The role of the α₂ phase in the transmission of slip in lamellar TiAl-based alloys

Abstract

Samples of a range of polycrystalline alloys based on TiAl have been examined using optical microscopy, analytical scanning and transmission electron microscopy in order to understand the role of the α₂ phase in controlling plastic deformation in alloys with a lamellar structure. It has been found that for grains in the hard orientation, that is when the stress axis is either parallel or perpendicular to [0001]_α2, fracture occurs preferentially along α₂—γ interfaces. Transmission electron microscopy has shown that twinning in the γ phase can be transmitted into the α₂ phase by the generation of dislocations with Burgers vector b along 〈1126〉 on {2021} planes. Slip along 〈1120〉 directions has also been observed in α₂ in grains where the stress axis is perpendicular to [0001]_α2. When crystals are stressed parallel with [0001]_α2, the only dislocations observed in α₂ have b along 〈1126〉. These observations are discussed in terms of the deformation of polycrystalline samples and of polysynthetically twinned samples.