Directional solidification of TiAl-Re-Si alloys with aligned γ/α2 lamellar microstructures

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 gamma/alpha(2) lamellar microstructure. Rhenium additions to TiAl were found to enlarge the primary beta composition range. Rhenium also tends to segregate to the beta-dendrite cores leading to the formation of the B2 phase. After the L + beta-->alpha 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 degrees C and tensile failure usually occurred by the coalescence of microcracks that nucleated from large silicide particles. Finally, it was found that the gamma/alpha(2) lamellar microstructure of Ti-43.5Al-3Si-0.5Re can successfully be aligned by using an appropriately oriented seed even though beta is the primary solidification phase indicating that the alpha-phase from both the L + beta-->alpha peritectic and the L-->alpha + Ti5Si3 eutectic reactions must be continuous with that of the seed material. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.