High-strain-rate superplasticity from nanocrystalline Al alloy 1420 at low temperatures

Superplasticity was investigated in nanocrystalline Al alloy 1420 to evaluate the scalability of conventional constitutive relationships in the nanocrystalline range. The parametric dependences of superplastic flow were obtained by constant-strain-rate tensile tests in the temperature range 200-300 degrees and a strain rate range of 3 x 10-(4)5 x 10(1) s(1). The nanocrystalline alloy exhibits superplasticity at low temperatures and higher strain rates compared with the microcrystalline alloy. The observation of high-strain-rate superplasticity coincided with the temperature range for microstructural instability. A comparison with the theoretical models for superplasticity and a constitutive relationship for superplasticity in microcrystalline alloys shows a transition to slower deformation kinetics on a normalized basis. The transmission electron microscopy of deformed specimens supports the slip accommodation models for superplasticity. It also shows a change in the intragranular dislocation density and dislocation configuration with grain size.