Shape Memory Alloys (SMA)

 

What are They?

 

Shape Memory alloys are metals that exhibit the properties of pseudo-elasticity and the Shape Memory Effect. 

Pseudo-elasticity- an almost rubber-like flexibility

Shape Memory Effect – unique ability of materials to be severely deformed and then return to their original shape through stimulus

Current popular Shape Memory Alloys include NiTi (Nickel Titanium), CuZnAl, and CuAlNi.

 

How do SMAs work?

 

A solid state phase change occurs which involves a molecular rearrangement.  The molecules are so tightly packed, however, that the metal remains in a solid state throughout the entire transformation.  A temperature increase of only 10 degrees Celsius is required for the phase change to occur.  The two phases that the alloy can exist in are:

Martensite – the lower temperature phase of a Shape Memory Alloy where the metal is deformable

Austenite – the higher temperature, strong phase of the Shape Memory Alloy

Pseudo-elasticity occurs in the Austenite phase without a change in temperature.  The load on the Smart Memory Alloy is increased until it turns into Martensite.  When the load is reduced, the SMA reverts back into Austenite (SMA/MEMS Research Group). 

 

Application of SMAs

Aeronautics:

Alloys can be used to create flexible wings in aircrafts using Shape Memory wires.  The wire can be heated with an electric current and can then be bent down or stretched out.  This eliminates the need for hinge as in used in aircrafts today.  It also eliminates large hydraulic lines which reduce weight, maintenance and the cost of aircrafts.  The main groups doing research in this field are currently the Defense Advanced Research Project Agency (DARPA) and Boeing (SMA/MEMS Research Group). 

 

Surgical Tools:

Bone plates can be made of NiTi molecules and used exactly like a current bone plate is used.  The Nickel Titanium is cooled during surgery and then as it heats up to the body temperature, it attempts to contract which applies pressure to the break site.  This steady pressure aids the healing process and reduces recovery time of a major bone break (Lenau).

 

Muscle Wires:

SMAs are very good at mimicking human muscles and tendons so they can be used to create humanlike movements in robots.  A computer can be used to control an electric current through the Shape Memory wires which will heat and cool the wire to bend and extend, mimicking something like the movement of a finger (Lenau).