Low-Temperature Actuation with High Work Output in Nanostructured Ti-Ni-Co Shape-Memory Alloys

Shape-memory alloys (SMAs) are novel materials that have the capability to recover significant inelastic deformation when exposed to heat, a phenomenon known as the shape-memory effect (SME). This unique property has made SMAs promising candidates for actuator applications with the advantages of high energy density, compactness, and lightness. The SME is driven by a reversible martensitic transformation between a low-temperature martensite phase and a high-temperature austenite phase. This transformation dictates the actuation temperature range of SMAs, making it a critical parameter for their functionality. SMAs based on TiNi, Cu, NiMn, and Ti, have martensitic transformations at temperatures above room temperature which make them useful materials in biomedical implants, aerospace structures, and automotive engines. However, the application of SMAs at lower temperatures which are essential in liquefied-gas valves, deep space missions, and polar explorations is still in early stages. To this end, new study published in Journal Advanced Engineering Materials and conducted by PhD candidate Pengfei Dang, engineer Lei Zhang, Professor Yumei Zhou, Cheng Li, Xiangdong Ding, Jun Sun, and led by Professor Dezhen Xue from the Xi’an Jiaotong University, focused on the development of low-temperature SMAs with high actuation work output. They introduced cobalt (Co) into the Ti–Ni alloy system to suppress the martensitic transformation to lower temperatures and created nanocrystalline structure with dense dislocations and nanoprecipitates which strengthen the matrix and avoided plastic deformation under large external loads.

The researchers prepared a series of post-deformation annealing Ti-Ni-Co alloys to investigate the effect of Co doping on martensitic transformation behavior. A quantitative phase diagram for the Ti-Ni-Co alloy system was established, which includes the B2 parent phase, intermediate R phase, and B19’ martensite phase. With the increase of Co doping content, the martensitic transformation is largely suppressed, which allows actuation at lower temperature. Moreover, the authors performed strain–temperature measurements under various biased tensile stresses in order to explore the shape-memory actuation properties of the alloys. They found that a high-biased stress gives rise to a large output strain, as more martensite variants are aligned along the favorable orientation, whereas excessive stress would cause plastic deformation during the phase transformation, leaving the irreversible strain after heating. The strain profiles of the alloys under different tensile stresses were analysed using in situ digital image correlation (DIC). The authors found that alloys with high Co content exhibit homogeneous strain distribution under biased stress, thereby facilitating precise control of actuation strain through temperature adjustments. The team also conducted transmission electron microscopy to study the microstructural changes and found that the doping of Co inhibits the recrystallization process and facilitates the precipitation of nanosized Ti₃Ni₄-like phase. The partially recrystallized state containing dense residual dislocations and nanoprecipitates remarkably strengthen the matrix. which enhances the recoverability of actuation strain under large biased stress.

The authors focused on comparing the work output among various SMAs. The work output is a core parameter for actuation applications, which can be simply defined as the product of the applied biased stress and the recoverable strain. High work output benefits the lightweight design of actuators, especially in microelectromechanical systems. The actuation response of Ti–Ni binary alloys is usually around the ambient temperature and exhibits work output below 25 MJ m^-3. Adding of Hf, Zr, and Pd elements increases the transformation temperature to achieve high-temperature applications. By doping of Co and post-deformation annealing, the present developed alloys exhibit good actuation performance in the low temperature range. Under a high biased stress up to 800 MPa, the best alloy shows a large recoverable strain of 4.5% and a resultant work output of about 36 MJ m^-3. Such a high value outperforms most reported SMAs, especially those at low temperatures.

In conclusion, Professor Dezhen Xue and colleagues provided a detailed understanding of how Co doping and nanostructure development can be utilized to manipulate the martensitic transformation temperatures and enhance the actuation properties of Ti-Ni alloys. The authors highlighted the ability to achieve significant actuation strains at temperatures significantly below room temperature which has not been well studied before and this advancement paves the way for the development of SMAs that can function reliably in cold environments. There are important implications for these developed low-temperature SMAs, for instance they are highly suitable for use in space exploration which makes these alloys ideal for applications such as actuators in spacecraft and satellites. They also can be used in equipment operating in the Arctic and Antarctic regions which will benefit from SMAs that maintain functionality and reliability in sub-zero temperatures and enhanced the efficiency and safety of scientific missions. Additionally, they can be used in medical field for cryogenic preservation or in industrial applications involving liquefied gases.  Finally, the new SMAs can be advantageous in automotive and aerospace industries for applications such as morphing structures, valve actuators, and adaptive systems that requires precise control and actuation under varying thermal conditions.