TEM characterisation of Widmanstätten microstructures in TiAl-based alloys

Widmanstätten lamellar microstructures in Ti-47A1-2Nb-1W-1Mn-0.2Si and Ti-46A1-8Nb (at.%) have been examined using transmission electron microscopy. Widmanstätten lamellar variants, with angles of about 30, 64 and about 90° with respect to the matrix lamellar colonies were observed with the majority of them making an angle of about 64°. The intersection of interfacial planes of the 64°-related Widmanstätten lamellar variants and the matrix's are in the 〈112¯〉γ directions which were 〈11¯00〉 in the parent α-phase at high temperature. These observations have been considered in terms of the angles expected between Widmanstätten variants from mechanisms put forward in earlier work, one involving nucleation of alpha lamellae in pre-existing gamma lamellae and the other involving twinning. The observations support the α twinning mechanism. The specific twinning mode was identified as the {1¯1¯22} twinning in the α-phase.