Achieving high strain rate superplasticity in an At-Li-Mg alloy through equal channel angular extrusion

An Al-Li-Mg-Sc-Zr alloy was fabricated by an ingot metallurgy technique and subjected to intense plastic straining through equal channel angular extrusion at three different temperatures, 240, 325 and 400 degrees C. The superplastic properties and microstructure evolution of the alloy were examined in tension in the temperature interval 250-500 degrees C at strain rates ranging from 1.4 x 10(-5) to 1.4 s(-1). Superior superplastic properties (elongation to failure of 3000% with a corresponding strain rate sensitivity coefficient m of similar to 0.6) were attained at 450 degrees C and (epsilon) over dot =1.4 x 10(-2) s(-1) in samples processed at 400 degrees C to a total strain of similar to 16. The alloy in this state had an average grain size of similar to 2.6 mu m and the recrystallisation fraction was about 90%. It was shown that the highest superplastic ductility appears in samples with more uniform microstructure containing the highest portion of high angle boundaries. It was established that the uniformity of structure and its stability under superplastic deformation is more important for achieving superior elongation to failure than the grain size if the latter varies from 0.5 to 2.6 mu m.