Large strain behaviour of a superplastic copper alloy—II. Cavitation and fracture

A quantitative study of cavitation damage and fracture of a superplastic copper alloy, Coronze 638, has been made. Cavities are found to nucleate at large particles present in the form of stringers. The size and shape of cavities, as well as the level of damage up to fracture are essentially independent of strain rate over regions I and II of the σ−ϵ curve, as are the true strain to fracture and the development of t instabilities. As the strain rate increases into region III, the level of damage to failure decreases, while the true failure strain increases and necks become sharper. Extensive cavity coalescence is observed up to strains of about 1.5, producing a number of large (> 100 μm) cavities which exhibit a high stability, and little tendency to coalescence. This allows the sample to sustain a very high level of cavitation without failure. The mechanism of cavity growth for small isolated cavities (< 10 μm) is thought to be diffusive growth constrained by matrix creep at low strain rates, with a transition to plasticity controlled growth at large strain rates. For larger cavities growth appears to be entirely creep controlled. Final fracture occurs by the material exhaustion in the ligaments between voids once the reduction in the cross section exceeds about 30%. No large instability either in flow or damage seems to be involved in this process.