Ageing response of sub-transus heat treated Ti-6.8Mo-4.5Fe-1.5Al alloy

- Phase transformations in α/β sub-transus heat treated Timetal LCB were investigated.
- ω particles promote precipitation of small, homogeneously distributed α lamellae.
- Without ω particles, rapid coarsening of α microstructure is observed.
- The highest hardness values are associated with very fine α particles.

In this study, phase transformations occurring during ageing of Timetal LCB (Ti-6.8Mo-4.5Fe-1.5Al in wt.%) and their influence on mechanical properties were investigated. The initial condition was prepared by solution treating above β-transus directly followed by heat treatment in α/β temperature regime. This treatment resulted in precipitation of α phase on β/β grain boundaries. The subsequent ageing response of this material was studied during isothermal ageing at 400 °C, 450 °C and 500 °C. The phase transformations were studied by X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Mechanical properties were investigated using microhardness measurements. During ageing at 400 °C, the ω phase was detected in all studied conditions. Due to its presence in the material, very fine and homogeneously distributed α particles were formed, leading to a high microhardness. At 450 °C, the ω phase disappeared already after a few hours of ageing. The highest microhardness values at this temperature were reached in samples aged for a few hours after the dissolution of ω particles. No ω was detected in samples aged at 500 °C. The α phase morphology after ageing at 500 °C consisted of large plate-shaped precipitates which coarsened rapidly with increasing ageing time. This coarse microstructure corresponded to a low microhardness.