Influence of grain size on the flow properties of an Al-Mg-Sc alloy over seven orders of magnitude of strain rate

Experiments were conducted to evaluate the flow properties of an Al-3Mg-0.2Sc alloy both without and with processing using equal-channel angular pressing (ECAP). The initial grain size was ~300 µm and this was reduced to ~250 nm by ECAP and then increased to ~600 nm by annealing at 673 K for 10 min. Tests were conducted to determine the mechanical properties over seven orders of magnitude of strain rate from ~10⁻⁴ to ~10³ s⁻¹. The results confirm the validity of the Hall-Petch relationship in the material processed by ECAP. Shear banding occurred in the coarse-grained material during dynamic testing but in the ECAP-processed alloy there was only minor grain coarsening. There was evidence for dynamic strain ageing in both the coarse and the ultrafine-grained (UFG) alloy with a transition in flow mechanism at high temperatures from dislocation climb in the coarse-grained material to superplasticity in the UFG alloy.