A comparative study on the microstructure and mechanical behavior of titanium: Ultrafine grain vs. coarse grain

To understand the effects of different factors such as grain size, strain rate and temperature on properties and deformation mechanisms of commercially pure titanium (CP-Ti), ultrafine grained (UFG) Ti was fabricated by room temperature equal channel angular pressing (ECAP). The microstructure was characterized using various techniques and the mechanical properties were evaluated within a wide range of temperature and strain rate. It has been shown that the microstructure change upon ECAP significantly impacts the mechanical properties. Strong dependence on grain size, temperature and strain rate of the mechanical properties of CP-Ti was observed. Ductility of UFG Ti increased at both cryogenic (77 K) and elevated temperatures (473 K), but with distinctly different deformation mechanisms. Other phenomena such as texture, deformation twinning, fracture, adiabatic shear banding, strain softening and hardening were observed and possible underlying mechanisms are given. Yield stress, elongation to failure, etc. and the related parameters, the Zener-Hollomon parameter, strain rate and temperature sensitivity, and activation volumes associated with plastic deformation were discussed. Comparison between CG and UFG CP-Ti sheds light on possible deformation mechanisms during tensile loading. Different rate controlling mechanisms such as dislocation slip and grain boundary sliding are proposed for this metal of hexagonal close-packed (hcp) structure.