X-Ray diffraction (XRD) analysis was used to study the influence of alloying with 5 at.% Nb, 5 at.% Zr and 5 at.% (Zr+Hf) on the lattice parameters of the γ(TiAl) and α 2 (Ti 3 Al) phase in the intermetallic alloys based on Ti-44Al-0.2B (at.%). Before XRD analysis duplex structures with near the same microstructural parameters were obtained in samples of the alloys. The XRD data were used to calculate the tetragonal distortion (c γ / a γ ratio) of the γ phase, the c α2 / a α2 ratio of the α 2 phase and the γ / α 2 lattice misfits of the alloys. The highest tetragonal distortion (c γ / a γ ) of the γ unit cell (c γ / a γ =1.0124) is observed for the base Ti-44Al-0.2B alloy, followed by the Nb-, (Zr+Hf)-and Zr-containing alloy (c γ / a γ =1.0116, 1.0075 and 1.0069, respectively). The c α2 / γ α2 ratio is insignificantly changed depending on alloying. Doping with Zr and Zr+Hf leads to a noticeable decrease in the γ / α 2 lattice misfits as compared with the alloy doped with Nb and the base alloy. For instance the γ / α 2 lattice misfits determined in both crystallographic directions of the γ phase in the Ti-44Al-5Zr-0.2B and Ti-44Al-5Nb-0.2B alloys were found to be ε 110 / ε 101 = 0.93 / 0.59 and ε 110 / ε 101 =1.38 / 0.79, respectively. It has been recently revealed that γ-TiAl alloys with near the same duplex structures based on Ti-44Al-0.2B and doped with Zr and Zr+Hf demonstrated appreciably higher creep resistance than the alloy doped with Nb and the base alloy. It is assumed that the lower c γ / a γ ratios obtained for the Zr-and (Zr+Hf)-containing alloys contribute to the reduction of creep resistance. The fact that the Zr-and (Zr+Hf)-containing alloys showed higher creep resistance than the Nb-containing alloy should be mostly attributed to the lower γ / α 2 lattice misfits in the Zr-and (Zr+Hf)-containing alloys and the higher solution hardening caused by doping with Zr and Hf. Therefore, the impact of alloying on the creep and heat resistance in β-solidifying γ-TiAl alloys should be considered taking into account the changes of the lattice parameters of the γ(TiAl) and α 2 (Ti 3 Al) phase, which influence the physical processes determining the creep behavior.
The present work is devoted to study of the oxidation behavior of two β-solidifying γ-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°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.
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