In order to investigate the microstructural behaviour of grain boundaries in L12-ordered Cu3Au during plastic deformation, in-situ deformation experiments were performed in a transmission electron microscope. The dislocation structure at a coherent twin boundary (CTB) before and after straining the specimen using a single-tilt straining holder is analysed in a double-tilt holder. During the in-situ deformation specific slip systems were active in both crystals from which dislocation transmission across the CTB was expected. However, the majority of the moving superdislocations are absorbed into the boundary, and change the boundary structure locally from a symmetrical structure into an asymmetrical one and vice versa. The implication is that grain-boundary fracture might occur if larger strains are applied to the specimen for which experimental evidence is given.
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