SUPERPLASTICITY IN VERY FINE-GRAINED AL-BASED ALLOYS PRODUCED BY MECHANICAL ALLOYING

Very high strain rate (also known as positive exponent) superplasticit y in three mechanically alloyed IN9021, IN9052 and IN905XL aluminum al loys has been characterized over a wide range of strain rates between 10(-3) to 300 s(-1) in air at temperatures from 698 to 873 K. The temp erature dependence of how stress, elongation and strain rate sensitivi ty exponent (m value) reveals that optimum superplasticity might occur at temperatures close to or above the melting point of each alloy. Th e presence of a liquid phase, resulting from the low melting point reg ions, as a result of solute segregation by mechanical alloying, is res ponsible for the observed positive exponent superplasticity. It is pro posed that superplastic how at high strain rates is controlled by a gr ain boundary sliding mechanism accommodated with relaxing the stress c oncentration by isolated liquid phases at grain boundaries. Mechanical ly alloyed processing is a powerful method to produce the desired micr ostructures with not only fine grain size but also optimizing segregat ion in solute along boundaries, required for positive exponent superpl asticity in aluminum alloys.