Superplasticity in nanocrystalline materials

High strain rate and/or low temperature superplasticity was observed in 1420 aluminum alloy and in Ni₃Al. The enhanced plasticity demonstrated in these materials is attributed to a refinement in grain size to the nano or near-nano range through the use of High Pressure Torsion (HPT) prior to elevated temperature tensile testing. Grain sizes after HPT were approximately 100 nm for the 1420 aluminum alloy, and 50 nm for Ni ₃Al. Tensile testing of the alloys was carried out at strain rates up to 10 and temperatures up to 725°C. Tensile curves typically exhibit high flow stresses and a degree of strain hardening that cannot be adequately described by conventional recovery mechanisms such as grain growth. Room temperature TEM in-situ straining experiments reveal some dislocation activity, and no evidence of dislocation pile-ups. It is suggested that the dominant deformation mechanism for elevated temperature plasticity in this study is grain boundary sliding and rotation.