Activation of slip in lamellae of α2-Ti3Al in TiAl alloys

The activation of slip at room temperature in lamellae of alpha(2)-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 gamma-TiAl, with the alpha(2) 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[<11(2)over bar 0>] 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 alpha(2)-Ti3Al may occur either by a slip transmission process or by stress-induced activation of sources in the interfaces between the two phases (alpha(2)-Ti3Al and gamma-TiAl). The resultant defects are superdislocations with Burgers vectors b = 1/3(<11(2)over bar 0>) gliding on prismatic {<1(1)over bar 00>} and/or pyramidal {<2(2)over bar 01>} planes. In contrast with the observations made on polycrystalline material, in samples of polysynthetically twinned crystals deformed in compression parallel to [111](gamma)//[0001](alpha 2) 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 gamma-TiAl. The results are discussed with reference to the influence of this shear transfer across alpha(2)-lamellae on tensile ductility in alloys of gamma-TiAl.