The transition from the martensite phase to the austenite phase is only dependent on temperature and stress, not time, as most phase changes are, as there is no diffusion involved. Similarly, the austenite structure gets its name from steel alloys of a similar structure. It is the reversible diffusionless transition between these two phases that allow the special properties to arise. While martensite can be formed from austenite by rapidly cooling carbon-steel, this process is not reversible, so steel does not have shape memory properties.
In this figure, ξ (T) represents the martensite fraction. The difference between the heating transition and the cooling transition give rise to the shape of the curve depends on the material properties of the shape memory alloy, such as the alloying and work hardening.
Shape memory alloys may have different kinds of shape memory effect. The two most common memory effects are the one-way and two-way shape memory. A schematic view of the two effects is given in the figure below.
In the figure above, the procedures are very similar: starting from martensite (a), adding a reversible deformation for the one-way effect or severe deformation with an irreversible amount for the two-way (b), heating the sample (c) and cooling it again (d).
When a shape memory alloy is in its cold state (below As), the metal can be bent or stretched into a variety of new shapes and will hold that shape until it is heated above the transition temperature. Upon heating, the shape changes back to its original shape, regardless of the shape it was when cold. When the metal cools again it will remain in the hot shape, until deformed again.
With the one-way effect, cooling from high temperatures does not cause a macroscopic shape change. A deformation is necessary to create the low-temperature shape. On heating, transformation starts at As and is completed at Af (typically 2 to 20 °C or hotter, depending on the alloy or the loading conditions). As is determined by the alloy type and composition. It can be varied between −150 °C and 200 °C.
The two-way shape memory effect is the effect that the material remembers two different shapes: one at low temperatures, and one at the high temperature shape. A material that shows a shape memory effect during both heating and cooling is called two-way shape memory. This can also be obtained without the application of an external force (intrinsic two-way effect). The reason the material behaves so differently in these situations lies in training. Training implies that a shape memory can "learn" to behave in a certain way. Under normal circumstances, a shape memory alloy "remembers" its high-temperature shape, but upon heating to recover the high-temperature shape, immediately "forgets" the low-temperature shape. However, it can be "trained" to "remember" to leave some reminders of the deformed low-temperature condition in the high-temperature phases. There are several ways of doing this.
If a shape memory alloy is heated up to very high temperatures (after it has been trained) then it may lose the two way memory effect, this process is known as "amnesia".
One of the commercial uses of shape memory alloy involves using the pseudo-elastic properties of the metal during the high temperature (austenitic) phase. The frames of reading glasses have been made of shape memory alloy as they can undergo large deformations in their high temperature state and then instantly revert back to their original shape when the stress is removed. This is the result of pseudo-elasticity; the martensitic phase is generated by stressing the metal in the austenitic state and this martensite phase is capable of large strains. With the removal of the load, the martensite transforms back into the austenite phase and resumes its original shape.
This allows the metal to be bent, twisted and pulled, before reforming its shape when released. This means the frames of shape memory alloy glasses are claimed to be "nearly indestructable" because it appears no amount of bending will result in permanent plastic deformation.
The first reported steps towards the discovery of the shape memory effect were taken in the 1930s. According to Otsuka and Wayman (1998), A. Ölander discovered the pseudoelastic behavior of the Au-Cd alloy in 1932. Greninger & Mooradian (1938) 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 (1949) and also by Chang & Read (1951).
The nickel-titanium alloys were first developed in 1962–1963 by the Naval Ordnance Laboratory and commercialized under the trade name Nitinol (an acronym for Nickel Titanium Naval Ordnance Laboratories). Their remarkable properties were discovered by accident. A sample that was bent out of shape many times was presented at a laboratory management meeting. One of the associate technical directors, Dr. David S. Muzzey, decided to see what would happen if the sample was subjected to heat and held his pipe lighter underneath it. To everyone's amazement the sample stretched back to its original shape.
There is another type of S.M.A., called a ferromagnetic shape memory alloy (FSMA), that changes shape under strong magnetic fields. These materials are of particular interest as the magnetic response tends to be faster and more efficient than temperature-induced responses.
Metal alloys are not the only thermally-responsive materials; shape memory polymers have also been developed, and became commercially available in the late 1990s.
Shape memory alloys are typically made by casting, using vacuum arc melting or induction melting. These are specialist techniques used to keep impurities in the alloy to a minimum and ensure the metals are well mixed. The ingot is then hot rolled into longer sections and then drawn to turn it into wire.
The way in which the alloys are "trained" depends on the properties wanted. The "training" dictates the shape that the alloy will remember when it is heated. This occurs by heating the alloy so that the dislocations re-order into stable positions, but not so hot that the material recrystallises. They are heated to between 400 °C and 500 °C for 30 minutes. Typical variables for some alloys are 500 °C and for more than 5 minutes.
They are then shaped while hot and are cooled rapidly by quenching in water or by cooling with air.
The yield strength of shape memory alloys is lower than that of conventional steel, but some compositions have a higher yield strength than plastic or aluminium. The yield stress for NiTi can reach 500 MPa. The high cost of the metal itself and the processing requirements make it difficult and expensive to implement SMAs into a design. As a result, these materials are used in applications where the superelastic properties or the shape memory effect can be exploited. The most common application is in actuation.
One of the advantages to using shape memory alloys is the high level of recoverable plastic strain that can be induced. The maximum recoverable strain these materials can hold without permanent damage is up to 8% for some alloys. This compares with a maximum strain 0.5% for conventional steels.
Harmeet D. Walia later utilized the alloy in the manufacture of root canal files in endodontics.
Materials having the memory effect at different temperatures and at different percentages of its solid solution contents.