Twinning nucleation in Cu-8 at.% Al single crystals

Transmission electron microscopy observations of the dislocation substructure in Cu-8 at.% Al single crystals deformed in tension have been carried out to elucidate the origin of the twinning dislocation which produces conjugate twinning in this alloy. It is argued that the 'correct' Heidenreich- Shockley dislocation is obtained in the reaction of the primary dislocation with the faulted (Frank) dipole. These interactions are expected to occur frequently during the deformation. Different configurations of the twinning sources are analysed with respect to the nucleation and the growth of the twin. Geometrical issues related to the pinning of the component dislocations at the nodes formed, the motion of the single twinning dislocation and the non-coherent twinning front and the passing barriers encountered during twin growth are analysed. It is shown that there are no geometrical and energetic obstacles to the development of a macroscopic twin by the pole mechanism from the source proposed here.