The effect of alloying and fluorination on the oxidation behavior of β-solidifying γ-TiAl based alloys

The present work is devoted to study of the oxidation behavior of two beta-solidifying gamma.-TiAl alloys (Ti-43.5Al-4Nb-1Mo-0.1B (TNM alloy) and Ti-44Al-6(Nb, Zr, Hf)-0.15B (TNZ alloy) (at.%)). The as-cast alloys were subjected to upset forging and heat treatment that resulted in similar microstructures in both alloys. Plate-shaped samples were cut from the obtained workpieces, mechanically polished and subjected to oxidation exposure at 800 degrees C (500 h). The samples during annealing were periodically removed from the furnace and weighed. After oxidation exposure the mass gain of the TNZ sample was found appreciably smaller than that of the TNM sample. The preliminary fluorination treatment in a diluted hydrofluoric acid (HF) provided a noticeable increase of the oxidation resistance in the case of the TNM alloy and a significant worsening of the oxidation resistance in the case of the TNZ alloy. At the same time, the non-fluorinated sample of the TNZ alloy showed near the same oxidation resistance as the TNM samples subjected to preliminary fluorination treatment. EDS analysis revealed the competitive formation of aluminum and titanium oxides on the surfaces of the oxidized TNM and TNZ samples. Predomination of the alumina formation contributed to higher oxidation resistance.