Large strain behaviour of a superplastic copper alloy—I. Deformation

The effect of strain hardening (which results from strain induced grain growth) on the flow behaviour of a superplastic copper alloy, Coronze 638, has been analyzed. To do this, we measure the strain induced grain growth as a function of strain and strain rate. Then, by using a constitutive equation relating grain size and flow stress, the effective hardening rate can be calculated. The work shows that grain growth at low strain rates causes sufficient hardening to compensate for the low strain rate sensitivity, thus preventing the development of sharp necks. The possible effect of cavitation on the hardening rate is also evaluated. A re-evaluation of the literature confirms our conclusion that a sharp neck develops at low strain rates (region I of the σ−ϵ curve) only for those materials which exhibit ver little strain-enhanced grain growth.