Activation of slip in lamellae of alpha2-Ti3Al in TiAl alloys

Abstract

The activation of slip at room temperature in lamellae of α2-Ti3Al in a Ti- 48 at.% Al alloy has been studied using transmission electron microscopy. In common with other studies, it appears that slip activity occurs mainly at, or near to, the intersection of certain cross-lamellar twins, formed by deformation in γ-TiAl, with the α2 lamellae. In polycrystalline samples, heat treated to a duplex microstructure with a significant lamellar component, only dislocations with Burgers vectors given by b = 1/3<1120> have been activated, and essentially no dislocations with b = 1/3<1126>, that is c -component dislocations, have been observed after deformation in tension at room temperature. It has been found that, for these samples slip activity in α2-Ti3 Al may occur either by a slip transmission process or by stress-induced activation of sources in the interfaces between the two phases (α2-Ti3Al and γ-TiAl). The resultant defects are superdislocations with Burgers vectors b = 1/3<1120> gliding on prismatic {1100} and/or pyramidal {2201} planes. In contrast with the observations made on polycrystalline material, in samples of polysynthetically twinned crystals deformed in compression parallel to [111]//[0001α2, activity of c-component dislocations has been observed. It is shown that, in all cases studied, the experimental results are consistent with the stress-induced activation of sources in the interfaces, rather than transmission of slip from γ-TiAl. The results are discussed with reference to the influence of this shear transfer across α2-lamellae on tensile ductility in alloys of γ-TiAl.