Shape Memory Alloys (SMA)

SHAPE MEMORY ALLOYS (SMA)

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 called shape memory alloys.

Phases of shape memory alloys

Martensite and austenite are two solid phases in SMA as shown in fig. 4.7

(i) Martensite is relatively soft. It is easily deformable yoпoupe phase which exists at low temperature (monoclinic) (fig 4.8).

(ii) Austenite is a phase that occurs at high temperature having a crystal structure and high degree of symmetry (cubic) (fig 4.8).

 

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

Note:

Now a days, shape memory alloys are also available in nanophase structures.

 

Methods of Processing

The shape memory alloys are generally prepared in vacuum or in an inert gas atmosphere due to the high reactivity of the titanium present in the compound.

The methods such as plasma arc melting, electron beam melting, vacuum induction, etc., are used for the commercial preparations of shape memory alloys.

 

Characteristics of SMAS

 

1. Shape memory effect

The change in 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 loys due to the chand 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 agnes (shape recovery) and upon cooling it gets transformed to twinned martensite (fig. 4.9).

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

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

4. Strees and termperature have a great influence on martensite transformation .

 

5. Pesudo elasticity

Pesudo-elasticity occurs in shape memory alloys when it is completely in austenite phase (temperature is greater than A_f austenitefinish 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 4.10)

As soon as the loading decreases the matensite begins to transform to austenite and results in shape recovery.

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

This pseudo elasticity is also known as super elasticity.

 

6. 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 transition temperature upon heating and cooling is called hysteresis. The hysteresis curve for SMAS is shown in fig. 4.11.

 

Applications of Shape Memory Alloys

Shape memory alloys have a wide range of applications.

 

1. Microvalve (Actuators)

One of the most common applications of SMAs is microvalves. Fig. 4.12 shows a microvalve made of Ni-Ti alloy actuator. Actuator is a microsensor which triggers the operation of a device. The electrical signal initiates an action.

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.

 

2. Toys and novelties

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

A butterfly using SMA as shown in fig. 4.13. It moves its wings in response to pulses of electricity.

 

3. Medical field

Blood clot filters

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

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

(ii) They are used in artificial hearts.  

(iii) Orthodontic applications

Ni-Ti 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. Ni-Ti is non-toxic and non-corrosive with body fluid.

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

 

4. Antenna wires

The flexibility of superelastic Ni - Ti wire makes it ideal for use as retractable antennas.

 

5. Thermostats

SMA are used as thermostat to open and close the valves at required temperature.

 

6. Cryofit hydraulic couplings

SMAS materials are used as couplings for metal pipes.

 

7. Springs, shock absorbers and valves

Due to the excellent elastic property of the SMAs, springs can be made which have varied industrial applications. Some of them are listed here.

• Engine micro valves

• Medical stents (Stents are internal inplant supports

• Firesafety valves and

• Aerospace latching mechanisms

 

8. Stepping motors

Digital SMA stepping motors are used for robotic control.

 

9. Titanium-aluminium shape memory alloys offer excellent strength with less weight and dominate in the aircraft industry. They are high temperature SMAS, for possible use in aircraft engines and other high temperature environments.

 

Advantages of Shape Memory Alloys

• They are simple, compact and highly 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 strong corrosion-resistance.

Disadvantages of Shape Memory Alloys

• They have poor fatigue properties.

• They are expensive.

• They have low energy efficiency.