Intermediate phases and phase relations in the composition range Al3(Ti, V) to TiAl in the Al–Ti–V system

Ternary Al75−xTi25Vx (x=4, 9) alloys, with compositions equivalent to binary alloys in the range Al3Ti–TiAl, have been examined using analytical electron microscopy with a view to establishing the intermediate phases possible and evaluating the potential for the development of useful multiphase intermetallic microstructures. Based on the observed microstructures, and previous observations in binary Ti–Al alloys, the evolution of microstructure during solidification and subsequent cooling of the chill-cast alloy Al71Ti25V4 is suggested to involve dendritic solidification of ξ-Ti5Al11 as a primary phase, followed by formation of δ-Al3(Ti, V) as the product of a peritectic reaction: L+ξ-Ti5Al→δ-Al3(Ti, V). There is evidence that the ξ phase subsequently decomposes in the solid state to form ξ′-Ti2Al5, perhaps as the result of a peritectoid reaction: ξ+δ→ξ′-Ti2Al5. In contrast to the irregular and complex dendritic microstructures in the chill-cast alloys, the Al71Ti25V4 alloy homogenized for 100 h at 1523 K possessed a simple two phase microstructure, identified to comprise δ-Al3(Ti, V) and ξ′-Ti2Al5 (M=1.75). The Al66Ti25V9 alloy homogenized for 100 h at 1523 K was, in contrast, essentially single phase ξ-Ti5Al11 (M=2) with lattice parameters a=∼0.389 nm and c=∼1.620 nm.