Influence of oxygen ingress on fine scale precipitation of α-Ti during oxidation of Beta21S β-Ti alloy

The formation of a surface oxide layer along with alpha precipitation in the subsurface oxygen-enriched zone, during the oxidation of a beta-Ti alloy, has been investigated using scanning electron microscopy, electron probe micro analysis, X-ray diffraction, (Scanning) transmission electron microscopy, 3D-Atom Probe studies, and nano-indentation. Immediately below the nanocrystalline oxide layer, a two-phase mixture consisting of nanoscale equiaxed alpha grains and rutile grains are formed. With increasing depth, the alpha morphology below the oxide layer varied from nanoscale equiaxed to lathlike, coupled with substantial changes in size-scale and nucleation density of alpha precipitates. A distinct change in the lattice parameters of alpha and beta phases below the oxide layer and the overall micro hardness of the material is also noted. The role of oxygen ingress on the scale and morphology of alpha precipitation has been discussed.