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

Very high strain rate (also known as positive exponent) superplasticity in three mechanically alloyed IN9021, IN9052 and IN905XL aluminum alloys 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 temperature dependence of how stress, elongation and strain rate sensitivity exponent (m value) reveals that optimum superplasticity might occur at temperatures close to or above the melting point of each alloy. The presence of a liquid phase, resulting from the low melting point regions, as a result of solute segregation by mechanical alloying, is responsible for the observed positive exponent superplasticity. It is proposed that superplastic how at high strain rates is controlled by a grain boundary sliding mechanism accommodated with relaxing the stress concentration by isolated liquid phases at grain boundaries. Mechanically alloyed processing is a powerful method to produce the desired microstructures with not only fine grain size but also optimizing segregation in solute along boundaries, required for positive exponent superplasticity in aluminum alloys.