Strength and fracture of ultrafine-grained titanium Grade 4

Processing of metallic materials by methods of severe plastic deformation (SPD) provides a possibility to reduce grain sizes to nanocrystalline (NC) or ultrafine-grained (UFG) conditions. Such a structure can lead to high strength of the material, but can also reduce material ductility. The majority of papers on this topic consider deformation parameters of NC and UFG materials only for the case of quasi-static tensile tests. Parameters of fracture toughness, dynamic strength, as well as fracture processes of UFG materials remain poorly understood. However, a reduction in ductility may result in low values of important constructional strength characteristics. This reduces the attractiveness of NC and UFG materials for industrial applications. This work presents the study an influence of SPD on the constructional strength parameters and fracture character of commercially pure titanium Grade 4. The UFG structure of the material was obtained by means of equal-channel angular pressing according to the Conform scheme (ECAP-Conform) and subsequent drawing. The combination of the utilized modes of ECAP and heat treatment makes it possible to obtain the maximal known degree of ductility for the UFG titanium. Quasistatic tensile tests, impact toughness tests on samples with U-shaped notch, and fracture toughness test for the case of three-point bending were carried out. The test results of the UFG titanium showed an increase in the tensile strength by 40%, an increase in the impact toughness by 15% and a reduction in the fracture toughness by 30% as compared to its coarse-grained analog. Fracture surface corresponding to localized plastic deformation is observed for all of the performed tests.