Superplastic behavior and microstructure evolution in a commercial Al-Mg-Sc alloy subjected to intense plastic straining

A commercial Al-6 pct Mg-0.3 pct Sc-0.3 pct Mn alloy subjected to equal-channel angular extrusion (ECAE) at 325 degreesC to a total strain of about 16 resulted in an average grain size of about 1 mum. Superplastic properties and microstructural evolution of the alloy were studied in tension at strain rates ranging from 1.4 x 10(-5) to 1.4 s(-1) in the temperature interval 250 degreesC to 500 degreesC. It was shown that this alloy exhibited superior superplastic properties in the wide temperature range 250 degreesC to 500 degreesC at strain rates higher than 10(-2) s(-1). The highest elongation to failure of 2000 pct was attained at a temperature of 450 degreesC and an initial strain rate of 5.6 x 10(-2) s(-1) with the, corresponding strain rate sensitivity coefficient of 0.46. An increase in temperature from 250 degreesC to 500 degreesC resulted in a shift of the optimal strain rate for superplasticity, at which highest ductility appeared, to higher strain rates. Superior superplastic properties of the commercial Al-Mg-Sc alloy are attributed to high stability of ultrafine grain structure under static annealing and superplastic deformation at T less than or equal to 450 degreesC. Two different fracture mechanisms were revealed. At temperatures higher than 300 degreesC or strain rates less than 10(-1) s(-1), failure took place in a brittle manner almost without necking, and cavitation played a major role in the failure. In contrast, at low temperatures or high strain rates, fracture occurred in a ductile manner by localized necking. ne results suggest that the development of ultrafine-grained structure in the commercial Al-Mg-Sc alloy enables superplastic deformation at high strain rates and low temperatures, making the process of superplastic forming commercially attractive for the fabrication of high-volume components.