Tailoring microstructure and chemical composition of advanced γ-TiAl based alloys for improved creep resistance

The TNM alloy with the nominal composition Ti-43.5Al-4Nb-1Mo-0.1B (in at.%) combines many advantages regarding applicable manufacturing techniques yielding balanced mechanical properties when compared to other gamma-TiAl based alloys presently used for industrial applications. However, the service temperature of the TNM alloy is limited to approximately 750 degrees C due to an insufficient creep resistance at temperatures beyond. Therefore, in the present work, options to improve the creep behavior by minor modifications of the chemical composition and the microstructure by designed heat treatments were explored. The resulting changes in phase equilibria by increasing the Al content or by the addition of the alpha-phase stabilizing element C provide the opportunity of eliminating the beta(o)-phase as well as globular gamma-grains in the final fully lamellar microstructure via a two-step heat treatment. Furthermore, lamellar alpha(2)/gamma-colonies with optimized grain size as well as homogenous size distribution could be adjusted using minor additions of Si. As demonstrated, stepwise reduction and elimination of detrimental microstructural constituents lead to a significant improvement of the creep resistance, thus increasing the application temperature significantly.