High strength titanium with a bimodal microstructure fabricated by thermomechanical consolidation of a nanocrystalline TiH2 powder

A titanium rod with a novel bimodal microstructure consisting of α-Ti plates of 1-5 µm in width and 10-15 µm in length and ultrafine grained (UFG) α-Ti regions with grain sizes in the range of 100-300 nm was fabricated by in-situ dehydrogenation and thermomechanical consolidation of a nanocrystalline TiH2 powder. The consolidation process combined spark plasma sintering at 800 °C for 5 min and hot extrusion at 1100 °C, and took less than 15 min in total. The titanium rod had a high tensile yield strength of 1052 MPa and a limited elongation to fracture of 2.0%. The high yield strength of the titanium rod can be attributed to the high strength of the UFG regions due to grain boundary strengthening and the increased strength of the α-Ti plates due to O solid solution strengthening associated with the high O content of 0.85 wt% in the titanium rod. A microstructural evolution model involving nucleation and growth of the α-Ti plates, partition of H between α-Ti and β-Ti phases and Ti-H eutectoid reaction has been proposed to elucidate the mechanism for the formation of the bimodal microstructure.