No abstract

The solidification pathways, subsequent solid-state transformations, and the liquidus surface in the Nb-Ti-Al system have been examined as part of a larger investigation of phase equilibria in Nb-TiAl intermetallic alloys. Fifteen alloys ranging in composition from 15 to 40 at. pct Al, with Nb to Ti ratios of 4:1, 2:1, 1.5:1, 1:1, and 1:1.5, were prepared by arc melting and the as-cast microstructures were characterized by optical microscopy (OM), microhardness, X-ray diffraction (XRD), differential thermal analysis (DTA), backscattered electron imaging (BSEI), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). The results indicate that the range of primary ␤ solidification is much wider than that indicated in previously reported liquidus surfaces, both experimental and calculated. Differential thermal analysis has identified the existence of a ␤ to ϩ ␥ transformation in three alloys where it was previously thought not to exist; confirmation was provided by hightemperature vacuum heat treatments in the single-phase ␤ region followed by rapid quenching. The location of the boundary between the ␤, , and ␦ primary solidification fields has been redefined. A massive ␤ → ␦ transformation, which was observed in the cast microstructure of a Nb-25Ti-25Al alloy, was repeatable through cooling following homogenization. A ␤ → ␦ ϩ eutectoid-like transformation in the 25 at. pct Al alloys, was detected by DTA and evaluated through microstructural analysis of heat-treated samples. Trends in the ␤ phase with variations in composition were established for both lattice parameters and microhardness. As a result of this wider extent of the primary ␤ solidification field, a greater possibility exists for microstructural control through thermal processing for alloys consisting of either ϩ ␥, ␤ ϩ , or ␤ ϩ ␦ phases.

No abstract

Many 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.