A quantitative analysis of cavitation in Al-Cu-Mg metal matrix composites exhibiting high strain rate superplasticity

Specimens of two Al-Cu-Mg (2124) composites, reinforced with 20 vol% of either Si3N4 particulates or Si3N4 whiskers, were tested under experimental conditions close to those for optimum high strain rate superplasticity. Both composites developed extensive internal cavitation during testing, but quantitative measurements show that significant cavity growth occurs throughout the test in the whisker-reinforced composite, but only at strains greater than or equal to 1.0 in the particulate-reinforced composite. This difference in behavior is attributed to differences in the extent of a discontinuous liquid phase at the grain boundaries and at the matrix/reinforcement interfaces. It is concluded that the presence of an extensive liquid phase in the particulate-reinforced composite is beneficial for attaining high ductility because it relieves the stress concentrations from grain boundary sliding and thereby limits the growth of cavities.