions in the alloys. Plate-shaped samples were cut from the forded and heat-treated workpieces, mechanically polished and subjected to oxidation exposure in air at 800°C for 500 and 1000 h. In the course of annealing, the samples were periodically removed from the furnace and weighed. It was found that alloying with 6 at.% (Nb, Zr, Hf) provided appreciably better oxidation resistance than alloying with 5 at.% (Nb, Mo). Besides, the near lamellar structure showed higher oxidation resistance than the near duplex one. Oxide scale analysis by SEM revealed that the layer enriched with oxygen was approximately 2 times thicker in sample of the TNM alloy than in sample of the TNZ alloy. The oxidation was accompanied by depletion in Al and Ti in the subsurface. However, the depletion in Ti was more pronounced in the case of the TNM alloy. XRD analysis of the oxide scale showed that the main oxidation products were alumina (Al2O3) and rutile (TiO2) but in the case of the TNM alloy the rutile was formed more preferably. The fundamental reason for better oxidation resistance of the TNZ / TNZ+ alloys is the enhanced thermal stability of the microstructure.