An Introduction to Shape Memory Materials

Shape memory materials (SMMs) are featured by the ability to recover their original shape from a significant and seemingly plastic deformation upon a particular stimulus is applied¹. This is known as the shape memory effect (SME). Superelasticity (in alloys) or visco-elasticity (in polymers) is also commonly observed if a SMM is deformed at the present of the stimulus. The SME can be utilized in many fields, from aerospace engineering (e.g., in deployable structures and morphing wings) to medical devices (e.g., in stents and filters).

Despite that the SME had been found as early as 1932 in AuCd alloy, this phenomenon was apparently not so attractive until 1971, when significant recoverable strain was observed in a NiTi alloy at the Naval Ordnance Laboratories, USA². Till today a range of shape memory alloys (SMAs) have been developed. Among them, only three alloy systems, namely NiTi-based, Cu-based (CuAlNi and CuZnAl) and Fe-based, are presently more of commercial importance.

A systematic comparison of NiTi, CuAlNi and CuZnAl SMAs, in terms of various performance indexes, which are of engineering application interest, has been done³. While NiTi should be the first choice due to its high performance and good biocompability, which is crucial in biomedical applications, for instance stents and guide wires in minimally invasive surgery, Cu-based SMAs have the advantages of low material cost and good workability in processing. The SME in Fe-based SMAs is relatively much weaker and they are most likely used only as fastener/clamp for one-time actuation largely due to the extremely low cost. All these SMAs are thermo-responsive, i.e., the stimulus required to trigger shape recovery is heat.

In recent years, good progress has been made in developing ferromagnetic SMAs, which are magneto-responsive. But from the real engineering application aspect, thermo-responsive SMA is more mature and many commercial applications have been realized so far^1,2,4. With the current trend toward Micro-Electro-Mechanical Systems (MEMS) and even Nano-Electro-Mechanical Systems (NEMS), thin film SMA (mainly NiTi based, produced by sputter deposition) has become a promising candidate for motion generation in these micron/submicron sized systems.

For the fundamentals and device applications of thin film SMAs, readers may refer to a recent book published by the Cambridge University Press⁵. In addition to the SME, some of the SMAs also have the temperature memory effect (TME), so that the highest temperature(s) in the previous heating process(es) within the transition range can be precisely recorded and revealed in the next heating process.

Tailoring the material properties of polymers is much easier as compared with that of metals/alloys. In addition, the cost (both material cost and processing cost) of polymers is traditionally much lower. A variety of shape memory polymers (SMPs) have been invented and well-documented, while new ones keep on emerging in every week, if not every day, at present⁶. In addition to the above-mentioned advantages, SMPs are much lighter, have much higher (an order higher at least) recoverable strain than SMAs, and can be triggered for shape recovery by various stimuli and even multiple stimuli simultaneously. Light (UV-light) and chemical (moisture, solvent and pH change) are two types of such stimuli, in addition to heat⁷. The underline mechanism for the SME in SMPs is the dual-segment system (one is always hard/elastic, while the other can be soft/ductile or stiff depending on whether a right stimulus is presented), which is different from the reversible martensitic transformation in SMAs.

SMP composites have remarkably widened the potential applications of SMPs⁸. For example, heating of thermo-responsive SMPs can also be realized by joule heating (by filling with conductive inclusions), induction heating (through energy dissipation due to hysteresis upon applying an alternating magnetic/electrical field, etc.). Opposite to SMAs, as SMPs normally become softened at the present of the right stimulus, most of SMPs are not suitable for cyclic actuation and cannot be trained to have the so-called two-way SME (which is the ability to repeatedly switch between two shapes, depending on whether the stimulus is applied). On the other hand, the shape recovery of SMPs can be accompanied by color change, and even the shape change sequence (not only two shapes/positions, i.e., dual-shape, during recovery, but also reaching three shapes, i.e., tri-shape, or even more) can be programmed by means of setting different recovery conditions (e.g., different stimuli or multiple (and even gradient) shape recovery temperatures).

As we can see, SMPs can be synthesized/designed to have the required properties for a particular application. However, strong background (professional knowledge and experience) is required atop trial and error. Shape memory composites (SMCs), which include at least one type of SMM, either SMA or SMP, as one of the components (e.g., Ref. 9), can be handled comfortably by design engineers, if the properties of SMA/SMP are well known.

Shape memory hybrid (SMH) is a more accessible and flexible approach to ordinary people, even with only limited science/engineering background. SMHs are made of conventional materials (properties are well-known and/or can be easily found, but all without the SME as an individual). Hence, one can design a SMM in a do-it-yourself (DIY) manner with the required function(s). For demonstration purpose, we have developed a SMH system. All features found in SMAs and SMPs, namely, dual-shape SME, multi-shape SME, two-way reversible SME, thermo-responsive (including by means of joule heating), moisture-responsive, etc. have been reproduced. A narrow shape recovery temperature range within 5oC has been achieved. In addition, a rubber-like (not only in high temperature range as in SMAs, but within the full concerned application temperature range, and more importantly with tiny hysteresis) SMH with self-healing function (not only for shape recovery, but also for repeat crack healing) has been demonstrated (Figure 1). Design of pressure-responsive SMH, thermo (upon cooling or at extremely high temperature)-responsive SMP is in progress.

Figure 1. Rubber-like SMH (cyclic loaded at low temperature and high speed).

References

1. Otsuka, K. and Wayman, C.M., Shape Memory Materials, Cambridge University Press, 1998.
2. Funakubo, H., Shape Memory Alloys, Gordon and Breach Science Publishers, 1987.
3. Huang, W., On the selection of shape memory alloys for actuators, Materials and Design, Vol. 23, 2002, pp11-19.
4. Duerig, T.W., Melton, K.N., Stöckel, D. and Wayman C.M., Engineering Aspects of Shape Memory Alloys, London: Butterworth Heinemann, 1990.
5. Miyazaki, S., Fu, Y.Q. and Huang, W.M., Thin Film Shape Memory Alloys: Fundamentals and Device Applications, Cambridge University Press, 2009.
6. Mather, P.T., Luo, X. and Rousseau, I.A., Shape memory polymer research, Annual Review of Materials Research, Vol. 39, 2009, pp445-471.
7. Leng, J., Lu, H., Liu, Y, Huang, W.M. and Du, S., Shape memory polymers-A class of novel smart material, MRS Bulletin, Vol. 34, 2009, pp848-855.
8. Gunes, I.S., Jana, S.C., Shape memory polymers and their nanocomposites: A review of science and technology of new multifunctional materials, Journal of Nanoscience and Nanotechnology, Vol. 8, 2008, pp1616-1637.
9. Tobushi, H., Pieczyska, E., Ejiri, Y. and Sakuragi, T., Thermomechanical properties of shape-memory alloy and polymer for their composites, Mechanics of Advanced Materials and Structures, 16, 2009, pp236-247.