SShape memory alloys enable development of simple, very compact,reliable actuators that can be integratedinto components and structures.

Francesco ButeraSAES Getters S.p.A.Milano, Italy

Shape memory alloys are metallic materials thatdemonstrate the ability to return to some previ-ously defined shape or size when subjected to theappropriate thermal procedure. Generally, thesematerials can be plastically deformed at some rel-atively low temperature, and upon exposure tosome higher temperature, they return to theirshape prior to the deformation.

The basis of the nickel-titanium system of al-loys is the binary, equiatomic intermetallic com-pound of Ni-Ti. This intermetallic compound isextraordinary because it has a moderate solubilityrange for excess nickel or titanium, as well as mostother metallic elements, and it also exhibits duc-tility comparable to that of most ordinary alloys.This solubility enables alloying with many ele-ments to modify both the mechanical propertiesand the transformation properties of the system.Excess nickel, in amounts up to about 1%, is themost common alloying addition. Excess nickelstrongly depresses the transformation tempera-ture and increases the yield strength of theaustenite. Other frequently used elements are ironand chromium (to lower the transformation temperature), and copper (to decrease the hys-teresis and lower the deformation stress of themartensite).

For actuators, the shape memory componentis designed to exert force over a considerablerange of motion, often for many cycles. Shapememory actuators represent an alternative to elec-tromagnetic actuators in a wide range of auto-motive applications. Figure 1 shows the functionsof several automotive actuators, divided ac-cording to category of use, whose characteristics

are within the area covered by shape memory al-loys. The most interesting actuation functions arethose in components used occasionally with non-rotary movements, such as rear-view mirrorfolding, movement of the climate control flaps forair flow adjustment, and lock/latch controls.

NiTi alloysThe equiatomic system NiTi has been estab-

lished as a standard alloy covering a wide rangeof application requirements. In fact, about 90% ofall shape memory applications involve the NiTipure binary alloy system.

NiTi shows the best combination of properties,especially in terms of the amount of work outputper material volume and the large amount of re-coverable strain. The obvious simplicity of me-

ADVANCED MATERIALS & PROCESSES/MARCH 2008 37

Inside ventilation ccontrols

Door llocks

Headrest aand cushion actuators

Rear vview mmirror actuators

Climate ccontrol

Dashboard actuators

Wiper aactuators

Engine ccontrol valves aand actuators

Adaptive headlights

Potential vehicle application for shape memory components.

Fig. 1 — This chart shows the main categories of actuators for automotive components.

FOR AUTOMOTIVE

APPLICATIONS

SHAPE MEMORY ACTUATORS

Spe

cific

wwor

k, JJ

/Kg

10 11000 1100,000Frequency, HHz

100,000

10,000

1000

100

10

1

0.1

0.01

chanical design and minimumnumber of moving parts is its primarybenefit as an actuator.

In particular, the mechanically sta-bilized binary NiTi SmartFlex wire ac-tuator, produced by SAES Getters,shows a very sophisticated profile ofproperties. This article examines theseproperties in depth to enable engineersto design the actuator so that the func-tional properties of the material can beoptimized and fully exploited.

The thermo-mechanical propertiesof NiTi wire can be investigated andmeasured by several methods. Themost common and useful tests are de-scribed here for a commercially avail-able wire called SmartFlex 76, a 76-�mhigh-temperature NiTi shape memorywire.

Hysteresis evaluationIn this test, the wire is subjected to

a constant load and its deformation ismeasured during a controlled temper-ature profile in an environmental cell.Figure 2 shows the test output forSmartFlex 76 under constant stress of300 MPa. As the graph shows, someimportant information can be gath-ered, such as the maximum stroke andthe transition temperatures. The max-imum stroke of the wire is around 5%,Mf at 65°C, and As at 96°C.

The applied load is an importantfactor affecting wire performance, asshown in Fig. 3. The martensite (M)-austenite (A) transformation temper-atures increase with load, as also expected from a modified Clausius-Clapeyron equation (shown on thegraph).

Wire transformation temperaturesare of course fundamental parameters.The main problem related to the hys-teresis test is the duration, because asingle cycle between 15 and 150°C ata rate of 1°C/min lasts more than fourhours.

Another problem is the maximumusable length: In a typical hysteresissystem, only samples of about 100 to150 mm can be analyzed. For thisreason, SAES Getters has developedand patented a new characterizationmethod in which quality control on thetotal length of the produced wire isperformed. This equipment will en-able an on-line 100% product qualitycontrol to measure and guarantee NiTiwire thermo-mechanical properties.

Fatigue lifeAnother very important feature that

defines the wire suitability for a spe-

38 ADVANCED MATERIALS & PROCESSES/MARCH 2008

Fig. 2 — Constant load

test on SmartFlex 76

shape memorywire.

Fig. 4 — Actuation times and strokes of SmartFlex 76at different heating currents.

Fig. 5 — Design example of a shape memory element with a bias spring.

Fig. 3 — Transition temperatures under different loads

50 1100 1150 2200Temperature, °°C

6

5

4

3

2

1W

ire cc

ontr

actio

n, %%

Smartflex 776

Smartflex 005

40 660 880 1100 1120 1140Temperature, °°C

500

400

300

200

100

Str

ess,

MMP

a

AsAf

Ms

Mf

Ms Af

Mf As

1 22 33 44 55 66 77 88 99 110 111 112�� %

���MPa

400

350

300

250 Final pposition

200

150

100

Austenite

Max fforce available

B TT MMartensite

Preload spring

A IInitial pposition

0.5 11 11.5 22 22.5 33.0Time, sseconds

3

2.5

2

1.5

1

0.5

Str

oke,

%%

120 130 140 150 160 170 180 190 200 210 220 230

Current, mmA