Microstructure evolution in tial alloys with b additions: Conventional solidification

Solidification microstructures of arc-melted, near-equiatomic TiAl alloys containing boron additions are analyzed and compared with those of binary Ti-Al and Ti-B alloys processed in a similar fashion. With the exception of the boride phase, the matrix of the ternary alloy consists of the same α₂ (DO₁₉) and γ (Ll₀) intermetallic phases found in the binary Ti-50 at. pct Al alloy. On the other hand, the boride phase, which is TiB (B27) in the binary Ti-B alloys, changes to TiB₂ (C32) with the addition of Al. The solidification path of the ternary alloys starts with the formation of primary α (A3) for an alloy lean in boron (∼1 at. pct) and with primary TiB₂ for a higher boron concentration (∼5 at. pct). In both cases, the system follows the liquidus surface down to a monovariant line, where both α and TiB₂ are solidified concurrently. In the final stage, the α phase gives way to γ, presumably by a peritectic-type reaction similar to the one in the binary Ti-Al system. Upon cooling, the α dendrites order to α₂ and later decompose to a lath structure consisting of alternating layers of γ and α₂.