SHAPE MEMORY ALLOYS

Shape Memory Alloys (SMA’s) are novel materials which have the ability to return to a predetermined shape when heated. When an SMA is cold, or below its transformation temperature, it has a very low yield strength and can be deformed quite easily into any new shape–which it will retain. However, when the material is heated above its transformation temperature it undergoes a change in crystal structure which causes it to return to its original shape. If the SMA encounters any resistance during this transformation, it can generate extremely large forces. This phenomenon provides a unique mechanism for remote actuation.

The most common shape memory material is an alloy of nickel and titanium called NiTinol. This particular alloy has very good electrical and mechanical properties, long fatigue life, and high corrosion resistance. As an actuator, it is capable of up to 5% strain recovery and 50,000 psi restoration stress with many cycles. By example, a NiTinol wire 0.020 inches in diameter can lift as much as 16 pounds. NiTinol also has the resistance properties which enable it to be actuated electrically by joule heating. When an electric current is passed directly through the wire, it can generate enough heat to cause the phase transformation. In most cases, the transition temperature of the SMA is chosen such that room temperature is well below the transformation point of the material. Only with the intentional addition of heat can the SMA exhibit actuation. In essence, NiTinol is an actuator, sensor, and heater all in one material.

Shape memory alloys, however, are not for all applications. One must take into account the forces, displacements, temperature conditions, and cycle rates required of a particular actuator. The advantages of NiTinol become more pronounced as the size of the application decreases. Large mechanisms may find solenoids, motors, and electromagnets more appropriate. But in applications where such actuators cannot be used, shape memory alloys provide an excellent alternative. There are few actuating mechanisms which produce more useful work per unit volume than NiTinol.

NiTinol is available in the form of wire, rod and bar stock, and thin film. Examples of SMA products developed by TiNi Alloy Company include silicon micro-machined gas flow microvalves, non-explosive release devices, tactile feedback device (skin stimulators), and aerospace latching mechanisms. If you are considering an application for shape memory alloys, TiNi Alloy Company can assist you in the design, prototyping, and manufacture of actuators and devices.

SHAPE MEMORY ALLOY PROPERTIES

A relatively new group of materials that exhibit an interesting phenomenon are the shape memory alloys or (SMAs) A shape memory alloy is polymorphic. It may two crystal structures or phases and shape memory effect involves phase transformation between them. One phase (termed an austenite phase) has a Face centered cubic crystal structure and other phase (termed as martensite) has Body centered Tetragonal structure.

Shape memory alloys are a type of material that uses to meet a growing demand for light weight, powerful actuators that can be scaled down. Memory materials exhibit the ability to induce large mechanical strains upon heating and cooling many shape memory materials are metal alloys; therefore, they can also produce large mechanical stress when thermally activated. These properties make them well suited for applications in controllable shape change, vibration control, and it can be used as a damper.

SMAs display the shape memory effect (SME) when annealed appropriately. This effect describes a material’s ability to return to a predetermined shape through heating after being plastically deformed from that shape. Heating can be achieved through Joule heating via electrical current.




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