Introduction

Introduction:

Shape memory alloys or in short form SMA are materials that can return its original state after deformation. SMAs are sensitive to heat changes. They can have 2 different crystal states when their temperature is upper or lower than their critical temperature. Materials that can be easily deformed at low temperatures and be easily turned into original states at high temperatures. The most commonly used ones are Ni-Ti and copper based alloys because these alloys can produce significant force during the shape-shifting. In these days, SMAs are used as simultaneous detectors and actuators and so they attract considerable attention.

The most commonly used SMAs in industry are Ni-Ti alloy and copper based alloys. Ni-Ti are perfect with their corrosion resistance and much more ductility when they are compared to copper based alloys as copper based alloys have less thermal determination and are more sensitive to corrosion. However copper based alloys are cheaper, easier to be melted and has a wider range of potential thermal transition. Ultimately, both alloys have their own advantages and disadvantages depending on in what environment they will be used.

Ni-Ti alloy is a dual alloy frame. Such a compound can dissolve extra nickel and titanium in acceptable limits and it has a ductility on a level that can be compared to conventional alloys. Due to this extraordinary ability of dissolving, other element can be add in order to change either the transition pattern or the mechanical properties at will. Even approximately 1% nickel addition will affect the properties of alloy frame. Contaminations, for instance oxygen and carbon, are not wanted in the structure as they change the transition heat and weaken the mechanical properties.

Copper based alloys may consist of three metals, such as CuZnAl and CuAlNi or they may appear in modification as they consist of four that contain manganese. In order to make thinner granular elements such as boron, cerium, cobalt, iron, titanium, vanadium and zirconium are added to the structure. Besides mangan lower the transition heat of both CuZnAl and CuAlNi and it changes the point of eutectoid of the alloys that contain high incidence of aluminum. Mangan is also added instead of aluminum for better ductility. Long duration of heats should be avoided since it causes zinc to vaporize and growth of grain. Feeding water is used for hardening. Cooling outdoor should be enough for alloys that contain high incidence of aluminum. However transition temperatures of cooled parts are generally undetermined and in order to make it become determined ageing should be done at temperature that higher than AF temperature. Thermal determination of copper based alloys are limited by their disintegration kinetics. Therefore the alloys, CuZnAl and CuAlNi, should not be exposed to temperatures higher than 150~200^oC. Ageing at temperature lower than that changes the transition temperatures. Ageing at beta phase will cause similar consequences. Alloys that were aged in martensitic state show martensitic stabilization effect which involves ageing. CuAlNi alloy, at high temperatures, are more determined than CuZnAl alloy. Therefore when strict control of transition temperatures is required in different temperature applications, these factors should be considered.