Abstract
In this work, the formation and thermal stability of the ω-Ti(Fe) phase that were produced by the high-pressure torsion (HPT) were studied in two-phase α-Ti + TiFe alloys containing 2 wt.%, 4 wt.% and 10 wt.% iron. The two-phase microstructure was achieved by annealing the alloys at 470 °C for 4000 h and then quenching them in water. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were utilized to characterize the samples. The thermal stability of the ω-Ti(Fe) phase was investigated using differential scanning calorimetry (DSC) and in situ high-temperature XRD. In the HPT process, the high-pressure ω-Ti(Fe) phase mainly formed from α-Ti. It started to decompose by a cascade of exothermic reactions already at temperatures of 130 °C. The decomposition was finished above ~320 °C. Upon further heating, the phase transformation proceeded via the formation of a supersaturated α-Ti(Fe) phase. Finally, the equilibrium phase assemblage was established at high temperatures. The eutectoid temperature and the phase transition temperatures measured in deformed and heat-treated samples are compared for the samples with different iron concentrations and for samples with different phase compositions prior to the HPT process. Thermodynamic calculations were carried out to predict stable and metastable phase assemblages after heat-treatments at low (α-Ti + TiFe) and high temperatures (α-Ti + β-(Ti,Fe), β-(Ti,Fe)).
Highlights
Titanium and titanium-base alloys are promising materials for numerous engineering applications, because they have several outstanding properties [1]
The ω-Ti(Fe) phase can be produced in different ways in a high-pressure torsion (HPT) process
The strongest exothermal effect was observed at around 380 ◦ C, whereas the released heat increases with increasing iron content within the investigated alloys. These results demonstrate that ω-Ti(Fe) possesses a lower thermal stability, if it is formed from α-Ti than if it is formed from β-(Ti,Fe)
Summary
Titanium and titanium-base alloys are promising materials for numerous engineering applications, because they have several outstanding properties [1]. The binary intermetallic phase TiFe has been presented as a possible material for solid-state hydrogen storage applications [5,6]. Metals 2020, 10, 402 material in order to improve the hydrogen storage properties of TiFe [7,8]. The SPD methods were utilized to generate bulk nanocrystalline materials that show, in many cases, better physical and mechanical properties than their microcrystalline counterparts [9,10]. It turned out that the unique properties are facilitated by diffusive and displacive (martensitic) phase transformations, which occur in the material during the HPT process [11,12,13]. The mechanism of the phase transformations and the influence of the initial microstructure on the phases formed after HPT are not fully understood yet
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