The effect of the presence of liquid phase on cavitation and fracture during superplastic flow for high strain-rate superplastic Si3N4P/2124 Al composite has been investigated. The conclusions obtained are as follows:
(1) At 723K, which is 60K lower than the partial melting point measured by DSC, a small elongation to fracture of 24% was obtained. This result is due to the development of microcracks at the interface between matrix and reinforcement which would be generated by high stress concentration at the interface. (2) At 783K, which is very close to the partial melting point, a large elongation to fracture of 550% was obtained. The large elongation resulted from the disappearance of microcracks. It is supposed that the nucleation of cracks is suppressed by low stress concentration at the interface due to the presence of liquid phase. (3) At 843K, which is 60K higher than the partial melting point, a small elongation to fracture of 18% was obtained. The small elongation could be explained by the excessive development of cavities due to too much liquid phase at the interface and grain boundaries. (4) It is suggested that the presence of adequate amount of liquid phase gives rise to the effective accommodation required for grain boundary sliding for the composite.
