Directional solidification of TiAl–Re–Si alloys with aligned γ/α 2 lamellar microstructures

Abstract

The effect of Re additions on the solidification behavior of Ti–Al–Re and Ti–Al–Re–Si alloys was investigated to determine if such additions would be compatible with a seeding technique developed previously to align the γ/ α 2 lamellar microstructure. Rhenium additions to TiAl were found to enlarge the primary β composition range. Rhenium also tends to segregate to the β-dendrite cores leading to the formation of the B2 phase. After the L+ β→ α peritectic reaction, Re tends to segregate to the interdendritic liquid. For the Ti–Al–Re alloys, the maximum Re content found in the lamellar microstructure was near 0.5 at% and the lamellar orientation in master alloys containing this amount of Re could successfully be grown from the seed material. However, such levels of Re had little effect on the measured yield stress either at room temperature or at 1000°C and tensile failure usually occurred by the coalescence of microcracks that nucleated from large silicide particles. Finally, it was found that the γ/ α 2 lamellar microstructure of Ti–43.5Al–3Si–0.5Re can successfully be aligned by using an appropriately oriented seed even though β is the primary solidification phase indicating that the α-phase from both the L+ β→ α peritectic and the L→ α+Ti 5Si 3 eutectic reactions must be continuous with that of the seed material.