Surface characterization of Ti and Ti–6Al–4V alloy by Ca vapor deoxidation: Relation between surface morphology and deoxidation reaction

Ca vapor deoxidation—which involves controlling oxygen through deoxidation on the surface of Ti—is important to ensure the quality of Ti and its alloys. We systematically investigated various surface properties before and after Ca vapor deoxidation of bulk Ti and Ti–6Al–4V alloy with small specific surface areas. The relationship between surface characteristics and deoxidation is discussed. The oxygen concentrations of Ti and Ti–6Al–4V decreased to 555 and 565 ppm, respectively, upon Ca vapor deoxidation. Moreover, the surface roughness increased, the surface oxide layer was slightly thinned, and the oxygen content of the matrix decreased. The surface chemical states of Ti before and after the deoxidation were mainly the tetravalent oxidation state (TiO₂) and metallic state toward the inside. The deoxidation produced a Ca oxide layer on the surfaces of Ti and Ti–6Al–4V and increased the surface roughness by creating craters on the surface. The deoxidation reactions included heterogeneous Ca/CaO nucleation and growth on the Ti surface, resulting in localized oxygen diffusion. Craters were formed on the surfaces of Ti and Ti–6Al–4V owing to the Ca oxide layer. Local oxygen diffusion and reactions with Ti and Al components enlarged the craters.