Dislocation structures in deformed Cu-Ni-Zn alloy single crystals

Abstract

It is known that Cu-Ni-Zn alloy has an ordered structure Cu2NiZn (Ll2) by annealing between 573 and 623 K. In the present experiments, the effects of annealing on the dislocation structure were studied on Cu-Ni-Zn single crystals with several compositions. Thin foils cut parallel to the {111} planes were observed in a transmission electron microscope. The results obtained are as follows. (i) In Cu-5Ni-5Zn, Cu-10Ni-10Zn and Cu-15Ni-15Zn (at%), the stress-strain behaviour, slip mode and dislocation structure did not change by annealing at 573 K. However, the slip mode became more concentrated and localized, and dislocations emitted from a source tended to stay on the same slip plane, as the nickel and zinc concentrations increased. (ii) However, those properties in Cu-20Ni-20Zn and Cu-25Ni-25Zn changed drastically by annealing. As the ordering proceeded, uniform distributions of superlattice dislocations were observed. A typical dislocation configuration, with long screw and wavy-edged superlattice dislocations, took the place of piled-up unit dislocations. (iii) The facts that edge-type superlattice dislocations formed dipoles and their clusters, and that the secondary dislocation density was much lower than the primary one, implied that the elastic interaction of the primary edge-type superlattice dislocations on the nearby parallel slip planes would control the work-hardening of ordered Cu2NiZn alloy single crystals.