EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Titanium and its alloys are widely used in various areas, from aerospace to medical implants to car industry and jewellery. Titanium alloys can be divided into several groups. We study the group called metastable β titanium alloys. These alloys contain a sufficient amount of elements which stabilize the high temperature β phase (body centred cubic), so that the transition to the low temperature α phase (hexagonal close packed) is prevented during quenching. Metastable β titanium alloys can also contain another, metastable phase with hexagonal arrangement, so-called ω phase.

We studied the effect of molybdenum content on phase transformations in binary titanium alloys (Ti-12Mo, Ti-15Mo and Ti-18Mo in wt%). We examined the alloys by in-situ methods during linear heating: dilatometry, differential scanning calorimetry (DSC) and electrical resistivity measurement (resistometry) and ex-situ methods after isothermal annealing: scanning electron microscopy (SEM) and microhardness measurement. Dilatometry is a suitable method to study formation, evolution and dissolution of ω phase. On the other hand, DSC is especially sensitive to formation of α phase, as transition from the high temperature β phase to the low temperature α phase is accompanied by heat release. Resistometry is complementary to the other two methods.

In Ti-18Mo, higher content of molybdenum does not allow for precipitation of α phase and ω phase only evolves in a narrow temperature range. Conversely, low content of molybdenum allows for formation of α phase and ω phase evolves in a wide temperature range in Ti-12Mo.

Alpha grain boundary phase and formation of α-lamellar microstructure was observed using SEM. Ti-12Mo after annealing at 400°C, owing to the mixture of β and ω phases, has the highest microhardness of all the studied alloys.