Creep behaviour of near γ-TiAl base alloys: Effects of microstructure and alloy composition

The strength and creep properties of fully transformed lamellar, duplex and globular microstructures of two γ-TiAl base alloys (Ti-47Al-1Cr-1Mn-1.5Nb-0.2Si (alloy 1) and Ti-46Al-1Mn-2Nb-0.5Si (alloy 2)) are investigated at 800 °C. It is shown that these properties are essentially controlled by the number of lamellar colonies, with the fully transformed lamellar structure having highest strength and creep resistance properties. Decreasing the Al content and increasing the Si content result in finer microstructures, i.e. finer lamellar colonies, interlamellar spacing and grain size, a higher stability against dynamic recrystallization and improved strength and creep resistance properties. It is also shown that alloy composition as no measurable effect on the extent of primary creep, which appears to be primarily controlled by the number of lamellar colonies. As ensues from microstructural observations, secondary creep is recovery-controlled in both alloys, in all microstructural conditions. However, the stress exponent has been found to be sensitive to the stability of microstructure against dynamic recrystallization.