Oxidation mechanism of a near β-Ti alloy

•The Ti–5Al–5Mo–5V–1Cr–1Fe alloy displayed parabolic oxidation kinetics.•The oxide layer consisted of Al2O3, TiO2, VO2, and Magnéli phases.•Polymorphic phase transformation and Magnéli phases affected oxide layer thickness.•Oxygen incorporation by Al2O3 and VO2 formed defects due to lattice distortion.•Continuous oxidation formed the higher-valence oxide V2O5 in VO2 particles.