The relationship between intergranular cavitation and superplastic flow in an industrial copper base alloy

Intergranular cavitation has been observed during the superplastic deformation of a fine grain sized (1 m) Cu-2.8% Al-1.8% Si-0.4% Co alloy when tested at temperatures 500° C. High voltage electron microscopy revealed that the cavities could be nucleated at twin boundary/grain boundary intersections. The maximum elongation occurs at a higher temperature than that of the maximum strain-rate sensitivity and this is explained in terms of grain-boundary migration, at the higher temperature, which restricts the cavitation process. This explanation was put forward on the basis of texture analysis which was used to study the deformation characteristics at the temperatures of maximum elongation and strain-rate sensitivity. The final fracture mode is shown to change with test temperature: (i) at 400° C no cavitation occurs and fracture is by ductile rupture, (ii) at 500 to 550° C cavitation occurs and fracture is by the interlinkage of voids by an intergranular void sheet (IVS) mechanism and (iii) at 800° C grain growth occurs and fracture occurs by the propagation and interlinkage of grain-boundary cracks along the grain boundaries.