Microhardness evolution and mechanical characteristics of commercial purity titanium processed by high-pressure torsion

An investigation was conducted to evaluate the microstructural evolution and mechanical properties of a commercial purity (CP) titanium (grade 2) processed by high-pressure torsion (HPT) at room temperature. Microstructural analysis was performed to provide detailed information on the effect of shear strain on the grain size. The results demonstrate that significant grain refinement is achieved through HPT processing with a reduction from ~45 μm in the initial annealed condition to a grain size of ~150 nm after 10 turns of HPT. Measurements of the Vickers microhardness show that the disks are essentially homogeneous after 5 or more turns of torsional straining. Analysis by X-ray diffraction (XRD) revealed an allotropic phase transformation from the α-phase (hexagonal closed-packed) to the ω-phase (hexagonal) during HPT processing. The results from mechanical testing at an elevated temperature of 673 K are consistent with the microstructural observations thereby showing that ultra-fine grained CP Ti exhibits excellent mechanical properties.