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What are shape memory alloys? How do they work? Give their applications.
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Shape Memory Alloys (SMAs) are metals that remember their original shapes. SMAs are useful for such things as actuators which are materials that change shape, stiffness, position, natural frequency, and other mechanical characteristics in response to temperature or electromagnetic fields.

 

Shape Memory Alloys 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.

 

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.

 

Working:

Shape memory alloys display two distinct crystal structures or phases. Temperature and internal stresses determine the phase that thein which the SMA will be. Martensite exists at lower temperatures and austenite exists at higher temperatures.

When a SMA is in martensite form at lower temperatures, the metal can easily be deformed into any shape. When the alloy is heated, it goes through transformation from martensite to austenite. In the austenite phase, the memory metal "remembers" the shape it had before it was deformed.

From the stress vs. temperature graph below, one can see that at low stress and low temperature, martensite exists. At higher temperature and higher stress, austenite exists.

Memory alloys also demonstrate great rates of super-elasticity. For example, eyeglass frames are in a martensite phase. Bending the arms in half (at room temperature) introduces a phase change at the bend to austenite. Austenite is not stable at room temperature, and because systems always seek lower energy states, the austenite will change back to the martensite phase, and to do this, the arm must bend back.

Applications:

  • Bones: 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.

     

  • Reinforcement for Arteries and Veins: For clogged blood vessels, an alloy tube is crushed and inserted into the clogged veins. The memory metal has a memory transfer temperature close to body heat, so the memory metal expands to open the clogged arteries.

     

  • Dental wires used for braces and dental arch wires

     

  • Anti-scalding protection: Temperature selection and control system for baths and showers. Memory metals can be designed to restrict water flow by reacting at different temperatures, which is important to prevent scalding.

     

  • Fire security and Protection systems: Lines that carry highly flammable and toxic fluids and gases must have a great amount of control to prevent catastrophic events. Systems can be programmed with memory metals to immediately shut down in the presence of increased heat.

     

  • GolfClubs: Shape memory alloys are inserted into the golf clubs. These inserts are super elastic which keep the ball on the clubface longer.

     

  • Helicopter blades: Performance for helicopter blades depend on vibrations with memory metals in micro processing control tabs for the trailing ends of the blades, pilots can fly with increased precision.

     

  • Eyeglass Frames: In certain commercials, eyeglass companies demonstrate eyeglass frames that can be bent back and forth and retain their shape.

     

  • Tubes, Wires, and Ribbons: For many applications that deal with a heated fluid flowing through tubes, or wire and ribbon applications where it is crucial for the alloys to maintain their shape in the midst of a heated environment, memory metals are ideal.

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