The structure and configuration of {111} twins, in polycrystalline films which had been prepared by laser ablation deposition, were investigated by means of high-resolution transmission electron microscopy. The number density of twins in the films was high, and the thickness of the twin lamellae was of the order of nm. In some grains, ordering of the nano-twins gave rise to a local structure which resembled that of the hexagonal phase. These twin lamellae usually extended so as to penetrate the whole grain. Twin lamellae which terminated within the grain were also found. Some of them were very small (less than 1nm in thickness) and were accompanied by partial dislocations with a Burgers vector of (a/3)<111>. These small twins were considered to represent an early stage of twin formation. It was deduced that the nucleation of a {111} twin via the dissociation of an a<111> edge dislocation was one of the mechanisms of twin formation in these polycrystalline films.

C.L.Jia, K.Urban, M.Mertin, S.Hoffmann, R.Waser: Philosophical Magazine A, 1998, 77[4], 923-39