Formation of ω phase in Ti–Fe alloys and its stability upon heatingWednesday (26.09.2018) 16:30 - 16:45 S1/01 - A3 Part of:
Titanium-rich Ti–Fe alloys are promising candidates for the development of new engineering materials, as they possess excellent mechanical properties and good corrosion resistances. Using mechanical and thermal treatments, these properties can be further improved via adjustment of the microstructure. In particular, the combination of the low-temperature α-Ti phase, the high-temperature β-Ti phase and the high-pressure ω-Ti phase play, together with the distribution of these phases, a key role. In the present work, severe plastic deformation by high-pressure torsion (HPT) was applied to refine the grains and to initiate the ω-Ti formation. To identify the phases present in the initial and deformed alloys, SEM, TEM and XRD measurements were performed. The results obtained from Ti–1Fe and Ti–2Fe (ma.%) show a martensitic microstructure in the initial state (quenched from the bcc β-(Ti,Fe) solid solution) and the ω phase formation during the high-pressure torsion. For the Ti–4Fe (ma.%) alloy, the formation of the athermal ω phase was detected in the initial state (sample annealed at 800°C for 100 h and subsequently quenched). During HPT, almost the whole sample was transformed to ω-Ti. In order to describe the phase distribution, crystallite sizes, phase transitions and the orientation relationships between the individual phases, TEM/SAED and HRTEM investigations were performed. The results show that the resulting amount of the ω phase after HPT strongly depends on the Fe content and the previously applied heat treatment. The stability of the ω phase upon heating was investigated for the Ti–4Fe alloy both in the initial state and after HPT by means of thermal analysis and in-situ/ex-situ XRD. Finally, thermodynamic calculation of the T0-lines were performed to describe the diffusion-less transformations of the high-temperature β phase into the low-temperature α´ phase and to predict the maximum Fe solubility in the martensitic α´ phase.