Mechanical Properties and Cavitation during Superplastic Deformation of Al–Mg Alloys

Abstract

The effect of temperature and strain rate on cavitation during tensile straining in Al–Mg alloys has been studied using precision density measurements, metallography and semi-empirical model of cavity growth. Cavitation occurs throughout the gauge length of the specimens, with most of the cavities being concentrated in regions near the fracture tip. The extent of cavitation has been found to increase with increasing temperature and decreasing strain rate. In fact, the cross-sectional area at fracture increased with increasing level of cavitation and its coalescence. However, cavitation is not found to be optimum around the superplastic temperature, i.e. 673 K and at an initial strain rate of ε=1.4×10−3 s−1. This is the combination of temperature and strain rate at which the alloys show a maximum elongation to failure and strain rate sensitivity index. Cavity growth at high temperatures may be controlled either by vacancy diffusion or the power law growth process. These two growth mechanisms are examined with reference to Al–Mg alloys. It is found that diffusion growth is favoured at low strains and there is a transition to the power law growth process at a critical cavity radius (rc). The value of (rc) increases with increasing temperature and decreasing strain rate.