The development of cavitation in superplastic aluminum composites reinforced with Si3N4

Two metal matrix composites, an Al-Cu-Mg (2124) alloy and an Al-Mg-Si (6061) alloy reinforced with Si3N4 particulates, exhibit superplastic elongations at a true stress of 8 MPa at high strain rates at 783 K and 833 K respectively. Internal cavities are formed in these composites at the interfaces between the particulates and the matrix even at strains of only 0.2. However, the rate of cavity growth after nucleation is very low because diffusion growth is of negligible importance at these high strain rates. Most of the cavities have diameters of less than 2 μm in the early stages of deformation. Elongated larger cavities are observed at relatively high strains as a result of cavity growth and interlinkage. The nucleation rate of cavities is low because stress concentrations are relieved by the presence of a liquid phase.