Abstract
Abstract High pressure torsion was conducted to obtain nano-sized β grains in a metastable Ti-5553 alloy. Much finer grains of <50 nm were achieved, compared to >100 nm in a stable Ti-20 wt.% Mo alloy. The more effective grain refinement was attributed to stress induced martensitic transformation in the former, leading to the formation of thin α" plates which divide β grains into smaller domains. Further deformation resulted in a reverse α" to β transformation with decreasing α" sizes, generating a completely nano β grain structure at very large straining. A detailed description of the β grain refinement mechanism is provided. The reverse transformation is shown to be caused by the significantly increased free energy below a critical α" size of ∼10 nm, consistent with experimental observations. It is also calculated that extremely high energies were required for the formation of α" in nano-sized β grains, making further martensitic transformation impossible. It is concluded that the stress induced martensitic transformation and the subsequent reverse transformation are critical to producing nano-grained metastable β Ti alloys.
Original language | English |
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Article number | 12227 |
Pages (from-to) | 146-155 |
Number of pages | 10 |
Journal | Acta Materialia |
Volume | 97 |
DOIs | |
Publication status | Published - 15 Sep 2015 |
Externally published | Yes |
Keywords
- Grain refinement
- High-pressure torsion
- Martensitic phase transformation
- Severe plastic deformation (SPD)
- β titanium alloys