Effect of cooling process on the α phase formation and mechanical properties of sintered Ti–Fe alloys

Titanium sintered compacts can be strengthened by adding iron due to both its high solid solution strengthening effect and its ability to stabilize the bcc β phase. However, these outcomes may be changed by the cooling rate and the amount of iron added, both of which influence the microstructures and properties of the sintered compact. This study presents the sintered properties of Ti–xFe (x = 3, 5, and 7 wt.%) sintered at 1150 °C under vacuum and then cooled with different cooling schedules. The results indicated that all alloys reached about 96% relative densities. Higher holding temperatures (740 and 640 °C) in the α + β region after sintering increased the tensile strength and hardness due to the higher amounts of the β phase and the acicular α precipitates inside the β. With holding at a lower temperature of 550 °C, the amount of β phase decreases and no secondary α precipitates were observed in the β phase. No TiFe compound was observed in any of the sintered parts, even with a slow cooling rate, indicating that iron is a very strong β phase stabilizer that can suppress the eutectoid reaction at 595 °C. These changes in microstructure result in significant changes in mechanical properties.

► With controlled cooling schedule, the mechanical properties of powder metal Ti–7Fe are approaching those of the annealed wrought Ti–6Al–4V.
► The mechanical properties of Ti–7Fe are improved by obtaining large amounts of β phase, high iron content in β, and fine α precipitates within β, which are determined by the cooling schedule applied.
► The TEM analysis confirms that the Ti(β) → Ti(α) + TiFe eutectoid reaction does not occur even after holding the specimen below the eutectoid temperature for 24 h.