Abstract
Shape memory alloys (SMAs) are a class of advanced materials known for their ability to remember their original shapes. The key functional properties of SMAs include the shape-memory effect (SME) and pseudoelasticity (PE). These properties make them highly versatile, enabling their widespread use in a great variety of industries, including aerospace, automobile, and biomedical fields. In today’s growing market of intelligent equipments and the global pursuit for energy efficiency, there’s an urgent need for mass reduction and environmental durability for SMAs. Conventional SMA classes, such as NiTi-, Cu-, and Fe-based alloys, encounter critical issues of excessive weight. Titanium alloys, in strong contrast, are renowned for their light weight, high specific strength, and excellent corrosion resistance. Nevertheless, Ti-based SMAs are generally susceptible to plastic deformation, resulting in poor recoverability that limits their applications. Departing from the traditional wisdom of eliminating grain boundaries or developing specific texture to optimize the shape-memory properties, in this thesis, we explored solutions in isotropic polycrystalline Ti-based alloys using more widely-applicable processing techniques.In the first part of this thesis, we studied the alloying effect on the shape-memory effect in a series of lightweight TiV-based SMAs. Alloys with nominal compositions of Ti-xV-3.5Al-1Hf-0.8Fe-0.01B (x = 9.5, 11, 12.5, 14, and 15.5 in at.%) were selected for this study. The The research began with the modulation of lattice parameters through varying vanadium content to predict the best shape-memory response. It was found that the lattice constant of the b' exhibited a linear dependence on the vanadium content. Accordingly, a higher SME could be anticipated in alloys with lower V concentration based on the theoretical calculation of transformation strains. Experimental results showed that the best SME of the TiV-based was achieved when the vanadium content was continuously reduced from 15.5 at.% to 12.5 at.%. However, the SME weakened when the vanadium content further decreased, which was attributed to the transition of deformation mechanisms and the appearance of α' martensite phase. The results indicated that the vanadium content of 12.5 at.%. was near the boundary for α' precipitation, which has a hexagonal close-packed (HCP) structured showing no shape-memory response. Meanwhile, optimal SME can be achieved via a complicated deformation process consisting of stress-induced martensitic transformation and reorientation. Thus, it is concluded that the lattice parameters can be theoretically modulated by controlling alloy composition, thereby optimizing the SME. Other important factors, such as phase stability and deformation behaviors, also play critical roles in influencing the SME.
Second, building upon the design principles outlined above, we reported a new type of lightweight SMA (Ti-12.5V-3.5Al-1Hf-0.8Fe-0.01B) with a mass density of ~4.73 g/cm3. We employed conventional thermal-mechanical treatment to attain a fine-grain structure with random grain orientation, which exhibited a maximum recovery strain of ~9.3%, a substantial improvement compared to its coarse-grain counterparts. Studies showed that the growth of α'' martensitic plates could cross grain boundaries both before and during deformation. This grain-boundary penetrating behavior of the α'' phase was investigated from the perspective of its unusual self-accommodation and non-Schmid behavior, both of which substantially influenced the microstructural evolution of the alloy during deformation. As a result, the deformation process became more complicated, consisting of stress-induced martensitic transformation and additional reorientation caused by the martensitic penetration. Such more complicated deformation mechanism was represented by a prolonged stress plateau, a typical feature representative of excellent shape-memory potential. Additionally, the mechanical property of the alloys was also improved by grain refinement to prevent early slip. These factors collectively contribute to improvement in SME.
Lastly, we studied the environmental durability of the Ti-12.5V-3.5Al-1Hf-0.8Fe-0.01B alloy in aqueous environment. The electrochemical and mechanical stability of the alloy were systematically investigated in the 3.5 wt.% NaCl solution. The alloy can generate a resilient passive film showing distinct but stable electrochemical responses. It is found that the reaction of the passive film can shift from the slow accumulation in the passive region, to the rapid buildup of passive layers in extreme anodic potentials. Consequently, the alloy exhibits an unprecedented pitting potential above 10 VSCE without being subjected to localized corrosion. Meanwhile, excellent mechanical reliability is also achieved during stress corrosion tests, owing to the fast repair of the passive film that substantially constrains the crack propagation. Such virtual immunity to seawater corrosion qualifies this titanium alloy as a potential candidate for long-term cost savings and sustainability.
We believe our studies are important to the understanding of deformation micro-mechanisms of high-performance lightwight SMAs and their electrochamical response in extreme environments, but also offer useful guidance for designing lightweight Ti-based SMAs for advanced functional applications.
| Date of Award | 12 Aug 2025 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Tao YANG (Supervisor) |