Correlation of microstructure with hardness and fracture properties of (TiC,SiC)/Ti–6Al–4V surface composites fabricated by high-energy electron-beam irradiation

Correlation of microstructure with hardness and fracture toughness of (TiC,SiC)/Ti–6Al–4V surface composites fabricated by high-energy electron-beam irradiation was investigated in this study. The mixtures of TiC, SiC, or TiC+SiC powders and CaF2 flux were placed on a Ti–6Al–4V substrate, and then the electron beam was irradiated on these mixtures using an electron-beam accelerator. The surface composite layers of 1.2∼2.1 mm in thickness were formed, and contained a large amount (up to 66 vol.%) of precipitates such as TiC and Ti5Si3 in the martensitic matrix. This microstructural modification including the formation of hard precipitates and hardened matrix in the surface composite layer improved hardness. In situ observation of the microfracture process revealed that microcracks primarily initiated at precipitates, and that shear bands formed in the matrix between these microcracks. Thus, fracture toughness was determined mainly by the volume fraction of precipitates working as fracture initiation sites and partly by the matrix property interrupting the crack propagation.