Development of Ti-Al-Ta-Nb-(Re) near-α high temperature titanium alloy: Microstructure, thermal stability and mechanical properties

doi:10.1016/j.jmst.2021.08.066

Abstract

Abstract:

Mechanical properties and microstructural stability under the service temperature are important to the high temperature titanium alloy. In order to evaluate the potential in increase the service temperature of Ti alloy, two near-α Ti alloys with high content of Al as α-stabilizer and Ta, Nb and/or Re as β-stabilizers were designed and prepared by ingot metallurgy and thermomechanical processing, and the microstructure and mechanical properties before and after thermal exposure at 650 °C for 100 h were characterized. The results indicated that due to the weak β-stabilizing ability of Ta and Nb elements, only a small amount of β phase was formed in Ti-10Al-4Ta-2Nb alloy. With a trace Re addition, the β phase was obviously increased in Ti-10Al-4Ta-2Nb-0.25Re, indicating that the Re was a strong β-stabilizer. Under the same thermomechanical conditions, the Re addition decreased the volume fraction of primary α (α_p) phase and refined the secondary α (α_s) phase evidently. The primary α phase presented an obvious core-shell structure in the Ti-10Al-4Ta-2Nb alloy, with higher Al concentration in the shell. While the core-shell structure was not obvious in the Re-containing alloy due to the Re decreases the diffusion of Al, Ta and Nb elements. A large number of ordered α₂ precipitates can be observed in the α_p and α_s phases of two alloys. The α₂ precipitates continuously grew up during thermal exposure, however, their growth rate in the α_s phase of Re-containing alloy were lower than that of Ti-10Al-4Ta-2Nb alloy. Although plenty of ordered α₂ precipitates formed in the Ti-10Al-4Ta-2Nb alloy, the alloy had a certain plasticity at room temperature. The trace Re addition evidently increased the tensile strength but caused the decrease of the plasticity. After thermal exposure, the strength was further increased, while the plasticity was decreased for both of alloys.

Key words: Titanium alloy, High temperature, Microstructure, Mechanical properties, a₂ precipitates, Thermal exposure

Juan Li, Yaqun Xu, Wenlong Xiao, Chaoli Ma, Xu Huang. Development of Ti-Al-Ta-Nb-(Re) near-α high temperature titanium alloy: Microstructure, thermal stability and mechanical properties[J]. J. Mater. Sci. Technol., 2022, 109: 1-11.

Figures/Tables 15

Fig. 1. DSC heating curves of Ti-10Al-4Ta-2Nb and Ti-10Al-4Ta-2Nb-0.25Re alloys.

Fig. 2. SEM image of Ti-10Al-4Ta-2Nb (a) and Ti-10Al-4Ta-2Nb-0.25Re (b) after homogenization treatment, and magnified SEM image (c) of Ti-10Al-4Ta-2Nb-0.25Re alloy with corresponding EDS elemental mapping images of Al, Nb, Ta, Re and Ti elements.

Fig. 3. TEM images of as-homogenized alloys: (a) BF image, (b) SAED pattern taken from α matrix, and (c) DF image showing the presence of α2 precipitates in the α lamellas of Ti-10Al-4Ta-2Nb alloy; (d) BF image, (e) SAED pattern taken from α matrix, and (f) DF image showing the presence of α2 precipitates in the α lamellas of Ti-10Al-4Ta-2Nb-0.25Re alloy.

Fig. 4. Optical microstructure of (a) Ti-10Al-4Ta-2Nb and (b) Ti-10Al-4Ta-2Nb-0.25Re alloys, and SEM image of (c) Ti-10Al-4Ta-2Nb and (d) Ti-10Al-4Ta-2Nb-0.25Re after stabilizing treatment.

Fig. 5. BSE image of Ti-10Al-4Ta-2Nb alloy and corresponding WDS elemental mapping images of Ti, Al, Nb and Ta elements.

Fig. 6. BSE image of Ti-10Al-4Ta-2Nb-0.25Re alloy and corresponding WDS elemental mapping images of Ti, Al, Nb, Ta and Re elements.

Fig. 7. TEM images of Ti-10Al-4Ta-2Nb alloy: (a) BF image and (b) SAED pattern taken from αs phase, respectively; (c) DF image showing the presence of α2 precipitates in the αs; (d) BF image and (e) SAED pattern taken from αp phase, respectively; (f) DF image showing the presence of α2 precipitates in the αp.

Fig. 8. TEM images of Ti-10Al-4Ta-2Nb-0.25Re alloy: (a) BF image and (b) SAED pattern taken from αs phase, respectively; (c) DF image showing the presence of α2 precipitates in the αs; (d) BF image and (e) SAED pattern taken from αp phase, respectively; (f) DF image showing the presence of α2 precipitates in the αp.

Fig. 9. BF images and DF TEM images showing the α2 precipitates after thermal exposure taken from (a, b) the αs and (c, d) the αp of Ti-10Al-4Ta-2Nb alloy, and (e, f) the αs and (g, h) the αp of Ti-10Al-4Ta-2Nb-0.25Re alloy.

Fig. 10. Micro-hardness of Ti-10Al-4Ta-2Nb and Ti-10Al-4Ta-2Nb-0.25Re alloys before and after thermal exposure.

Fig. 11. Tensile engineering stress-strain curves and corresponding mechanical properties of Ti-10Al-4Ta-2Nb and Ti-10Al-4Ta-2Nb-0.25Re alloy: (a) without thermal exposure; (b) after 650 °C/100 h thermal exposure.

Fig. 12. BSE image of Ti-10Al-4Ta-2Nb-0.25Re alloy and corresponding WDS elemental mapping images of Ti, Al, Nb, Ta and Re elements after thermal exposure.

Fig. 13. BF images and DF TEM images showing α2 precipitates in (a-c) the αp and (d-f) the αs of as-solutionized Ti-10Al-4Ta-2Nb alloy.

Fig. 14. BF images and DF TEM images showing α2 precipitates in (a-c) the αp and (d-f) the αs of as-solutionized Ti-10Al-4Ta-2Nb-0.25Re alloy.

Fig. 15. Fracture surfaces after tensile tests at different conditions: (a, b) Ti-10Al-4Ta-2Nb and (c, d) Ti-10Al-4Ta-2Nb-0.25Re alloy without thermal exposure; (e, f) Ti-10Al-4Ta-2Nb and (g, h)Ti-10Al-4Ta-2Nb-0.25Re alloy after 650 °C/100 h thermal exposure.

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