TEM characterisation of Widmanstatten microstructures in TiAl-based alloys

Widmanstatten lamellar microstructures in Ti-47Al-2Nb-1W-1Mn-0.2Si and Ti-46Al-8Nb (at.%) have been examined using transmission electron microscopy. Widmanstatten lamellar variants, with angles of about 30, 64 and about 90degrees with respect to the matrix lamellar colonies were observed with the majority of them making an angle of about 64degrees. The intersection of interfacial planes of the 64degrees-related Widmanstatten lamellar variants and the matrix's are in the <11 (2) over bar >gamma directions which were <1 (1) over bar 00> in the parent alpha-phase at high temperature. These observations have been considered in terms of the angles expected between Widmanstatten 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 a twinning mechanism. The specific twinning mode was identified as the {(1) over bar(1) over bar 22} twinning in the alpha-phase. (C) 2004 Elsevier Ltd. All rights reserved.