Iron-Titanium (Fe-Ti) Phase Diagram
A study of the precipitation hardening of iron-titanium (Fe-Ti) alloys usually starts with the iron-titanium binary system. Fe-Ti-based alloys are also used for storage of hydrogen in the form of metal hydride.
Figure 1 shows the iron-titanium phase diagram (up to 30 wt. % Ti) calculated with Thermo-Calc, coupled with TCFE3 thermodynamic database. The diagram has the following eutectic reaction at 1289 °C: Liquid —> Ferrite + Fe2Ti.
The crystal structure of Fe2Ti intermetallic phase is of the MgZn2 type. Another stable intermetallic compound that forms in the iron-titanium binary system, and which is of great importance for hydrogen storage, is FeTi (not shown in Figure 1, as FeTi forms when the amount of titanium in the Fe-Ti binary system is about 50 at. %).
Figure 1. Fe-Ti phase diagram shows which phases are to be expected at equilibrium for different combinations of titanium content (up to 30 wt. %) and temperature. The Fe-Ti phase diagram was calculated with Thermo-Calc, coupled with TCFE3 thermodynamic database.
The limits of the austenite field and the austenite + ferrite field are placed at 0.69 and 1.24 wt. % Ti, respectively. The solubility of titanium in ferrite (alpha iron) reaches its maximum at 8.4 wt. % Ti at the eutectic temperature of 1289 °C, but decreases significantly at lower temperatures (e.g., 0.53 wt. % Ti at 600 °C and 0.15 wt. % Ti at 400 °C). This difference in the solubility of titanium enables the precipitation hardening of Fe-Ti alloys.
• The phase diagram of Iron-Titanium binary system (up to 30 wt. % Ti)
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