Nucleation and thermal stability of carbide precipitates in high Nb containing TiAl alloys

• Published in: Intermetallics Vol. 66; pp. 111 - 119
• Main Authors: Wang, Li, Gabrisch, Heike, Lorenz, Uwe, Schimansky, Frank-Peter, Schreyer, Andreas, Stark, Andreas, Pyczak, Florian
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-11-2015
• Subjects: Alloys; Carbides; Carbon; Diffraction/scattering (electron, neutron and X-ray); Electron microscopy, transmission; Homogenizing; Intermetallic compounds; Intermetallics; Nucleation; Nucleation and growth; Precipitates; Precipitation; Titanium aluminides; Nucleation and growth; Intermetallics; Electron microscopy, transmission; Diffraction/scattering (electron, neutron and X-ray); Precipitates
• ISSN: 0966-9795; 1879-0216
• DOI: 10.1016/j.intermet.2015.07.001

This work monitors the nucleation, growth and coarsening of carbides in powder metallurgically processed, high Nb containing TiAl alloys. The effects of carbon content, annealing conditions and internal defects on the precipitation and stability of carbides were systematically investigated by high energy X-ray diffraction and transmission electron microscopy. In general, at 800 °C the carbide microstructure can still change significantly up to 1000 h of annealing. It is found that a higher carbon concentration promotes the carbide precipitation process and increases the thermal stability of carbides. Internal interfaces and other crystallographic defects act not only as heterogeneous nucleation sites for perovskite Ti3AlC carbides but also as carbon sinks. This retards the carbide nucleation in the interior of γ-TiAl grains. By homogenising the carbon distribution through solution heat treatment the nucleation of carbides in the γ-TiAl matrix is significantly accelerated as an effect of higher matrix carbon content.
•P-type carbides in high Nb containing TiAl alloys are stable at 800 °C for over 1000 h.•No H-type carbides form at 800 °C for over 1000 h or at 1000 °C for 168 h.•Feasibility of carbide hardening in high Nb containing TiAl alloys is restricted to temperatures of about 800 °C.•It can take more than 1000 h at 800 °C to reach the equilibrium carbide fraction.•Higher carbon content increases thermal stability and accelerates precipitation of carbides.