The most common SMA is Nitinol (Ti3Ni4), and research into it dates back to research by the United States Navy, hence it’s name, Nickel Titanium Navy Ordinance Labs (Nitinol). The morphing affect of SMAs relies on the temperature dependent phase transformation between austenite and martensite. The shape memory effect is possible through reversibility of the martensite transformation via self-accommodation of martensitic plates.
Shape memory alloys rely on a transformation sequence between martensite and austenite. A shape memory material may be mechanically deformed while exhibiting a martensitic phase, and return to its original shape when heated above its transformation temperature. When heated, the material goes through a reverse transformation, the phase changing from martensite to austenite. Upon cooling from the austenite phase, the material crystallography returns to martensite. The transformation is rather versatile, enabling the manufacture of three different types of shape memory effects: one-way, two-way, and mechanical shape memory.
The narrow composition tolerance of Nitinol, and cost of the raw materials makes it difficult to manufacture in large bulk. Advances are being made in the area of iron and copper based SMAs, which offer a lower material cost. Super elastic tendons and texture-changing surfaces are common applications of Nitinol-based smart material system designs.
However, the phase transformation dependence on temperature renders a poor actuation response time unless the material can be heated and cooled at high rates. This implies that the surface area to volume ratio of a Nitinol actuator should be very high, such as occurs with a Nitinol coating as opposed to a Nitinol beam actuator. Despite a great deal of research expenditure, Nitinol based smart material designs have seen limited success. One drawback of SMAs such as Nitinol is the accumulation of dislocations in the crystal lattice, which can limit the service life and long-term reliability of Nitinol actuators. Over millions of fatigue cycles degradation in shape recovery can be seen.
