Fundamental Characteristics of Shape Memory Alloys

History of shape memory alloys

The three main types of SMA are the copper-zinc-aluminum-nickel, copper-aluminium-nickel, and nickel-titanium (NiTi) alloys and the most commonly seen is the NiTi. The first reported steps towards the discovery of the shape memory effect were taken in the 1930s. According to Otsuka and Wayman A. Ölander discovered the pseudoelastic behavior of the Au-Cd alloy in 1932. Greninger & Mooradian observed the formation and disappearance of a martensitic phase by decreasing and increasing the temperature of a Cu-Zn alloy. The basic phenomenon of the memory effect governed by the thermoelastic behavior of the martensite phase was widely reported a decade later by Kurdjumov & Khandros and also by Chang & Read. In the early 1960s, Buehler and his co-workers at the U.S. Naval Ordnance Laboratory discovered the shape memory effect in an equiatomic alloy of nickel and titanium, which can be considered a breaktrought in the field of shape memory materials. This alloy was named Nitinol (Nickel-Titanium Naval Ordnance Laboratory). Since that time, intensive investigations have been made to elucidate the mechanics of its basic behavior. The use of NiTi is fascinating because of its superelasticity and shape memory effect, which are completely new properties compared to the conventional metal alloys.

General principles

NiTi shape memory metal alloy can exist in a two different temperature-dependent crystal structures (phases) called martensite (lower temperature) and austenite (higher temperature or parent phase). Several properties of austenite NiTi and martensite NiTi are notably different. When martensite NiTi is heated, it begins to change into austenite.
The temperature at which this phenomenon starts is called austenite start temperature (As). The temperature at which this phenomenon is complete is called austenite finish temperature (Af). When austenite NiTi is cooled, it begins to change onto martensite. The temperature at which this phenomenon starts is called martensite start temperature (Ms). The temperature at which martensite is again completely reverted is called martensite finish temperature (Mf).

Composition and metallurgical treatments have dramatic impacts on the above transition temperatures. From the point of view of practical applications, NiTi can have three different forms: martensite, stress-induced martensite (superelastic), and austenite. When the material is in its martensite form, it is soft and ductile and can be easily deformed (somewhat like soft pewter). Superelastic NiTi is highly elastic (rubber-like), while austenitic NiTi is quite strong and hard (similar to titanium). The NiTi material has all these properties, their specific expression depending on the temperature in which it is used.


Shape memory effect

NiTi senses a change in ambient temperature and is able to convert its shape to a preprogrammed structure. While NiTi is soft and easily deformable in its lower temperature form (martensite), it resumes its original shape and rigidity when heated to its higher temperature form (austenite). This is called the one-way shape memory effect. The ability of shape memory alloys to recover a preset shape upon heating above the transformation temperatures and to return to a certain alternate shape upon cooling is known as the two-way shape memory effect.

Superelasticity

Superelasticity (or pseudoelasticity) refers to the ability of NiTi to return to its original shape upon unloading after a substantial deformation. This is based on stress-induced martensite formation. The macroscopic deformation is accommodated by the formation of martensite. When the stress is released, the martensite transforms back into austenite and the specimen returns back to its original shape. Superelastic NiTi can be strained several times more than ordinary metal alloys without being plastically deformed, which reflects its rubber-like behavior. It is, however, only observed over a specific temperature area.

Mechanical properties of NiTi

The mechanical properties of NiTi depend on its phase state at a certain temperature. NiTi has an ability to be highly damping and vibration-attenuating below As. For example, when a martensic NiTi ball is dropped from a constant height, it bounces only slightly over half the height reached by a similar ball dropped above the austenite temperature. NiTi has unique high fatigue and ductile properties, which are also related to its martensitic transformation. Also, very high wear resistance has been reported compared to the CoCrMo alloy. NiTi is a non-magnetic alloy. Electrical resistance and acoustic damping also change when the temperature changes.