EXPERIMENTAL INVESTIGATION OF CAVITATION FRACTURE AT VERY HIGH-STRAIN RATES IN SUPERPLASTIC ALUMINUM-ALLOY MATRIX COMPOSITES

The level and rate of cavitation with superplastic strain were investigated for a powder metallurgically processed 20 vol.% Si3N4p-6061 composite (where the subscript p denotes particulate) and a mechanically alloyed 15 vol.% SiCp-IN9021 composite, in which large superplastic elongations of more than 500% were found at very high strain rates of 2 s(-1) and 5 s(-1) respectively. Cavities initiated at the poles of particulate reinforcements and parallel to the applied stress direction within initial small strains, and their subsequent growth and coalescence invariably leads to premature failure. Also, cavity growth seems to be plasticity controlled. It is concluded that cavitation in superplastically deformed Si3N4p-6061 and SiCp-IN9021 composites is very low, owing to the accommodation process to relieve the stress constraints from inter facial sliding by the presence of the liquid phase at interfaces at optimum superplastic temperatures.