Dynamic flow stress of fine grain material processed using equal channel angular pressing

Abstract

The dynamic deformation of a fine grained aluminum alloy generated using equal channel pressing techniques is examined using a split Hopkinson pressure bar at strain rates of 102–104 s−1 and temperature levels 20, 50, 100 and 200°C. A constitutive dynamic flow relationship has been formulated and its parameters are generated as an explicit function of strain, strain rate and temperature as well as the grain size. This relationship is derived by considering the flow stress to be composed of two physically based components; athermal and thermal, each of which is associated with a different set of dislocations barriers. The athermal component of stress is shown to be related to grain size through the typical Hall-Petch relationship while also relating the hardening parameter to the grain size. The thermal component of the flow stress is shown to be increasingly dependent on the grain size due to the transition in deformation mechanisms from coarse-grain regime into the ultra-fine grain. The material studied is shown to exist in the fine grain regime, in which competing deformation mechanisms exists. The constitutive model has been used to obtain the true stress-strain relationship for the fine grain material at different strain, strain rate, temperature and grain size. Results of the model simulation compared well with the experimentally generated curves.