Shape memory alloys (SMAs)
Shape memory alloys are a unique class of metal alloys that can recover apparent permanent strains when they are heated above a certain temperature. The shape memory alloys have two stable phases - the high–temperature phase, called austenite, and the low–temperature phase, called martensite. There are different kinds of SMAs, and the most famous one is the NiTi alloys and because of the low prices of the FeMnSi systems, it is considered as promising alloys.

FeMnSi based shape memory alloys have been widely studied in recent years. It has been found that the efficiency of the shape memory FeMnSi based alloys is affected by many factors such as pre-strain [1,2] deformation temperature [3, 4], annealing treatment [5, 6] and thermo-mechanical training [5-8]. All of these effects are highly correlated with ↔ transformation, which governs the shape memory effect in FeMnSi based shape memory alloys.

Shape memory effect (SME)

The SME is a unique property of certain alloys exhibiting martensitic transformations. If the alloy is deformed in the low temperature phase, it recovers to its original shape by the reverse transformation upon heating above a critical temperature called the reverse transformation temperature (see Figure 1). This effect was first found by Chang and Read in an AuCd alloy [9]. SMAs could also have the super-elasticity (SE) property at a higher temperature, which is associated with a large nonlinear recoverable strain upon loading and unloading.

One- way shape memory effect
The one-way shape memory effect refers to the memorization of one shape, i.e. the original ‘hot shape’, which is recovered on reheating a deformed sample. The only restriction is that the deformation may not exceed a certain strain limit (up to 8%) [10]; as long as the total strain does not induce permanent plastic flow. The deformation may be of any type (e.g. tension, compression, bending or more complex combinations).

During the one–way shape memory effect internal structural changes take place. When a load is applied to the self–accommodated martensite, the structure becomes deformed through variant rearrangement, resulting in a net macroscopic shape change. When the alloy is unloaded this deformed structure remains, resulting in an apparent permanent strain. If the alloy is now reheated to a temperature above the martensitic transformation range, the original parent phase microstructure and macroscopic geometry is restored. This is possible because no matter what the post–deformation distribution of martensite variants are, there is only one reversion pathway to the parent phase for each variant [11]. If the alloy is cooled again under the martensitic finish temperature, a self–accommodated martensite microstructure is formed and the original shape before deformation is retained. Thus a one–way shape memory is achieved.


Two- way shape memory

In the one-way shape memory effect there is only one shape remembered by the alloy. That is the parent phase shape (so-called hot shape). The two-way shape memory effect is the effect that the material remembers two different shapes: one at the low temperature, and one at the high temperature. This can be achieved without the application of an external force (intrinsic two way effect). The two-way shape memory effect is only obtained after a specific thermo-mechanical treatment, called training.

Pseudo-elasticity or the super-elastic effect

It is possible to induce a phase transformation by applying a pure mechanical load isothermally at a temperature above austenite finish temperature. The result of this load application is fully detwinned martensite and very large strains are observed. The martensite formed in this way is known as stress–induced martensite and is only stable under the application of stress. On unloading, the reduction in stress and surrounding elastic forces generated during the transformation cause the martensite to return back to the original parent phase. This effect is known as pseudo-elastic or super-elastic effect [11]. Reversible strains up to 8% of the initial length can be obtained, compared to 0.2% elastic strain of a common metallic material. Figure 2 shows the mechanical behavior of such super-elastic material, and compares it with a conventional metallic alloy.

Figure 2 Comparison of the stress–strain curves of conventional and super-elastic alloys [11].

Applications of shape memory alloys

Shape memory alloys have a wide range of applications in different fields. NASA uses them in the space industry. They are used in many biomedical applications.


Super-elastic Devices: One Way SME: Two Way SME:

Medical Guide wires

Medical Guide pins

Surgical Localization-
Hooks

Eyeglass Frames

Cellular Telephone-
Antennas

Pipe Coupling

Vibration Dampers

Bendable Surgical

Electrical Connectors

Coffee pot thermostats

Water temperature controller

Green house window actuator

Satellite Release Bolts

Aero-space Actuators

Table 1 The applications of the One-Way SME, the Two-Way SME and the Super-elastic shape memory alloys.

Some other applications:

Broken bones can be mended with shape memory alloys. The alloy plate has a memory transfer temperature that is close to body temperature, and is attached to both ends of the broken bone. From body heat, the plate wants to contract and retain its original shape, therefore exerting a compression force on the broken bone at the place of fracture. After the bone has healed, the plate continues exerting the compressive force, and aids in strengthening during rehabilitation.

Dental wires: used for braces and dental arch wires, shape memory alloys maintain their shape since they are at a constant temperature, and because of the super elasticity of the shape memory alloys, the wires retain their original shape after stress has been applied and removed.

Golf Clubs: A new line of golf putters and wedges has been developed using shape memory alloys are inserted into the golf clubs.


Table 1 summarizes the applications of the One-Way SME, the Two-Way SME and the Super-elastic shape memory alloys.

[1] J. S. Robinson and P. G. McCormick. Scripta Metall. 23, (1989) p.1975.
[2] J. S. Robinson and P.G. McCormick, J. Mater. Sci. Forum56– 58, (1990) p.649.
[3] H. Inagaki. Z. Metall. 83 (1992) p.90.
[4] A. Sato, E. Chishima, T. Mori. Acta Metall. 32, (1984) p.539.
[5] Z.Nishiyama, Martensitic Transformation, Academic Press, (1978) p.306.
[6] G. Ghosh, Y. Vandereken, J. Van Humbeeck, M. Chandraekaran, L. Delaey and W. Van Moorleghem. Proc. MRS Int. Meet. Adv. Mater.9, (1989) p.457.
[7] H. Otsuka, M. Murakami, S. Matsuda. Proc. MRS Int. Meet. Adv. Mater.9, (1989) p.451.
[8] D. Dunne and H. Li. J. Phys. (France) IV 5 (1995) p.C8– 415.
[9] L. C. Chang, T. A. Read. Trans. AIME, 189, (1951) p. 47.
[10] J. Van Hambeeck Advanced engineering Materials 3, No11, (2001) p.837.
[11] K. Worden, W. A. Bullough, J. Haywood, Smart technologies, World Scientific, (2003) p.109.