HIGH-STRAIN-RATE SUPERPLASTICITY IN ALUMINUM MATRIX COMPOSITES

Superplasticity has been observed in several whisker-reinforced aluminum composites at extremely high strain rates of approximately 0.1-1 s-1. These composites include beta-SiC-2124Al, beta-Si3N4-2124Al, alpha-Si3N4-7064Al and beta-Si3N4-6061Al. The materials generally exhibited a strain rate sensitivity value of about 0.3 and a maximum elongation of about 300%. The exact deformation mechanism leading to superplasticity is apparently a unique one. Experimental results indicated that the ability of a whisker-reinforced composite to undergo high-strain-rate superplasticity depends upon the morphology, crystal structure and chemistry of the whiskers, as well as on the chemical composition at the whisker-matrix interfaces. It is proposed that the presence of a low melting point region, or in some cases a liquid phase, at the whisker-matrix interfaces is responsible for the observed phenomenon of superplasticity at very high rates. These regions are present at the deformation temperature as a result of solute segregation. It is noted that the phenomenon is not observed in all whisker-reinforced composites despite the fact that they contain fine grain sizes. Thus a fine matrix grain size is a necessary but insufficient condition for the observation of high-strain-rate superplasticity in whisker-reinforced composites.