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					                                                Climate control                                                              Inside
                                                                                                                             ventilation controls
                                                   Dashboard                                                                            Door locks
                                                Wiper actuators

SHAPE                                            Engine control
                                                    valves and
                                                                                                                       Rear view mirror
                                                                                                                                          Headrest and

MEMORY                                                                                   Potential vehicle application for shape memory components.

Shape memory alloys enable                                                     100,000
development of simple, very compact,
reliable actuators that can be integrated
into components and structures.
Francesco Butera                                                                 1000
SAES Getters S.p.A.
Milano, Italy
                                                         Specific work, J/Kg

Shape memory alloys are metallic materials that
demonstrate the ability to return to some previ-
ously defined shape or size when subjected to the
appropriate thermal procedure. Generally, these                                    10
materials can be plastically deformed at some rel-
atively low temperature, and upon exposure to
some higher temperature, they return to their                                       1
shape prior to the deformation.
   The basis of the nickel-titanium system of al-
loys is the binary, equiatomic intermetallic com-                                  0.1
pound of Ni-Ti. This intermetallic compound is
extraordinary because it has a moderate solubility
range for excess nickel or titanium, as well as most
other metallic elements, and it also exhibits duc-                                0.01
                                                                                            10                1000                100,000
tility comparable to that of most ordinary alloys.                                                        Frequency, Hz
This solubility enables alloying with many ele-         Fig. 1 — This chart shows the main categories of actuators
ments to modify both the mechanical properties          for automotive components.
and the transformation properties of the system.
Excess nickel, in amounts up to about 1%, is the        are within the area covered by shape memory al-
most common alloying addition. Excess nickel            loys. The most interesting actuation functions are
strongly depresses the transformation tempera-          those in components used occasionally with non-
ture and increases the yield strength of the            rotary movements, such as rear-view mirror
austenite. Other frequently used elements are iron      folding, movement of the climate control flaps for
and chromium (to lower the transformation               air flow adjustment, and lock/latch controls.
temperature), and copper (to decrease the hys-
teresis and lower the deformation stress of the         NiTi alloys
martensite).                                               The equiatomic system NiTi has been estab-
   For actuators, the shape memory component            lished as a standard alloy covering a wide range
is designed to exert force over a considerable          of application requirements. In fact, about 90% of
range of motion, often for many cycles. Shape           all shape memory applications involve the NiTi
memory actuators represent an alternative to elec-      pure binary alloy system.
tromagnetic actuators in a wide range of auto-             NiTi shows the best combination of properties,
motive applications. Figure 1 shows the functions       especially in terms of the amount of work output
of several automotive actuators, divided ac-            per material volume and the large amount of re-
cording to category of use, whose characteristics       coverable strain. The obvious simplicity of me-
ADVANCED MATERIALS & PROCESSES/MARCH 2008                                                                                                         37
      Fig. 2 —                               6                                                                                   chanical design and minimum
 Constant load                                                                                                                   number of moving parts is its primary
        test on                              5                                                                                   benefit as an actuator.

                      Wire contraction, %
  SmartFlex 76                                                                                                                      In particular, the mechanically sta-
 shape memory                                4                                                                                   bilized binary NiTi SmartFlex wire ac-
                                                                                                                                 tuator, produced by SAES Getters,
                                                                                                       Smartflex 76              shows a very sophisticated profile of
                                             2                                                         Smartflex 05
                                                                                                                                 properties. This article examines these
                                                                                                                                 properties in depth to enable engineers
                                             1                                                                                   to design the actuator so that the func-
                                                                                                                                 tional properties of the material can be
                                                                50                   100                150               200    optimized and fully exploited.
                                                                                 Temperature, °C                                    The thermo-mechanical properties
                                                                                                                                 of NiTi wire can be investigated and
                                                                                                                                 measured by several methods. The
                                                                        Ms                                            As         most common and useful tests are de-
                                                                                                                                 scribed here for a commercially avail-
                                                                                                                      Mf         able wire called SmartFlex 76, a 76-Pm
                  Stress, MPa

                                            300                                                                                  high-temperature NiTi shape memory
                                                                                                                                 Hysteresis evaluation
                                            100                                                                                     In this test, the wire is subjected to
                                                          Mf                     As                                              a constant load and its deformation is
                                                                                                                                 measured during a controlled temper-
                                               40              60                 80       100             120            140    ature profile in an environmental cell.
                                                                                 Temperature, °C
                                                                                                                                 Figure 2 shows the test output for
                                                          Fig. 3 — Transition temperatures under different loads                 SmartFlex 76 under constant stress of
                                                                                                                                 300 MPa. As the graph shows, some
             3                                                                                                                   important information can be gath-
                                                                                                               Current, mA       ered, such as the maximum stroke and
            2.5                                                                                                     120          the transition temperatures. The max-
                                                                                                                    140          imum stroke of the wire is around 5%,
             2                                                                                                      150          Mf at 65°C, and As at 96°C.
Stroke, %

                                                                                                                                    The applied load is an important
            1.5                                                                                                     170
                                                                                                                    180          factor affecting wire performance, as
                                                                                                                    190          shown in Fig. 3. The martensite (M)-
             1                                                                                                      200          austenite (A) transformation temper-
                                                                                                                                 atures increase with load, as also
            0.5                                                                                                     220
                                                                                                                    230          expected from a modified Clausius-
                                                                                                                                 Clapeyron equation (shown on the
                  0.5                                 1                 1.5                   2           2.5              3.0   graph).
                                                                    Time, seconds
                                                                                                                                    Wire transformation temperatures
                                                          Fig. 4 — Actuation times and strokes of SmartFlex 76                   are of course fundamental parameters.
                                                                                  at different heating currents.                 The main problem related to the hys-
                                                                                                                                 teresis test is the duration, because a
                                                                                                                                 single cycle between 15 and 150°C at
                                            V MPa                               Austenite
                                                                                                                                 a rate of 1°C/min lasts more than four
                                              400                                                                                   Another problem is the maximum
                                              350                                                                                usable length: In a typical hysteresis
                                                                        Max force                                                system, only samples of about 100 to
                                              300                        available
                                                                                                                                 150 mm can be analyzed. For this
                                              250                                                                                reason, SAES Getters has developed
                  Final position
                                                     B              T                             Martensite
                                                                                                                                 and patented a new characterization
                                              200                                                                                method in which quality control on the
                                                                                                                                 total length of the produced wire is
                                              150    Preload                                                                     performed. This equipment will en-
                                              100                   A Initial position
                                                                                                                                 able an on-line 100% product quality
                                                                                                                                 control to measure and guarantee NiTi
                                                                                                                                 wire thermo-mechanical properties.
                                                      1     2 3         4    5     6    7     8   9 10 11 12
                                                                                      H %                                       Fatigue life
                                                                         Fig. 5 — Design example of a shape memory                 Another very important feature that
                                                                                         element with a bias spring.             defines the wire suitability for a spe-
38                                                                                                         ADVANCED MATERIALS & PROCESSES/MARCH 2008

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