Shape Memory  Alloys

Shape Memory Alloy (SMA)is an alloy that "remembers" its shape. SMA is also known as smart alloy is memory metal or muscle wire. SMA has the ability to return to its previous shape after being deformed by applying heat or in some cases releasing the stress.

The first SMA, nickel-titanium (NiTi)alloy was discovered in 1962 by the naval Ordinance LABORATORY, u.s.a.  It is commercialized under the trade name Nitinol, the acronym for Nickel Titanium Naval Ordnance Laboratories.  Copper-Zinc.  Aluminum-Nickel and Copper-Aluminium- Nickel are also two main types of SMAs.

Ferromagnetic Shape Memory Alloy 9FSMA0is another type of SMA that changes shape under string magnetic fields.  FSMAs are of particular interest as the magnetic response trends 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.

In Ni Ti alloy, the high temperature phase is called 'authentic', which has cubic crystal structure and the low temperature phase is called 'martensite'. which has monoclinic crystal structure.  The martensite can be in one of the two forms; twinned and detwinned.  In martensite phase, the material has the ability to stretch beyond the normal elastic limit, as it can go from twinned state to detwinned state.  This property is known as pseudo elasticity.

Materials which exhibit shape memory effect only on heating are called one-way shape memory alloys (one-way trained SMAs) and those which undergo change in shape on both heating and cooling are called two-way shape memory alloys (two-way trained SMAs).

If a two-way trained SMA is heated up to very high temperatures, it may lose the two-way  memory effect. This process is called 'amnesia'

Shape memory alloys are called by suitable heat treatments like casting, vacuum are melting or induction to the alloys.  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.

If the temperature is increased the material changes from martensite phase to authenite  phase .  On cooling, the material changes to martensite phase and regains  its origional shape.  The SMAs in martensite phase exhibits pseudoelasticity but austenite phase does not.  SMAs exhibit self heating effect.

SMAs are used as Actuators (microsensors that can trigger the operation of a device,), cryofit hydraulic couplings, springs, shock absorbers, valves etc. SMAs have orthodontic applications- NiTi wire holds yhe teeth tight with a constant stress irrespective of the strain produced by the teeth movement.  They used in stepping motors for Robotic control.