The nucleation and growth of cavities in a superplastic quasi-single phase copper alloy

Experiments were conducted on a superplastic quasi-single phase copper alloy, Coronze CDA 638, to provide information on the nucleation and growth of internal cavities during deformation. It is shown that, at a temperature of 823 K, the cavities are generally associated with the presence of large Co-rich particles on the grain boundaries, with nucleation occurring at the particle/grain boundary interfaces. The cavities tend to form in stringers and these stringers are always oriented along the rolling direction regardless of the direction of the tensile axis. Thus, the cavity stringers are clearly associated with the Co-rich particles which also lie in stringers parallel to the rolling direction. A detailed series of tests at an initial strain rate of 1.3 × 10⁻⁵s⁻¹ shows that the cavities grow by a diffusion-controlled mechanism for cavity radii <15 μm and by the power-law mechanism for radii >20μm. It is demonstrated that this conclusion is consistent both with direct measurements of the cavity growth rates and with estimates of the increases in cavity size due to superplastic diffusion growth. The results therefore emphasize the importance of diffusion-controlled cavity growth at low strain rates.