Relaxation mechanisms of near – α titanium alloy during stress relaxation with synchronous aging

Abstract

Stress relaxation with synchronous aging (SRSA) could not only reduce springback but as well improve the performance of titanium alloy components. To reveal the relaxation mechanisms during SRSA, solution treatments of Ti-6.5Al–2Zr–1Mo–1V alloy under different temperatures were carried out to obtain diverse initial microstructures. The evolutions of microstructure including phase transformation, dislocation movement and stress-induced twinning were meticulously characterized. The progress of activation energy was analyzed quantitatively by the Kohlrausch-Williams-Watts equation. Results show that SRSA under all different conditions exhibits a rapid stress decline initially followed by a quasi-static stage, due to a shift in the driving force from stress-driven to thermally activated driven. The activation energy changes from a quasi-static state to a rapid rise with relaxation time. After two-phase solution treatment, the primary α (αp) dominant mechanism is the dislocation movement, including dislocation tangles and then recovery. The relaxation mechanism of the supersaturated metastable β phase is phase transformation (precipitation and growth of secondary α (αs)) and stress-induced twinning, and the αs realizes stress release through the globularization in the later stage. The full martensite microstructure was obtained by solution treatment in a single-phase region, and the main relaxation mechanism involves stacking faults, stress-induced twinning and then globularization. The enhancement in strength following SRSA is primarily attributed to the formation of αs, dislocation strengthening, and stress-induced twinning.