Nanocrystalline titanium produced by hydrostatic extrusion

Commercial purity titanium (CP-Ti) grade 2 was hydrostatically extruded (HE) in 20 consecutive passes with cumulative true strain of 5.47 without intermediate annealing. HE results in a significant grain refinement. The grain size was reduced from ∼33 μm to nanometre scale of equivalent grain diameter 47 nm. The refinement of the structure was accompanied by enormous enhancement of the mechanical properties with the ductility retained at the level characteristic of other bulk severe plastic deformation (SPD) treated materials (∼8%). The ultimate tensile strength, UTS > 1300 MPa and yield stress, YS ∼ 1250 MPa, the highest ever reported for bulk CP-Ti samples, were measured after HE. These values are higher than those of the solution treated (hardened) Ti–Al–V commercial alloy. The Hall-Petch (H-P) strength and hardness relations have shown to be not congruent. For the YS dependence the linear fit seems to be appropriate, while the hardness dependence reveals the slope change from positive (material hardening with decreasing grain size) to negative (material softening). The threshold falls at ∼120 nm, around the point commonly considered to be the UFG/NC microstructure transformation border. The results obtained revealed the possibility fabricating long-length high strength nanocrystalline CP-Ti semi-products using the hydrostatic extrusion (HE) process. Such semi-products can be used in medicine, e.g. for dental implants, where high strength and good biocompatibility are required.