In situ straining experiments on α-Al₂O₃/Nb/α-Al₂O₃ single crystal sandwiches have been performed in a high-voltage electron microscope in order to observe the dislocation motion connected with the fracture of these samples. The tricrystals had two different orientation relationships at the interfaces, both possessing high fracture energies. The in situ studies showed that the fracture occurs along the interfaces but more frequently inside the Nb sheet. The fracture is connected with strong plastic deformation. The crack opening and the dislocation processes are analysed.