Abstract
The behaviour of screw-type superdislocations upon entering a coherent twin boundary in L12-ordered Cu3Au was studied by in situ deformation in a transmission electron microscope. From the observed slip systems during deformation, transmission of the glissile superdislocations across the boundary was expected to occur. However, the majority of the superdislocations are absorbed into the boundary. Each superdislocation changes the boundary structure locally from a symmetrical structure into an asymmetrical structure or vice versa. Different geometrically possible configurations for a dissociated superdislocation in the twin boundary are discussed and compared with experiment. The driving force for absorption is presumably a lower energy of the superdislocations in the boundary compared with superdislocations in the grain. The mechanism can be described as a {111} cross-slip event. When more dislocations arrive at the same position in the boundary, absorption continues and some dislocations are transmitted into the other crystal, leaving behind ribbons of antiphase boundaries (APBs), and also stacking faults are formed. In view of these processes occurring at the boundary it is concluded that a coherent twin boundary in ordered Cu3Au forms an obstacle to dislocation motion, even when the dislocations are glissile in the boundary.
It was found that the movement of superdislocations and of single superpartials during deformation was not impeded by the grown-in APBs. All superdislocations analysed before and after the in situ deformation experiment underwent cross-slip to cube planes. This result is compared with conflicting results in the literature and discussed in terms of the driving force for cross-slip. A superlattice intrinsic stacking fault and a complex stacking fault present in the deformed specimen were found to be separated from an APB by a Shockley partial and a Frank partial respectively.
