SHAPE MEMORY ALLOYS

SHAPE MEMORY ALLOYS is explained in detail in this page.

SHAPE MEMORY ALLOYS

A group of metallic alloys which shows the ability to return to their original shape or size

i.e., alloy appears to have memory

when they are subjected to heating or cooling are called shape memory alloys.

Phase of shape memory alloys

Martensite and austenite are two solid phases in SMA as shown in fig.

Fig. Phases of SMA

Martensite is relatively soft and it is easily deformable phase which exists at low temperature (monoclinic).

(i) Austenite is a phase that occurs at high temperature having a crystal structure and high degree of symmetry (cubic).

TYPES OF SHAPE MEMORY ALLOYS

There are two types of shape memory alloys

(i)One-way shape memory alloy

(ii)Two-way shape memory alloy

A material which exhibits shape memory effect only upon heating is known as one-way shape memory.

A material which shows a shape memory effect during both heating and cooling is called two-way shape memory.

Examples of shape memory alloys

Generally, shape memory alloys are intermetallic compounds having super lattice structures and metallic-ionic-covalent characteristics.

Thus, they have the properties of both metals and ceramics.

Ni –Ti alloy (Nitinol)

Cu –Al –Ni alloy

Cu –Zn –Al alloy

Au –Cd alloy

Ni –Mn –Ga and Fe based alloys

CHARACTERISTICS OF SHAPE MEMORY ALLOYS – SMAS

Shape memory effect

The change of shape of a material at low temperature by loading and regaining of original shape by heating it, is known as shape memory effect.

The shape memory effect occurs in alloys due to the change in their crystalline structure with the change in temperature and stress.

While loading, twinned martensite becomes deformed martensite at low temperature.

On heating, deformed martensite becomes austenite (shape recovery) and upon cooling it gets transformed to twinned martensite (fig.).

SMAs exhibit changes in electrical resistance, volume and length during the transformation with temperature.

The mechanism involved in SMA is reversible (austenite to martensite and vice versa.)

Stress and temperature have a great influence on martensite transformation.
Pseudo elasticity

Pseudo –elasticity occurs in shape memory alloys when it is completely in austenite phase (temperature is greater than A_faustenite finish temperature).

Unlike the shape memory effect, Pseudo-elasticity occurs due to stress induced phase transformation without a change in temperature.

The load on the shape memory alloy changes austenite phase into martensite (Fig.).

As soon as the loading decreases the martensite begins to transform to austenite.

This phenomenon of deformation of a SMA on application of large stress and regaining of shape on removal of the load is known as pseudo elasticity.

This pseudo elasticity is also known as super elasticity

Hysteresis

The temperature range for the martensite to austenite transformation which takes place upon heating is somewhat higher than that for the reverse transformation upon cooling.

The difference between the transition temperature upon heating and cooling is called hysteresis. The hysteresis curve for SMAs is shown in fig.

The difiference of temperature is found to be 20-30^oC,

COMMERCIAL SHAPE MEMORY ALLOYS

The only two alloy systems that have achieved any level of commercial exploitation are,

(i) Ni-Ti alloys, and

(ii) Copper base alloys.

Properties of the two systems are quite different.

Nickel-Titanium Alloys

The basis of the Nickel-Titanium alloy is the binary, equi-atomic inter-metallic compound of Ti-Ni. The inter-metallic compound is extraordinary because it has moderate solubility range for excess Nickel or Titanium, as well as most other metallic elements. This solubility allows alloying with many of the elements to modify both the mechanical properties and the transformation properties of the system. Excess Nickel strongly depresses the transformation temperature and increases the yield strength of the austenite. The contaminants such as Oxygen and Carbon shift the transformation temperature and degrade the mechanical properties. Therefore, it is also desirable to minimize the amount of such elements.

Properties:

(i) The Ni-Ti alloys have greater shape memory strain upto 8.5% tend to be much more thermally stable.

(ii) They have excellent corrosion resistance and susceptibility, and have much higher ductility.

(iii) Machining by turning or milling is very difficult except with special tools.

(iv) Welding, brazing or soldering the alloys is generally difficult.

(v) The material do respond well to abrasive removal such as grinding, and shearing.

(vi) Punching can be done if thicknesses are kept small.

ADVANTAGES OF SHAPE MEMORY ALLOYS

They are simple, compact and high safe.

They have good bio –compatibility.

They have diverse applications and offer clean, silent and spark-free working condition

They have good mechanical properties and are strong corrosion-resistant.

DISADVANTAGES OF SHAPE MEMORY ALLOYS

They have poor fatigue properties.

They are expensive.

They have low energy efficiency.

APPLICATIONS OF SHAPE MEMORY ALLOYS

Microvalve (Actuators)

One of the most common applications of SMAs is mocrovalves. Fig. shows a microvalve made of Ni –Ti alloy actuator. Actuator is a microsensor that can trigger the operation of a device. The electrical signal initiates an action.

Fig. Schematic of microvalves that open and close according to temperature

When an electrical current of 50 to 150 mA flows in Ni-Ti actuator, it contracts and lifts the poppet from the orifice and opens the valve.

Toys and novelties

Shape memory alloys are used to make toys and ornamental goods.

A butterfly using SMA. Moves its wings in response to pulses of electricity.

Medical field Blood clot filters

(i) Blood clot filters are SMAs, properly shaped and inserted in veins to stop the passing blood clots.

When the SMA is in contact with the clot at a lower temperature, it expands and stops the clot and blood passes through the veins.

(ii) They are used in artificial hearts.

(iii) Orthodontic applications

NiTi wire holds the teeth tight with a constant stress irrespective of the strain produced by the teeth movement. It resists permanent deformation even if it is bent. NiTi is non-toxic and non-corrosive with body fluid.

(iv) SMAs (NiTi) are used to make eye glass frames and medical tools. Sun-glasses made from superelastic Ni-Ti frames provide good comfort and durability.