Shape Memory Alloys (SMA)
Renishaw Advanced Materials has created a number of novel applications for these intriguing materials and has through its staff, more than twenty years experience in the application and qualification of shape memory alloys.
What are SMAs?
Metal alloys and polymers can exhibit shape recovery, consequent to the removal of a load that deforms the material. In the case of the metal alloys, thermally activated shape recovery and elastic recovery can be seen. A stretched elastic band when released is an example of elastic recovery or release of the energy stored in the stretched form. Renishaw Advanced Materials uses the well known, near equi-atomic nickel-titanium alloys known as Nitinol.
There are two properties that can be exploited from shape memory alloys; thermally induced shape recovery and elastic shape recovery. In both cases, the “memory” is established by heating the alloy in a desired shape between 450 - 550 ˚C. It is then rapidly cooled and retains the recoverable memory, when heated to a transition point.
Process and Properties of SMAs
Ni/Ti is a very tough material, difficult to machine using traditional methods. Its corrosion resistance is good: although hot acids will attack the metal, normal environmental conditions have little effect. Renishaw Advanced Materials has more than 25 years experience in the processing and application of shape memory alloys and has introduced a number of novel methods of exploiting the metal, such as achieving proportional movement, (compared to its essentially “digital” response) when heated. A composite of textiles and shape memory alloy have been devised and a method of heat treating the in-situ textile at temperatures that would normally cause irreparable damage. This textile-shape memory composite has diverse applications in the fashionable apparel and technical textile arenas.
The alloys can and are being used in many industrial sectors; the medical field being the most active, with stents, guide-wires, graft reinforcement, orthodontics, surgical staples and blood-clot filters being some of the current commercial applications. Aerospace (both space and aircraft) also utilise the materials in the form of actuators, for example, satellite solar panel deployment or locking, aircraft fuel and hydraulic line connectors. The elastic, or super-elastic version of the shape memory alloys have found extensive use for spectacle frames; the frame is able to be severely deformed and recover its normal shape without permanent deformation. Compared to other springy metals, like spring steel, super-elastic Ni/Ti has approximately 10 times the recoverable strain.