Characterization of a New Microstructure in a β-Solidifying TiAl Alloy after Air-Cooling from a β Phase Field and Subsequent Tempering
Abstract
:1. Introduction
2. Materials and Methods
3. Results and Discussion
3.1. Phase Constituents and Morphology
3.2. Crystallography and Variant Selection
3.2.1. α2′ Martensite
3.2.2. γ Laths
3.3. Microstructure Decomposition during Tempering
4. Conclusions
- Owing to the extraordinary stability of the β phase in the Ti-40Al-10V alloy, well-developed α2′ lath martensite was produced after being heat-treated in the single β phase field followed by air cooling, rather than the common α2/γ lamellar colonies. The martensitic morphology and crystallography were very similar to that produced during quenching, but much larger in size. The obtained α2′ laths accorded a strict Burger orientation relationship ({110}β//{00.1}α2′ and <111>β//<11.0>α2′) with the β matrix and hence a total of twelve variants were produced. Local variant selection was detected, meaning that three (six) α2′ variants sharing one (two) common [11.0] axes were predominant in a local region, while no global variant selection could be noted.
- Subsequent to the martensitic transformation, the retained β phase decomposed via a β→γ transformation. The γ laths always nucleated at the α2′/β interface and mainly grew into the β matrix. Each α2′ lath produced two twin-related γ variants conforming to the Blackburn orientation relationship ({00.1}α2′//{111}γ and <11.0>α2′//<110>γ). Hence there were twenty-four γ variants for a given β grain, according to a K-S orientation relationship ({110}β//{111}γ and <111>β//<110>γ) as a result of the combination of the Burgers and Blackburn orientation relationships.
- During tempering, the air-cooled microstructure decomposed mainly via an α2′→γ transformation according to the Blackburn orientation relationship. The martensite was almost completely decomposed after tempering at 1000 °C for 4 h. The volume fraction of the γ phase was significantly increased and became homogenous with sizes of about 0.5 μm. Meanwhile, due to the local variant selection, the misorientation angle distribution of the γ grain boundaries presented only several independent misorientations. Such a treatment, resembling the quenching-tempering in steels, may be a new strategy for the microstructural design of TiAl alloys.
Acknowledgments
Author Contributions
Conflicts of Interest
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Element | α2′ | β/B2 | γ |
---|---|---|---|
Ti (at. %) | 50.1 ± 0.7 | 50.2 ± 0.4 | 49.9 ± 0.9 |
Al (at. %) | 39.7 ± 1.1 | 39.0 ± 1.5 | 40.2 ± 2.6 |
V (at. %) | 10.1 ± 0.9 | 10.7 ± 1.0 | 9.8 ± 1.2 |
Specific OR with Parent β | Variant No. | Misorientation Axis/Angle between γ1 and Other Variants |
---|---|---|
<110>γ//[111]β | γ1 | - |
γ2 | [0.577 0.577 ]/60° | |
γ3 | [0 0.707 0.707]/60° | |
γ4 | [0 ]/10.53° | |
γ5 | [0 ]/60° | |
γ6 | [0 0.707 0.707]/49.47° | |
<110>γ//[11]β | γ7 | [ 0.577]/49.47° |
γ8 | [0.577 0.577 [ ]/10.53° | |
γ9 | [ 0.186 /50.51° | |
γ10 | [ 0.490]/50.51° | |
γ11 | [0.933 0.354 0.065]/14.88° | |
γ12 | [ 0.603 0.714]/57.21° | |
<110>γ//[11]β | γ13 | [0.354 ]/14.88° |
γ14 | [ 0.462 /50.51° | |
γ15 | [ 0.628]/57.21° | |
γ16 | [0.659 ]/20.61° | |
γ17 | [ 0.363 /51.73° | |
γ18 | [ 0.626]/47.11° | |
<110>γ//[11]β | γ19 | [ 0.767 0.615]/50.51° |
γ20 | [0.357 0.714 ]/57.21° | |
γ21 | [0.955 0 ]/20.61° | |
γ22 | [ 0.626 0.719]/47.11° | |
γ23 | [ /57.21° | |
γ24 | [0.912 0]/21.06° |
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Chen, Y.; Cheng, L.; Sun, L.; Lu, Y.; Yang, G.; Kou, H.; Bouzy, E. Characterization of a New Microstructure in a β-Solidifying TiAl Alloy after Air-Cooling from a β Phase Field and Subsequent Tempering. Metals 2018, 8, 156. https://doi.org/10.3390/met8030156
Chen Y, Cheng L, Sun L, Lu Y, Yang G, Kou H, Bouzy E. Characterization of a New Microstructure in a β-Solidifying TiAl Alloy after Air-Cooling from a β Phase Field and Subsequent Tempering. Metals. 2018; 8(3):156. https://doi.org/10.3390/met8030156
Chicago/Turabian StyleChen, Yi, Liang Cheng, Lingyan Sun, Yalin Lu, Guang Yang, Hongchao Kou, and Emmanuel Bouzy. 2018. "Characterization of a New Microstructure in a β-Solidifying TiAl Alloy after Air-Cooling from a β Phase Field and Subsequent Tempering" Metals 8, no. 3: 156. https://doi.org/10.3390/met8030156