Dependences of grain size and strain-rate on deformation behavior of commercial purity titanium processed by multi-directional forging

Strain rate dependencies of deformation behavior of commercial purity titanium specimens having different grain sizes were systematically investigated. Ultrafine-grained titanium with an average grain size of 0.07 µm (UFG-Ti) fabricated by multi-directional forging followed by conventional thermo-mechanical processing, and fine-grained (FG-Ti) and coarse-grained (CG-Ti) specimens with an average grain sizes of 0.8 and 12 µm attained by its, respectively, annealing at 773 and 973 K for 1.8 ks were prepared. The FG- and UFG-Ti specimens exhibited strong strain-rate dependence of 0.2% proof stress, while that of CG-Ti ones were almost constant regardless of applied strain-rate. In-situ X-ray diffraction measurements during tensile tests were also conducted at synchrotron radiation facility, SPring-8. Using the modified Williamson-Hall and the modified Warren-Averbach methods, the activated slip systems and change in dislocation density during deformation were estimated. As a result, it was found that <a> and <c + a> slips were activated in FG- and UFG-Ti specimens. On the other hand, the activation of <c + a> slip was never observed in the CG-Ti ones. It can be, thus, concluded that the different strain-rate dependency of deformation behaviors of specimens with different grain sizes were ascribed to the difference in the deformation mechanisms.