The influence of carbon on precipitation of α in Ti–5Al–5Mo–5V–3Cr

The effect of carbon-doping on the ageing response of solution heat-treated and quenched Ti–5Al–5Mo–5V–3Cr, a metastable β-Ti alloy, has been characterised using scanning and transmission electron microscopy, and microhardness testing. Alloys containing 0–0.1 wt% added carbon were processed and parameters including solution temperature, ageing temperature, time and heating rate were varied to study the resultant microstructural evolution. Results show that small carbon additions have a significant effect on alloys that were β-solutionised and subsequently aged at 500–600 °C, both refining and homogenising the precipitating α phase. Importantly, carbon reduces the tendency for coarse α to form at grain boundaries and, at lower ageing temperatures, strongly affects formation of isothermal ω. It is also demonstrated that considerable α refinement in undoped alloys can be achieved through slow or step heating. It appears that the responsible refinement mechanisms resulting from carbon-doping are different from those resulting from slower heating.

Research highlights▶ Added carbon greatly reduces grain boundary α lath formation in Ti-5553. ▶ Carbon strongly influences the isothermal β-to-omega transformation. ▶ The effect of carbon is strongest in β-annealed structures.