Strain rate sensitivity and deformation kinetics of ECAPed aluminium over a wide range of strain rates

The strain rate sensitivity and apparent activation volume of aluminium processed by equal channel angular pressing for 1-8 passes are investigated at strain rates from 10^-3 to 10³/s. Constant strain rate tests performed in compression under both quasi-static and dynamic loading conditions show a continuously increasing strain rate sensitivity for the material subjected up to 8 passes of pressing. However, for the specimens subjected to the same number of ECAP passes, the dynamic strain rate sensitivity factor exhibits a 3- to 6-fold increase compared to the quasi-static value. This result suggests that the rate-controlling mechanism at high strain rates is different from that at low strain rates. Although the apparent activation volume under both quasi-static and dynamic loading conditions decreases with increasing pass number, the quasi-static activation volume for the Al ECAPed up to 8 passes is greater than 100b³ (where b is the Burgers vector of aluminium) while the dynamic value is ~30b³. Our experimental findings and analysis suggest that the deformation kinetics are controlled by the dislocation activations. The dominant rate-controlling mechanisms are suggested to be the forest dislocation cutting mechanism at quasi-static strain while viscous drag also plays an important role in the case of dynamic deformation.