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Hochdruckdrehbeanspruchungsinduzierte Phasenumwandlung in Titan und Titanlegierungen

Fachliche Zuordnung Metallurgische, thermische und thermomechanische Behandlung von Werkstoffen
Mechanische Eigenschaften von metallischen Werkstoffen und ihre mikrostrukturellen Ursachen
Förderung Förderung von 2015 bis 2020
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 267921614
 
Erstellungsjahr 2021

Zusammenfassung der Projektergebnisse

3. Summary In titanium alloys containing iron, cobalt or niobium as alloying elements, the formation and thermal stability of the ω phase having the space group P6/mmm was investigated for different concentrations of the alloying elements (≤ 7 wt.%) and for different sample states, i.e., in samples quenched from a homogenization temperature and in samples, which were additionally subjected to a severe plastic deformation using the high-pressure torsion (HPT). In order to disclose the effect of the kind and distribution of the alloying elements on the microstructure of the titanium alloys and on the stability of the ω phase, a combination of microstructure analytical techniques and thermal analysis was employed. The experimental methods of the microstructure analytics comprised post mortem and in situ X-ray diffraction at elevated temperatures, transmission electron microscopy, selected area electron diffraction (in TEM) and local chemical analyses using X-ray spectroscopy. The thermal analyses were carried out using differential scanning calorimetry. The experimental data regarding the phase stabilities were used for thermodynamic modelling using the CalPhaD approach. It was shown that the ω phase develops preferentially from β-Ti (space group 𝐼𝐼𝐼𝐼3�𝐼𝐼). The phase transition β→ω is facilitated by the crystallographic orientation relationship (111)β || (0001)ω and [11�0]β || [112�0]ω. As the lattice parameters of ω-Ti and β-Ti depend differently on the concentration of the alloying elements, the phase transition β→ω takes place preferentially at the concentrations of alloying elements, which lead to the best match of the interatomic distances in both phases. Additionally, an α/α’ → ω phase transition was observed, which is made possible by the martensitic transformation of oversaturated α-Ti (space group 𝑃𝑃63/𝐼𝐼𝐼𝐼𝑚𝑚) into α’-Ti, by the orientation relationship (110)β || (0001)α’ and [11�1]β || [112�0]α’ between the α’- martensite and β-Ti, and by the orientation relationship between β-Ti and ω-Ti mentioned above. The in-plane orientation relationship between α-Ti and ω-Ti was described as (0001)α || (011�0)ω and as (0001)α || (112�0)ω. The alternative orientation relationships result from the development of two ω-Ti variants that is forced by the twinning of the intermediate phase. Formation of ω-Ti was observed in samples with different original microstructures, i.e., in samples with a low density of microstructure defects, which were quenched from a homogenization temperature to room temperature, and in samples with a high defect density that were additionally subjected to severe plastic deformation. However, as ω-Ti is a metastable phase, it decomposes at elevated temperatures (between 130°C and 250°C depending on the kind and concentration of the alloying element). Still, the amount and, in particular, the thermal stability of ω-Ti are different in deformed and in non-deformed samples. The amount of ω-Ti is higher in the deformed samples, because the β → ω transformation is accelerated by the shearing during the high-pressure torsion. Athermal ω-Ti, which forms upon quenching in non-deformed samples, is stable to higher temperatures than the deformation-induced ω-Ti. The faster decomposition of the deformation-induced ω-Ti is caused by a faster redistribution of the alloying atoms through the microstructure defects that destabilizes the metastable ω phase. Additionally, the formation of ω-Ti is inhibited, when a Ti-rich intermetallic phase containing the respective alloying element precipitates in the titanium matrix. Also in this case, the alloying atoms are redistributed in the microstructure of the titanium alloy, their concentration in the matrix decreases and the formation of ω-Ti is hindered. The relevant intermetallic phases in the systems Ti–Fe and Ti–Co are TiFe and Ti2Co, respectively. In the Ti–Nb system, only α/α’- Ti(Nb) and β-Ti(Nb) are present, as this system contains no ordered intermetallic phase. In general, a lower amount of ω-Ti forms in samples with intermetallic precipitates. The pressure-induced changes in the homogeneity ranges of the terminal solid solutions and intermetallic phases in the Ti–Fe system were investigated experimentally in the Ti–Fe diffusion couples, which were prepared in a multianvil press at 2.5 GPa and at 1173 and 1273 K. External pressure extends the homogeneity ranges of the terminal solid solutions β-Ti(Fe) and γ-Fe. The homogeneity range of the intermetallic phase TiFe extends towards higher titanium concentrations. The homogeneity range of TiFe2 also shifts to higher titanium concentrations, but its width does not change significantly. The homogeneity range of δ-Fe(Ti) expands on the Ti-rich side and shifts slightly towards lower Fe concentrations on the Fe-rich side. These findings were implemented into the thermodynamic modelling of the phase stabilities in the Ti–Fe system under pressure.

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