Phase equilibria and microstructural control of gamma TiAl based alloys

Phase equilibria among the β (bcc or B2), (hcp) and γ (L1₀) phases in Ti---Al---M ternary systems at elevated temperatures have been studied, where the M is β stabilizing element for pure titanium. The β(B2) + + γ three-phase coexisting region exists at temperatures above 1473 K, and it moves towards a direction of high aluminum and high M concentrations with increasing temperature. The change in the phase equilibria by the addition M is associated with the ↔ β allotropic transformation temperature of pure titanium and can be thermodynamically interpreted in terms of the lattice stability ratios of M to Ti. The change in the phase equilibria results in new reaction pathways peculiar to the ternary systems, thereby opening more possibility for microstructure control of the gamma based alloys. The vertical section drawn based on these studies demonstrates that the γ phase can be in equilibrium with the β(B2) phase at relatively low temperatures in the ternary systems and create a reaction pathway of → β(B2) + γ. Upon cooling along the same pathway of → γ as in the binary alloy, the addition of third element affects the transformation rate, and the massive γ structure is found to be formed even under slow cooling rate.