Establishing the Influence of Oxygen on Constrained Groove Pressing—Microstructure—Mechanical Properties Correlation in β‐21S Titanium Alloy Sheets

High‐temperature processing of β‐21S titanium alloy is highly susceptible to oxygen diffusion, which degrades ductility and restricts the alloy's ability to undergo severe plastic deformation (SPD). This study examines how delta‐glaze, a silica‐based glass‐frit, surface protection influences oxygen diffusion during solution treatment and, consequently, the constrained groove pressing (CGP) response of β‐21S Ti‐alloy sheet. CGP, a SPD technique that imposes alternating shear using grooved and flat dies, was used to evaluate the deformation response of the alloy. Sheets treated with and without the delta‐glaze protective coating were characterized for oxygen content, microstructure, and mechanical behavior. Electron probe microanalysis confirmed a substantial increase in oxygen concentration in the nonprotected sheets, leading to premature cracking and failure after the CGP pass‐1 (effective plastic strain of 1.16). In contrast, delta‐glaze‐protected sheets deformed up to CGP pass‐2 (effective plastic strain of 2.32) without major cracks. Microstructural analysis revealed the development of deformation‐accommodating mechanisms, including {332}⟨113⟩ mechanical twinning and kink‐band formation in delta‐glaze‐protected sheets whereas excess oxygen suppressed twinning and promoted slip‐dominated deformation. Although nonprotected samples exhibit higher yield strength owing to interstitial solid‐solution strengthening, however, this was accompanied by reduced ductility and poor CGP formability. The findings highlight that delta‐glaze protection is essential for maintaining surface integrity, mitigating oxygen embrittlement, and enabling large‐strain SPD processing in β‐21S Ti‐alloy, in sheet form, thereby improving its suitability for advanced aerospace forming applications.