Cavity growth during superplastic flow in an Al-Mg alloy: I. Experimental study

Cavitation caused by superplastic straining of a fine-grained Al-Mg-Mn-Cu alloy under uniaxial tension has been quantitatively evaluated. Tensile tests were conducted at constant true strain rate in a range of 10^-4s^-1 to 10^-2s^-1 at constant temperatures between 450°C and 550°C. Care was exercised to achieve precision in strain-rate control in these tests since strain-rate and temperature history do affect the extent of cavitation. Measurements of the number and size of cavities were made by using image analysis on tested specimens viewed by optical microscopy and further supported by SEM. With increasing strain, the cavity population density increases, a result seen previously but not studied in detail. Cavity growth was also monitored carefully and found to be due to the plastic deformation of the matrix surrounding the cavity. The total volume fraction of cavities which is the product of the above two components was found to increase exponentially with strain. The dependencies of cavity volume fraction on strain-rate and temperature are not straightforward, however. Based on experimental observations of decohesion between matrix and non-deformable particles, continuous nucleation of new cavities, and data related to plasticity-based growth of cavities, attempts are made to explain these complex effects.