What is a shape memory alloy?
Shape memory alloys or SMA's are metals that exhibit shape memory properties.
The shape memory effect was first discovered in 1932 in a silver-cadmium alloy. It allows materials possessing shape memory properties to return to their original shape after having suffered some form of deformation after they are heated to temperatures above their transformation temperature.
It is possible to have a transformation temperature below ambient, in which case the alloy will behave like a spring.
At the phase transformation temperature, the alloy undergoes a crystalline reversible solid state phase change from martensite to austenite. It should be noted that both phases have different properties.
How do they work?
Over a range of temperatures, starting at the transformation temperature, the alloy undergoes a reversible solid state transformation. The transformation temperature is influenced by composition and other factors.
How much deformation can a shape memory alloy take and still recover?
Shape memory alloys can recover from large amounts of bending and torsional deformations as well as small amounts of strain. Provided the deformations are within recoverable ranges, the process of deformation and shape recovery can be repeated millions of times.
What metals exhibit shape memory characteristics?
Commercially available shape memory alloys include:
• Nickel/Titanium alloys such as Nitinol and Tinel
• Copper/Zinc/Aluminium Alloys
• Copper/Aluminium/Nickel Alloys
Other alloys that are known to display shape memory properties are:
• Silver/Cadmium Alloys
• Gold / Cadmium alloys
• Copper / Tin alloys
• Copper / Zinc alloys
• Indium / Titanium alloys
• Nickel / Aluminium alloys
• Iron / Platinum alloys
• Manganese / copper alloys
• Iron / Manganese / Silicon alloys
Where are the applications for shape memory alloys?
The medical and aerospace and marine industries are the largest consumers of shape memory components. Some of their applications are outlined below, together with some less well know application areas:
Medical Industry -
• Stents - A device used to treat coronary disease. It would be inserted in the deformed shape and would expand upon reaching body temperature to open arteries and increase blood flow.
• Vena-Cava Filters - A device used to trap blood clots. Inserted as a small cylinder, it reverts to an umbrella shaped filter to trap small blood clots and prevent them from travelling to parts of the body where they may have a detrimental effect.
• Dental and Orthodontic Archwires - These work similar to a spring. They apply a continuous and gentle force correcting misaligned teeth, as opposed to the periodic and uncomfortable tightening required by stainless steels.
Aerospace and Marine -
• Fluid Fittings - SMA couplings are available that seal metal to metal with large radial clamping forces. Supplied in cold couplings are simply placed over the pipe to be connected and they shrink as they warm up.
Other areas of Application -
Other areas where shape memory alloys are use include:
• Spectacle frames
• Underwired brassieres
• Pipe jointing systems
• Temperature control systems