Evolution of boride morphologies in TiAl-B alloys

The solidification of γ-TiAl alloys with relatively low (<2 at. pct) additions of boron is discussed. Binary Ti-Al alloys containing 49 to 52 at. pct Al form primary α-(Ti) dendrites from the melt, which are subsequently surrounded by γ segregate as the system goes through the peritectic reactionL + α →γ. Alloys between 45 and 49 at. pct Al go through a double peritectic cascade, forming primary β-(Ti) surrounded by α-(Ti) and eventually by γ in the interdendritic spaces. Boron additions to these binary alloys do not change the basic solidifi-cation sequence of the matrix but introduce the refractory compound TiB₂ in a variety of mor-phologies. The boride develops as highly convoluted flakes in the leaner alloys, but needles, plates, and equiaxed particles gradually appear as the B content increases above ∼1 at. pct. Increasing the solidification rate initially promotes the formation of flakes over plates/needles and ultimately gives way to very fine equiaxed TiB₂ particles in the interdendritic spaces of the metallic matrix. Furthermore, the primary phase selection in the 49 to 52 at. pct Al range changes from α-(Ti) to β-(Ti) at supercoolings of the order of 200 K. The different boride morphologies are fully characterized, and their evolution is rationalized in terms of differences in their nucleation and growth behavior and their relationship to the solidification of the inter-metallic matrix. Formerly Research Assistant, University of California-Santa Barbara (UCSB) Formerly Professor of Materials and Dean of the College of Engineering at UCSB