OVERVIEW NO-119 - SUPERPLASTICITY IN POWDER-METALLURGY ALUMINUM-ALLOYS AND COMPOSITES

Superplasticity in powder metallurgy aluminum alloys and composites ha s been reviewed through a detailed analysis. The stress-strain curves can be put into four categories: a classical well-behaved type, contin uous strain hardening type, continuous strain softening type and a com plex type. The origin of these different types of stress-strain curves is discussed. The microstructural features of the processed material and the role of strain have been reviewed. The role of increasing miso rientation of low angle boundaries to high angle boundaries by lattice dislocation absorption is examined. Threshold stresses have been dete rmined and analyzed. The parametric dependencies for superplastic flow in modified conventional aluminum alloys, mechanically alloyed alloys and aluminum alloy matrix composites is determined to elucidate the s uperplastic mechanism at high strain rates. The role of incipient melt ing has been analyzed. A stress exponent of 2, an activation energy eq ual to that for grain boundary diffusion and a grain size dependence o f 2 generally describes superplastic flow in modified conventional alu minum alloys and mechanically alloyed alloys. The present results agre e well with the predictions of grain boundary sliding models. This sug gests that the mechanism of high strain rate superplasticity in the ab ove-mentioned alloys is similar to conventional superplasticity. The s hift of optimum superplastic strain rates to higher values is a conseq uence of microstructural refinement. The parametric dependencies for s uperplasticity in aluminum alloy matrix composites, however, is differ ent. A true activation energy of 313 kJ mol(-1) best describes the com posites having SiC reinforcements. The role of shape of the reinforcem ent (particle or whisker) and processing history is addressed. The ana lysis suggests that the mechanism for superplasticity in composites is interface diffusion controlled grain boundary sliding.