It is recognized that when shape memory alloys are transformed from complex rhombus crystal structure to simple cubic structure, it will produce memory of shape recovering. And when memory alloy restore the original shape, huge force will be produced. Nitinol alloy can produce 60kg/mm force, which is much bigger than the force of previous shape changing. Normally, it can be 10 times bigger. So that means the outcoming energy is much bigger than the input energy.
Memory alloy already been used in pipeline combination and automation control. Casing made by memory alloy can replace welding. The method is to expand the inner tube about 4% under low temperature, then after heating, the casing shrinks to original shape to form a close joint. The US marine’s airplanes used 100,000 pcs casing for hydraulic system and never have any oil leaking or broken problem in many years. If the tubes of vessel or submarine oil field has been damaged, it is quite easy to be repaired by memory alloy parts. In some inconvenient position, we can use memory alloy to make dowels, put into holes and heat them, the ends will split and become curvy to form single side assembly parts.
Memory alloy are specially suitable for thermal mechanical and automatic control in constant temperature. We have developed automatic arms which can open the windows during daylight and close them when temperature drops at night. The design project of memory alloy heat engine also have many different kinds. All of them can be used between two transmission mediums which have a low temperature difference. This can be a new way to make use of industrial cooling water, nuclear reactor waste heat, ocean temperature difference and solar energy. The ubiquitous problem is low efficient which is only 4%~6% and need to be improved.
The application of memory alloy in medical field is also quite noticeable. Such as bone lamella to combine the broken bones, it can only not fix the two broken bones, but also produce compression during the recovering time to force the broken bones joint together. The wire used for orthodontics, the long clip used for brain aneurysm and vas deferens and the support plate for manipulative restoration are all boosted by the human temperature after being planted into human body. The thrombus filter is also a new product of memory alloy. The straight filter will become webbed slowly after being planted into vein. This can stop 95% sludged blood to flow to heart and lung.
Artificial heart is more complicated. Muscle fiber made by memory alloy and stretchy film ventricular work together and imitate the ventricular contractions. Now the pump can deliver water.
Since memory alloy is an alloy with life, we can design many different kinds of automatic control device using the advantage that memory alloy can be reformed under certain temperature. The application of memory alloy is now wider and wider.
Superelasticity means the test sample produced far more strain than the elastic limit, but can be recovered by itself when unloading. That is, under parent phase, external stress causes martensite phase transformation. So the alloy appears to show different mechanical behavior than other normal materials. Its elastic limit is far more beyond normal material and do not abbey Hooke’ law. Compare to shape memory character, superelasticity does not involve with heat. In all, superelasticity refers to the stress in a certain deformation range increases with the increase of strain. It can be separated to linear superelasticity and non-linear superelasticity. The stress - strain curve of the former one is close to the linear relationship. Non-linear superelasticity means the results of stress induced martensite phase transition and inverse phase change in the process of loading and unloading of the above temperature interval are obtained. So non-linear superelasticity is also called as transformation pseudo-elasticity. The transformation pseudo-elasticity of nitinol can reach around 8%. The superelasticity of nitinol will be changed along with heat treatment condition. When the arch wire is heated to above 400C, the superelasticity begins to decline.