Shape Memory Alloys

Physics

By: Taher K D

INDEX

Introduction Properties Applications with examples Limitations with examples List of shape memory alloys Conclusion

Definition of a Shape Memory Alloy

Shape Memory Alloys (SMAs) are a class of metal alloys that can recover apparent permanent strains when they are heated above a certain temperature.

Shaped Memory EffectAt a low temperature, a SMA can be seemingly “plastically” deformed, but this “plastic” strain can be recovered by increasing the temperature. This is called the Shape Memory Effect (SME). At a high temperature, a large deformation can be recovered simply by releasing the applied force. This behavior is known as Super elasticity (SE).

(a) Shape Memory Effect and (b) Super elasticity

Stress Free Shape Re-covery

TEMPERATURE

STR

ESS

Mf Ms As Af

TEMPERATURE

STR

ESS

Mf Ms As Af

Twinned Martensite

(unstressed)

Detwinned Martensite (stressed - deformed)

Detwinned Martensite (stressed - deformed)

Detwinned Martensite

(unstressed - deformed)

Austenite (undeformed)

Shape Recovery Under Stress

TEMPERATURE

STR

ESS

Mf Ms

Detwinned Martensite(stressed) Austenite

As Af

The Shape Memory Effect

s

e

T

Cooling

Detwinning

Heating/Recovery

Basic working principle

SMAs have two stable phases - the high-temperature phase, called austenite and the low-temperature phase, called martensite.

the martensite can be in one of two forms: twinned and detwinned, as shown in Figure 1.

A phase transformation which occurs between these two phases upon heating/cooling is the basis for the unique properties of the SMAs.

Thermally Induced Phase Transformation in SMAs

Mf Ms As AfAustenite

Martensite

TEMPERATURE

Mf Ms As AfAustenite

Martensite

TEMPERATURE

(twinned)

(twinned)

Characteristic temperatures:Mf=Martensitic FinishMs=Martensitic StartAs=Austenitic StartAf=Austenitic Finish

Pseudoelastic Behavior

It is also possible to induce a phase

transformation by applying a pure

mechanical load. The result of this load

application is fully detwinned martensite

and very large strains are observed. If the

temperature of the material is above A0f, a

complete shape recovery is observed upon

unloading, thus, the material behavior

resembles elasticity. Thus the above-

described effect is known under the name of

Pseudoelastic Effect.

The Pseudoelastic EffectS

TR

ESS

TEMPERATURE

Mf Ms As Aff s s f

Austenite

Detwinned Martensite(stressed)

The Pseudoelastic Effect

s Mf

s Ms

s Af

s As

s

e

SMA Demonstrations and Ap-plications

Floral Arrangement SMA Actuated Butterfly: SMA Linear Actuator

Thermobile™ Demonstrator:SMA Properties/Thermodynamics

Where Are SMAs Used?

SMAs in Bio-medical Devices

(a)

(b)

SMA Space Applications

Landis G.A., NASA Lewis Research Center, “Materials Adherence Experiment Results from Mars Pathfinder”, 26th Photovoltaic Specialists Conference, 1997

SMA RibbonBolt

released

Links

Flexible tail

Joints

Model withoutskin

Model withskin

Rigid nose

SMAs as Linear Actua-tors

Miniaturized Walking Robot

The implementation of SMA

wires coupled with a sim-ple DC

control system can be used

to drive small objects without

the addition of relatively heavy

motors, gears, or drive mechanisms.

Advantages:

Bio-compatibility Diverse Fields of Application Good Mechanical Properties (strong,

corrosion resistant)

What materials are SMA’sa)Nickel-titanium alloysb) Copper-base alloys such as CuZnAl and

CuAlNic) Ag-Cd 44/49 at.% Cd) Au-Cd 46.5/50 at.% Cde) Cu-Al-Ni 14/14.5 wt.% Al and 3/4.5 wt.% Nif) Cu-Sn approx. 15 at.% Sng) Cu-Zn 38.5/41.5 wt.% Zh) Cu-Zn-X (X = Si,Sn,Al) a few wt.% of i)In-Ti 18/23 at.% Tij) Ni-Al 36/38 at.% Alk) Ni-Ti 49/51 at.% Nil) Fe-Pt approx. 25 at.% Ptm) Mn-Cu 5/35 at.% Cun) Fe-Mn-Si o) Pt alloys

Thank You