Cyclic behavior of ultrafine-grain titanium produced by severe plastic deformation

The cyclic behavior is investigated of commercial purity massive ultrafine-grained titanium obtained by severe plastic deformation through equal channel angular pressing (ECAP). Both stress- and strain-controlled experiments were carried out to access the fatigue performance in terms of Wöhler diagram, fatigue limit, Coffin–Manson plot, cyclic hardening curves and cyclic stress-strain curves in a range of plastic strain amplitudes from 7.5×10−4 to 10−2. A significant enhancement of fatigue limit and fatigue life in the ultrafine-grained state is found under constant stress testing. No cyclic softening and degradation in the strain-controlled experiments is noticed in titanium contrary to wavy slip materials such as Cu subjected to ECAP. A simple one-parameter dislocation-based model is proposed to account for experimental results. It is shown that many cyclic properties of severely predeformed materials with fine grains can be rationalized in terms of Hall–Petch grain boundary hardening and dislocation hardening.