Transformation Pathway upon Heating of Ti-Fe Alloys Deformed by High-Pressure Torsion

The current work presents the results of a study of the thermal stability of metastable omega-Ti(Fe) produced by a high-pressure torsion process and describes the phase transformations of omega-Ti(Fe) upon heating. The titanium alloys under study contain between 1 and 7wt% of iron, the phase transitions are investigated using a combination of in situ high-temperature X-ray diffraction and differential scanning calorimetry. The high-temperature X-ray diffraction reveals the phase sequence omega -> alpha' -> alpha + beta -> beta upon heating. The differential scanning calorimetry shows that the first phase transformation is exothermal and that the temperature of this phase transition is independent of the iron concentration within the composition range under study. Subsequent phase transitions are endothermal and the respective transition temperatures depend on the iron concentration. The differences between the phase stabilities conclude from the phase diagram and the phase stabilities observe experimentally are explained by the partial coherence of the alpha/alpha'-Ti and beta-Ti grains.