Nature is full of magic materials, which are to be discovered in forms suitable to our needs. Such magical materials, also known as intelligent or smart materials, can sense, process, stimulate and actuate a response. Smart materials have one or more properties that can be dramatically altered. Most everyday materials have physical properties, which cannot be significantly altered; for example if oil is heated it will become a little thinner, whereas a smart material with variable viscosity may turn from a fluid which flows easily to a solid. A variety of smart materials already exist, and are being researched extensively. Some everyday items are already incorporating smart materials (coffeepots, cars etc) and the number of applications for them is growing steadily.
What Are Shape Memory Alloys?
Shape memory alloys (SMA's) are metals, which exhibit two very unique properties, pseudo-elasticity(An almost rubber-like flexibility demonstrated by shape memory alloys), and the shape memory effect(The unique ability of shape memory alloys to be severely deformed and then returned to their original shape simply by heating them). The most effective and widely used alloys include NiTi (Nickel - Titanium), CuZnAl, and CuAlNi.
Working of Shape memory alloys
The two unique properties described above are made possible through a solid-state phase change, that is a molecular rearrangement, which occurs in the shape memory alloy. A solid-state phase change is similar in that a molecular rearrangement is occurring, but the molecules remain closely packed so that the substance remains a solid. The two phases, which occur in shape memory alloys, are Martensite, and Austenite.
Martensite is the relatively soft and easily deformed phase of shape memory alloys, which exists at lower temperatures. The molecular structure in this phase is twinned as shown Figure 2. Upon deformation this phase takes on the second form shown in Figure 2, on the right. Austenite, the stronger phase of shape memory alloys, occurs at higher temperatures. The shape of the Austenite structure is cubic.
Applications of Shape Memory Alloys
The unusual properties mentioned above are being applied to a wide variety of applications in a number of different fields.
Aircraft maneuverability depends heavily on the movement of flaps found at the rear or trailing edge of the wings. The efficiency and reliability of operating these flaps is of critical importance.
Most aircraft in the air today operate these flaps using extensive hydraulic systems. These hydraulic systems utilize large centralized pumps to maintain pressure, and hydraulic lines to distribute the pressure to the flap actuators. In order to maintain reliability of operation, multiple hydraulic lines must be run to each set of flaps. This complex system of pumps and lines is often relatively difficult and costly to maintain.
Many alternatives to the hydraulic systems are being explored by the aerospace industry. Among the most promising alternatives are piezoelectric fibers, electrostrictive ceramics, and shape memory alloys.
"Smart" wings, which incorporate shape memory alloys, are typically like the wing shown in Figure 6, this system is much more compact and efficient, in that the shape memory wires only require an electric current for movement.