EVOLUTION OF BORIDE MORPHOLOGIES IN TIAL-B ALLOYS

The solidification of gamma-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 alpha-(Ti) dendrites from the melt, which are subsequently surrounded by gamma-segregate as the system goes through the peritectic reaction L + alpha --> gamma. Alloys between 45 and 49 at. pct Al go through a double peritectic cascade, forming primary beta-(Ti) surrounded by alpha-(Ti) and eventually by gamma in the interdendritic spaces. Boron additions to these binary alloys do not change the basic solidification sequence of the matrix but introduce the refractory compound TiB2 in a variety of morphologies. 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 approximately 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 alpha-(Ti) to beta-(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 intermetallic matrix.