Microstructural Evolution and Microhardness Variations in Pure Titanium Processed by High-Pressure Torsion

A grade 2 pure titanium with an initial grain size of ≈50 μm is processed by high-pressure torsion (HPT) at room temperature under an imposed pressure of 6.0 GPa. The microhardness variations are examined and the results show that the disks are reasonably homogeneous after 10 turns of torsional straining. The microstructural evolution is systematically characterized by optical microscopy, X-ray diffraction, and transmission electron microscopy to provide information on the effect of shear strain on grain size and microstructure. The results demonstrate that the initial coarse structure is gradually refined from the edge to the center of the disk under the shear stress during HPT processing and an ultrafine-grained pure Ti is achieved with an average grain size of ≈96 nm after 10 turns. A model is developed by considering the formation of subgrain boundaries, twins, and high-angle grain boundaries for the grain refinement of pure Ti processed by HPT.