TEM observations of mechanical twins in a hot-deformed Ti-45Al-10Nb alloy

Two types of {111} 〈112̄] twins in a hot-deformed Ti-45Al-10Nb alloy were investigated by high resolution transmission electron microscopy (HREM). Type P ((111) [112̄]) twins can be formed either through dislocation mechanisms or at grain boundary. The former can be further described by two mechanisms. One is the pole mechanism and the other is the dissociation of a perfect dislocation mechanism. For the formation of twins at the grain boundary, twins can be nucleated by bowing out of twinning dislocations at the grain boundary and propagated by the sequential emission of twinning dislocations on every adjacent twinning plane. 1/3[111] Frank partial dislocations can be observed at steps on the incoherent twin boundary of some twins. A mechanism has been proposed to illustrate the propagation of the twins with or without 1/3[111] Frank partials at the steps on the incoherent twin boundary. Type Q ((1̄11)[1̄12̄]) twins usually formed at the lamellar interfaces. The steps on these lamellar interfaces are the common sites for the nucleation of type Q twins. Deformation twinning in lamellar TiAl alloys was suggested as a stress relaxation process for the pile-up of interfacial dislocations. Possible dislocation mechanisms for the generation of twinning dislocations on the lamellar interface are discussed.