Effect of Nb, Zr and Zr + Hf on the microstructure and mechanical properties of β-solidifying γ-TiAl alloys

Abstract

The present work has been devoted to study of β-solidifying γ-TiAl alloys based on Ti–44Al-0.2B and doped with Nb, Zr and Zr + Hf. The phase transformation sequences were established for the alloys. On this basis, upset forging followed by heat treatment was performed that resulted in formation of near the same duplex type microstructures in the alloys. Tensile tests in the temperature range of 20–900 °C and creep tests at 700 °C were carried out for the alloys in the duplex conditions. The alloys doped with Zr and Zr + Hf showed appreciably higher strength, higher temperatures of the brittle-ductile transition (BDT) and enhanced creep resistance while retaining near the same ductility below the BDT temperatures as compared to the Nb-containing alloy. Enhanced creep resistance and higher temperatures of the BDT in the alloys doped with Zr and Zr + Hf in contrast to the Nb-containing alloy were attributed to higher solid solution hardening due to larger atomic radii of Zr and Hf versus Nb and to different partitioning behaviors of Zr and Zr + Hf versus Nb leading to higher concentrations in the γ phase of Zr and Zr + Hf in the Zr- and (Zr + Hf)-containing alloys in contrast to that of Nb in the Nb-containing alloy. Both of these factors contributed to lower diffusivity (especially in γ grains) in the alloys doped with Zr and Zr + Hf. In particular, alloying with Zr and Zr + Hf shifted the development of dynamic recrystallization processes towards high temperatures that slowed down the increase of ductility with increasing the test temperature in the BDT range and led to higher BDT temperatures.