Preparation of Multiscale α Phase by Heat Treatments and Its Effect on Tensile Properties in Metastable β Titanium Alloy Sheet
Abstract
:1. Introduction
2. Materials and Methods
3. Results and Discussion
3.1. Microstructure Observation
- (1)
- During the HLDA process, with the prolonging of the second-step aging time, the morphology of αT in PFZs changes from dot-like to needle-like, and its number and volume fraction increase gradually. In addition, increasing the second-step aging temperature can promote the precipitation of αT, and the size of αT is slightly increased.
- (2)
- The amount and size of αS obtained by single aging do not change significantly after duplex aging, which indicated that it had certain stability.
3.2. Mechanical Properties
3.3. Effect of Multiscale α on Tensile Deformation Behavior
4. Conclusions
- The microstructure composed by multiscale α phase can be obtained by high/low-temperature two-step aging heat-treatment in a descending order. In first-step high-temperature aging, the purpose of the coarse α phase obtained is to maintain a certain plasticity of the alloy, and the reserved precipitate-free zones are aimed at precipitating nanoscale (width) tertiary α phase in the second-step low-temperature aging process and enhancing the strength of the alloy. Finally, a needle-like mixture morphology, which is composed of secondary α phase (micron size in width) and tertiary α phase (nanoscale size in width), is formed after duplex aging.
- For the microstructure, a certain amount of lath secondary α precipitated from the matrix and from the PFZs after first-step aging. During the second-step aging process, with the prolonging of second-step aging time, the morphology of tertiary α phase in PFZs changes from dot-like to needle-like, and its number and volume fraction increase gradually. In addition, increasing the second-step aging temperature can promote the precipitation of tertiary α phase, and the size of tertiary α phase is slightly increased. After second-step aging for 4 h at 550 °C, the concentration of the tertiary α phase distribution of the alloy is the densest.
- For mechanical properties, secondary α that precipitates during first-step aging has a very obvious strengthening effect, but it can be seen that the plasticity is slightly decreased. At the same second-step aging temperature, with the increase of aging time, the alloy macroscopically shows the characteristics of strength increase and plasticity decrease. At the same second-step aging time, increasing the aging temperature can further increase the strength of the alloy. After second-step aging at 550 °C for 4 h, the alloy properties of tensile strength and yield strength reach 1115 MPa and 1020 MPa, respectively, and the elongation is 12%.
- In situ tensile testing of solution treatment, first-step aging, and second-step aging of alloys were carried out in order to understand the relationship between microstructure and mechanical properties. After solution treatment, a microstructure of the alloy was composed of β matrix and a small amount of primary α phase. During the stage of tensile testing, a large number of intersecting and parallel slip lines appear in the β-grains, and the cracks initiate along the slip system with large deformation and mainly exhibit transgranular fracture. Therefore, the alloy can exhibit excellent plasticity. After first-step aging, the secondary α phase has appeared in some grains. The crack passes through these grains to form a transgranular crack, and other grains will also form secondary cracks. Finally, the crack propagates in a mixed mode of transgranular fracture and intergranular fracture. Coarse secondary α phase precipitates can increase the resistance of crack growth, so the alloy still has good ductility. After second-step aging, a large number of tertiary α phase precipitates in the PFZs region of the grain, and the crack nucleates and grows in the region where the tertiary α phase precipitation is dense. A large number of small and dispersed tertiary α phase increases the strength level of the alloy. It is because the coarse secondary α phase precipitates during first-step aging and forms a multiscale structure with tertiary α phase that the alloy can maintain a certain plasticity while improving its strength.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
References
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Element | Mo | Al | Nb | Si | O | Fe | H | N | C | Ti |
---|---|---|---|---|---|---|---|---|---|---|
Ti-15Mo-3Al-2.7Nb-0.2Si | 14.9 | 3.06 | 2.74 | 0.21 | 0.005 | 0.02 | 0.0017 | 0.091 | 0.009 | Bal. |
Heat Treatment | Parameters of Heat Treatment | Coding Designation of Heat Treatment | |
---|---|---|---|
Temperature/°C | Time | ||
Solution treatment | 780 | 0.5 h | HT |
Plus first-step aging | 650 | 4 h | SA |
Plus second-step aging | 450 | 5 min, 0.5 h, 2 h, 4 h | HLDA |
500 | |||
550 |
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Jiang, H.; Du, Z.; Wang, D.; Gong, T.; Cui, X.; Liu, F.; Cheng, J.; Chen, W. Preparation of Multiscale α Phase by Heat Treatments and Its Effect on Tensile Properties in Metastable β Titanium Alloy Sheet. Metals 2021, 11, 1708. https://doi.org/10.3390/met11111708
Jiang H, Du Z, Wang D, Gong T, Cui X, Liu F, Cheng J, Chen W. Preparation of Multiscale α Phase by Heat Treatments and Its Effect on Tensile Properties in Metastable β Titanium Alloy Sheet. Metals. 2021; 11(11):1708. https://doi.org/10.3390/met11111708
Chicago/Turabian StyleJiang, Hanyu, Zhaoxin Du, Da Wang, Tianhao Gong, Xiaoming Cui, Fei Liu, Jun Cheng, and Wenzhen Chen. 2021. "Preparation of Multiscale α Phase by Heat Treatments and Its Effect on Tensile Properties in Metastable β Titanium Alloy Sheet" Metals 11, no. 11: 1708. https://doi.org/10.3390/met11111708