NITINOL

The most famous Shape Memory Alloy used in industrial applications is Nitinol (NiTi) than CuZnAl alloy. The properties of NiTi and CuZnAl alloys are different, due to their different microstructure. NiTi alloys have much higher strength, larger recoverable strain, better corrosion resistance and most importantly higher reliability than CuZnAl, they are the standard choice for use in space and several other applications [1].

Table 2‑1 Comparison of SMA Materials

	NiTi	Cu Al Zn
Recovery strain	max 8%	Max 6%
Recovery stress	max  400 Mpa	Max 300 Mpa
Number of cycles	105 ( e = 0.02) 107 ( e = 0.005)	102 ( e = 0.02) 105 ( e = 0.005)
Corrosion resistance	good	Poor
Workability SM processing	Poor fair processing comparatively easy	Fair easy fairly difficult

1.1.1        Shape Memory Effect and Super Elasticity

At lower temperatures, SMA can be seemingly plastically deformed, but this plastic strain can be recovered by increasing the temperature which is called as the Shape Memory Effect. At higher temperatures, a large deformation can be recovered simply by releasing the applied force, and this behaviour is called as Super Elasticity [2] .

                                                Figure 2‑1 SME vs Supererelastisity [2]

1.1.2        Phase Transformation

In order to induce the austenite to martensitic transformation, the chemical free energy of the martensitic phase must be lower than that of the austenite phase. However, since the transformation requires also non-chemical free energy, such as transformation strain energy, friction energy, etc.; the transformation can happen only when the difference between the chemical free energies of the two phases is greater than the necessary non-chemical free energy. Similarly, the transformation from martensite to austenite will occur when the difference between the above energies, with their signs reversed, is sufficiently large. There is a clear difference between the forward and reverse transformation paths [3]

                     

Figure 2‑2 Phase Transformation [2]

Another crucial characteristic of SMAs is the thermoplastic nature of these phase transformations, which means;

• The transformations can be induced either by a temperature change, or by an applied stress both of which change the free energy of the material. The volume change associated with them is very small. All deformation in transformations is recoverable.
• Once martensitic crystals have nucleated, they grow at a rate proportional to the cooling rate or the rate of increase of the applied stress. Similarly, these crystals shrink upon heating and it has been found that in thermally induced transformations, the martensitic crystals that were formed first are the last to undergo the reverse transformation [4].

1.1.3        Two-way Shape Memory Alloys

A special type of austenite, martensitic transformation, called Two-way Shape Memory Effect, is obtained by a special process normally consisting of a series of thermal cycles, during which high-temperature and low-temperature shapes are imposed on a SMA component. Thus, one obtains a Two-way Shape Memory Alloy, which can remember both its low temperature shape (martensitic) and high temperature shape (austenite). TWSMAs

switch from their low temperature shape to their high temperature shape, but their recoverable strain is usually about half of the corresponding one-way recoverable strain for the same material. It is believed that the internal stress field produced in the material during the training process influences the growth of martensitic. Thus, during cooling; the induced martensitic variants are infamous of both internal stress and external stress, plus boundary constraints [5].

Figure 2‑3 Schematic Illustration of the mechanism of shape memory effect [2]

1.1.4        Physical Properties of Nitinol

Table 2‑2 Physical properties of Nitinol [2]

Melting point	1,240 - 1310oC
Density	6,450 - 6,500 Kg/m3
Thermal conductivity	Austenite       18 W/moC Martensite     8.6 - 10 W/moC
Coefficient of thermal expansion	Austenite       11x10-6  oC Martensite     6.6x10-6  oC
Specific heat	490 J/KgoC
Electrical resistivity	Austenite         0.82x10-6  Ωm Martensite      0.76x10-6  Ωm

1.1.5        Mechanical Properties of Nitinol

Table 2‑3 Mechanical properties of Nitinol  [2]

Young's Modulus	Austenite         75-83 GPa Martensite      28 -41 GPa
Poisson's ratio	0.33
Ultimate tensile strength	Fully annealed        895 MPa Work hardened    1900 MPa
Fatigue strength (N=106)	350 MPa