Structure and deformation behaviour of Ti-Al alloys containing nearly 60at.% Al

Abstract

An alloy of composition 38 at.% Ti, 57 at.% Al and 5 at.% Cr has been prepared both by casting and by spray forming, and its structure and deformation mechanisms examined. This alloy is essentially a high-Al-content γ-TiAl inter-metallic susceptible to decomposition by forming Al₂Ti phase. Microstructures are very different for cast and sprayed material because of casting segregation leading in one case to a homogeneous but unstable matrix with a Cr-rich interdendritic phase and in the other case to an intimately mixed two-phase γ-TiAl+ Al₂Ti structure. The two-phase material is very strong but both cast and sprayed states are brittle. Microstructural instabilities during high-temperature testing lead to deformation instabilities seen as serrations on the stress-strain curves. Dislocation structures at room and elevated temperatures are the same as those seen in lower-Al-content γ-TiAl alloys but show significant variations in character and extent of dissociation with test temperatures. Variations in fault energies are examined over a wider range of Al content (48–75 at.%) and as a function of temperature and are discussed in terms of atomic bonding and local chemical segregation.