MICROSTRUCTURES OF RAPIDLY-SOLIDIFIED BINARY TIAL ALLOYS

The microstructures of rapidly-solidified binary TiAl alloys containing 46-70 at.% Al have been studied using optical and analytical transmission electron microscopy (AEM). The phases present in the alloys and their distribution were found to be a sensitive function of composition. Essentially single-phase microstructures were seen for alloys with 46 at.% Al, 50-52 at.% Al and 60-65 at.% A. The primary solidification phases present in these alloys were α-Ti, ordered γ-TiAl and disordered cubic TiAl, respectively. The 60-65 at.% Al alloys showed indications of the solid-state formation of long-period superlattice structures based upon γ-TiAl, due to the excess Al. In other composition ranges, two-phase microstructures were seen. The 48 at.% Al alloy contained α2-Ti3Al + γ-TiAl, with α2-Ti3Al as the primary solidification phase. Alloys from 53 to 55 at.% Al were also α2-Ti3Al + γ-TiAl, but with γ-TiAl as the primary solidification phase. The 70 at.% Al alloy was two phase TiAl2 + TiAl3. A strong effect of interstitial oxygen content on the α2-Ti3Al + γ-TiAl phase relations was also seen. Comparison of these results with the equilibrium phase diagram and with ingot studies of the same alloys showed that most of the microstructures produced by rapid solidification were metastable. A possible metastable phase diagram for TiAl which is consistent with the results is proposed. © 1990.