Effects of aluminum and titanium additions on the formation of nonmetallic inclusions in nickel-based superalloys

To study the effects of Al and Ti additions on the formation of inclusions in nickel-based superalloys, laboratory experiments were carried out at 1823 K. We observed mainly pure Al2O3 inclusions and some Al2O3–TixOy and Al2O3(–TixOy)–titanium nitride (TiN) composite inclusions in the nickel-based alloy with initial deoxidants of 0.3–0.7% Al and 0.7–1.2% Ti (wt%). The total Ti content in the inclusions increased slightly with an increasing amount of added Ti. Thermodynamic calculations were conducted to understand the formation mechanism of the inclusions and control the inclusions. The predominance diagram obtained for the Al–Ti–O inclusions in the nickel-based superalloy system was based on Factsage and classical thermodynamic calculations. The inclusions obtained from experiments were not completely consistent with the Factsage calculated results; however, they agreed well with the classical thermodynamic calculation. The calculated results indicated that the thermodynamic stable inclusion was Al2O3 in liquid K4169 alloy at 1823 K. The oxygen content should be controlled to< 10 ppm to avoid the formation of Al2O3 inclusions in the superalloy. Thermodynamic results indicated the initial precipitation of TiN in the FCC phase of the superalloy from 963 to 1403 K. The precipitation temperature of TiN decreased with the decrease of N content and increase in Ti content in the superalloy. The precipitation of TiN was not only dependent on the contents of Ti and N in a sample but also on the cooling conditions.