Titanium-Aluminide Alloys Between the Compositions Ti3 Al and TiAl

ABSTRACTMany questions still remain about the Ti-Al phase diagram, particularly for the compositions between the intermetallic compounds Ti3Al and TiAl. In an experimental study of the phase equilibria, titanium-aluminum alloys with 44, 46, 48, 50 at.% aluminum were produced by drop casting, HIP, and a double forging process method. Differential thermal analysis (DTA), optical metallography, and residual oxygen analysis were performed in order to characterize the low and high temperature phase equilibria of the alloys. The experimental results are compared with the calculated Ti-Al phase diagram which is being modeled concurrently. For the bcc, hcp and liquid phases, the Margules type of equations are used to represent the excess Gibbs energies. A maximum of six parameters are used for each of the phases. For the TiAl (Llo) and Ti3Al (D019 ) phases, the Wagner-Schottky type of equations are used to represent the Gibbs energies. All of the other phases are treated as line compounds. Values of the solution parameters were obtained by optimization using existing thermochemical and phase boundary data reported in the literature. The calculated results show that the high temperature hcp phase field is stable between approximately 34 at% Al, in equilibrium with β and α2 up to about 48 at% Al in equilibrium with γ and L and forms from the liquid by a peritectic reaction β+L-α. The experimental results obtained to date for the four alloys are consistent with the calculated equilibria which is being refined and also allows for an estimate of the metastable equilibria.