Critical reinforcement size for cavity nucleation during superplastic deformation of metal matrix composites

A high strain rate superplastic composite Al - Cu - Mg/Si3N4p has been investigated to estimate the critical particle size for cavity nucleation. Specimens superplastically deformed to a strain of 0.2 at 773 K contained no liquid phase whereas those deformed at 783 K contained liquid phase. The critical particle size, i,e. the smallest size to nucleate a cavity, obtained experimentally was 0.5 mum dia. for deformation at 773 K and 0.4 mum for deformation at 783 K. The critical particle size for deformation at 773 K can be estimated accurately using Stowell's equation for critical strain rate, However, for deformation at 783 K when liquid phase was present, the predicted critical strain rate was much lower than the experimental strain rate for critical particle size for cavity nucleation, It is suggested that the actual critical strain rate was increased by an increase in diffusivity at the interfaces and grain boundaries due to the presence of liquid phase. MST/4777.