Unveiling the effects of oxidation on the microstructure and microchemistry of the oxygen-rich layer in a near-α Ti alloy

A combination of electro-probe microanalysis, transmission electron microscopy and atom probe tomography was employed to investigate the effects of high-temperature oxidation on the gradient of the microstructure and microchemistry of the oxygen-rich layer on a near-α Ti alloy, Ti6242s, with a bimodal microstructure. α₂ precipitation occurred within the oxygen-rich layer, more pronounced inside the primary α grains than within the secondary α laths. The degree of α₂ precipitation increases with increasing oxygen content, and the critical amount of oxygen triggering α₂ is ∼3 at. % in the primary α grains, and even higher within the secondary α laths. These values are far greater than the reported value, ∼0.6–0.75 at. %, for the bulk alloy. Heterogeneous Al segregation was observed within the secondary α laths while bcc-β phase persisted even at the very surface where the oxygen content was highest. These findings indicate that oxygen ingress accelerates precipitation of α₂ at elevated temperature, and faster O diffusion along the dislocations and boundaries stimulates heterogeneous Al segregation. The lower degree of ordering within the secondary α laths was probably owing to the lower Si content and Al supersaturation.