Fracture behaviour of Cu-Zn-Co and Cu-Zn-Ni alloys during superplastic deformation

Abstract

Cavitation and fracture behaviour in two commercial α/gb brasses, one modified with 2wt% Co (Cu-Zn-Co) and the other with 2wt% Cr (Cu-Zn-Cr), have been investigated in Region II of superplastic flow. These alloying elements form cobalt-rich (0.3 μm average diameter) and chromium-rich (5 μm average diameter) precipitate particles which are distributed uniformly in the matrix and which play an important role in cavitation and inhibiting grain growth during deformation. Void size distributions, volume fraction of voids and the number of voids per unit area have been measured as a function of strain in Region II and the results show a very marked difference in the degree of cavitation in Cu-Zn-Co and Cu-Zn-Cr alloys. Experiments show that the deformation is quasi-uniform with little or no necking in the specimens of Cu-Zn-Co alloy in Region II, and the final fracture occurred by the growth and interlinkage of internal voids. On the other hand, in the specimens of Cu-Zn-Cr alloy a sharp or localized neck developed early in the deformation in Region II and the specimen pulled down to a fine point leading to failure by necking. The importance of diffusion or slip accommodation of grain boundary sliding in void formation during superplastic flow is discussed and a criterion for failure is suggested.