The cavitation and fracture characteristics of a superplastic Al---Cu---Li---Zr alloy

Abstract

An Al---Cu---Li---Zr alloy processed to recrystallize statically exhibits superplastic characteristics at a testing temperature of 723 K, with an optimum elongation to failure of 560% at a strain rate of 3 × 10^-4s^-1. Under optimum conditions the alloy exhibits diffuse necking, whereas at strain rates higher and lower than the optimum value there is an increasing tendency for sharp necking. It is shown that the optimum superplastic elongation is obtained by a suitable combination of the strain rate sensitivity and the strain-hardening coefficient. The alloy exhibits considerable cavitation under all experimental conditions, with cavitation being restricted to regions near the fracture tip in specimens which failed with a sharp neck, and cavitation being more uniformly distributed in specimens with a diffuse neck. Cavities are nucleated during superplastic deformation at coarse Al---Fe---Cu particles, and cavity growth occurs predominantly by a power-law mechanism. It is shown that surface diffusion at elevated temperatures may lead to the observation of rounded cavities, even when cavity growth has occurred by a power-law growth mechanism.