Titanium-Aluminide Alloys Between the Compositions Ti 3 Al and TiAl
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TITANIUM-ALUMINIDE ALLOYS BETWEEN THE COMPOSITIONS Ti 3 Al AND TiAl J. C. MISHURDA, J. C. LIN, Y. A. CHANG AND J. H. PEREPEZKO, Department of Materials Science and Engineering, University of Wisconsin, Madison, WI 53706. ABSTRACT Many questions still remain about the Ti-Al phase diagram, particularly for the compositions between the intermetallic compounds Ti 3 Al 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 Ti 3 Al (D0 1 9 ) 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 P and a 2 up to about 48 at% Al in equilibrium with I and L and forms from the liquid by a peritectic reaction #+L-a. 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. INTRODUCTION In the development of effective materials processing strategies in order to optimize the properties and performance of any alloy an accurate and reliable knowledge of the phase equilibria and phase transformation sequences is essential. For the Ti-Al system there are several alloy phases Ti 3 Al (D0 1 9 ), TiAl (L1 0 ), and TiAI 3 (D0 2 2 )) that have attracted considerable interest for high temperature structural applications due to their high specific strength and stability at elevated temperature. Much of the current work has focused on alloy compositions between Ti 3 Al and TiAl with selected minor alloying additions. A recent critical evaluation and thermodynamic modeling of the phase equilibria in Ti-Al alloys resulted in a proposed phase diagram which is shown in Figure 1 [1]. For the region between Ti 3 AI (a 2 ) and TiAl (-y) a single peritectic reaction: P-Ti+L*7 is indicated during solidification. However, over the past two years several reports have appeared which demonstrate that an hcp a phase forms from the liquid during both powder processing and conventional ingot solidification [2,3,4]. Similarly, other recent studies in the solid state have Mat. Res. Soc. Symp. Proc. Vol. 133.
1989 Materials Research So
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