SHAPE MEMORY ALLOYS

(SMA)

INTRODUCTION

Some Major Contributions

Discovered by Arne Olander (1932).

Described by Vernon(1941).

Recognised with the discovery of shape memory

effect in Ni-Ti alloy (Nitinol).

Shape Memory AlloysThese materials have the

tendency to regain their pre-

deformed shape and size when

subjected to certain stimulus.

Example :

Nickel-Titanium 50-50%

Alloy (Nitinol)

Copper base alloys (Cu-Zn-

Al & Cu-Al-Ni).

How does It Work?

Solid State phase

transformation.

The internal structure of a

solid material changes back

and forth between two

crystalline forms.

Properties of SMA

Tendency to possess different crystal structure

at same composition.

Tendency to revert back to its original shape

after heating.

Relatively lightweight and bio compatible.

Easy to manufacture.

High force to weight ratio.

Properties of SMA

NITINOL

Combination of NiTi and Naval Ordinance

Laboratory.

Equal amount of Ni and Ti

(50% each by weight).

Exhibits shape memory and superelasticity.

SMA exhibits differing properties

including :

1. Shape memory effect

2. Super elasticity

SMA shows super-elastic

behaviour over large strain ranges

of up to about 8%.

Shape Memory Effect

Describes the effect

of restoring the

original shape of a

plastically deformed

sample by heating it.

Superelasticity

The SMA reverts to

its original shape after

removal of

mechanical loading ,

without the need for

any thermal

activation.

One-Way Memory EffectWhen the alloy is deformed,

it will hold that shape until

heated .

Upon heating it changes to

its original shape and when

the it cools again it will

remain in its hot shape.

(T2>T1)

T1

T1

T2

T1

Two-Way Memory Effect

The material remember two

different shapes.

The reason the material behaves

so differently in these situations

lies in training .

T1

T1

T2

T1

A shape memory alloy can "learn"

to behave in a certain way.

It is "trained" to "remember" or

to leave some reminders of the

deformed low-temperature

condition in the high-temperature

phases.

Training of materials

Theory of phase

transformation

Phases in SMA

Parent phase :Austenite phase

Daughter Phase : Martensite phase

Reverse transformation between these two

phases takes place on temperature change.

Description of phase

transformation

Austenite Phase

Crystal Structure:

Face Centred Cubic.

Exists at higher temperature

Harder material

Difficult to deform

Martensite Phase

Crystal Structure:

Body Centred Tetragonal

Exists at lower temperatures

Relatively soft

Plastic and easy to shape.

A TO M TRANSFORMATION

Martensite phase is obtained by cooling of

austenite to low temperatures.

Results from diffusion less transformation of

austenite.

Cooling rate should be high to prevent diffusion.

Large number of atoms co-operative movements

with respect to their neighbours.

Also called as interstitial or substitutional solid

solution.

Phase transformation in SMA occurs when:

chemical free energy of martensite phase is less

than that of parent phase.

Eparent-Emartensite>Non-chemical Free Energy

Non-chemical Free Energy includes strain and

interface energy.

PLOT OF G VS T

Phase Transition

3-D VIEW

Microscopic point of view

Austenite phase

Twinned Martensite.

Detwinned Martensite

Twinned martensite

Occurs by re-

arrangement of atoms

by simple shear.

.

Does not cause

breaking of atomic

bonds.

Comparison of Twinned

and Detwinned

martensite phase

DETWINNED FORM TWINNED FORM

No volume change

Shape change occurs

No volume change

No shape change

Phase transformation curve

In the figure ,

(T) represents the martensite

fraction.

Transformation temperature

Not unique as transformation begins at one temperature and ends at another.

There are 4 transformation temperature:

1. Ms - Martensite start

2. Mf Martensite finish

3. As Austenite start

4. Af Austenite finish

Characteristics of phase

transformation

Reversible As heating above transition

temperature will revert the crystal back to its

austenitic phase.

Transformation is instantaneous in both the

directions.

Transformation hysterisis

Difference between

temperatures at

which the material is

50% transformed on

either phase.

For Ni-Ti : 25-50C

The difference

between the heating

and cooling

transition gives rise

to hysteresis where

some of the energy

is lost.

The shape of the curve depends on the material

properties of the shape-memory alloy.

Although the deformation experienced by shape-

memory alloys is semi-permanent, it is not truly

plastic deformation neither is it strictly

elastic . It is termed thermo elastic.

Types of Martensitic

transformation Thermo-Elastic : Occurs when interface energy

and energy required for plastic deformation are

negligible.

Non-Thermo elastic : Occurs when interface

energy and energy required for plastic

deformation are high.

Stress-strain behaviour

comparison

Comparison of SME and

Superelasticity

Shape memory polymers

They are inexpensive

plastics with

properties similar to

shape-memory alloys.

They are likely to

expand the list of

applications for

SMAs.

APPLICATIONS

PIPING

Weld less shrink-to-fit pipe couplers

Oil line pipes for industrial applications, water

pipes.

EYEGLASS FRAMES

Allows the frames to undergo large deformation

under stress , yet regain their intended shape when

unloaded.

DENTISTRY

Orthodontic wires that reduce the need to

retighten and adjust the wire.

FABRICATION

A shirt which moulds to the shape of your

body, or shortens and lengthens the sleeves to

match the temperature.

ROBOTICS

Used to create very light robots or parts of them.

Example Robotic arm.

BIOMEDICAL

Nano-muscles

Surgical instruments:

1. Tissue Spreader

2. Stents(angioplasty).

3. Coronary Probe

4. Brain Spatula

Endoscopy: miniature zoom device, bending

actuator

Force sensor.

Smart skin (wing turbulence reduction)

AEROSPACE

General Electric

Aircraft Engines.

Connection of hydraulic

tubing.

ACTUATORS

SMA actuators are typically actuated electrically

by Joule heating.

AUTOMOTIVE

Automotive valve application-to run low

pressure pneumatics in a car seat to adjust the

contour.

STRUCTURES AND COMPOSITES

For vibration control in

structures.

For design of structure

capable of extremely

large , recoverable

deflections.

MISCELLANEOUS

APPLICATIONS

THE ICEMOBILE

A heat engine, that has a

loop of Nitinol which you

immerse in warm water, to

make it spin (which then

cuts up ice cubes).

NO MORE OIL BURNS

A deep fryer that senses the

right temperature for

when to lower the basket

into the oil.

FIRE ALARM SPRINKLER

SYSTEM.

When there is a fire the

temperature will affect the

electrical circuit and

trigger the sprinkler.

SMA REINFORCED

COMPOSITES

Used for active

vibration control of

large flexible aerospace

and space structures.

GLOBAL

FORECAST