Page 247 - Mechanics of Microelectromechanical Systems

234 Chapter 4
martensite (since this phase is more compliant), with relatively low levels of
external intervention. By further increasing the temperature over a critical
value, which triggers the reversed martensite-austenite transformation,
whereby the higher-symmetry crystallographic orientation of the parent
(austenite) phase becomes stable, the component changes its shape to its
original condition, and thus it remembers it. The reversed transformation will
take place upon heating when the martensite becomes unstable.
Usually, the shape memory alloys produce the one-way SME, as depicted
in Fig. 4.46 (a), and therefore the cyclic martensite-austenite transformation
is not possible, as the deformed martensite state cannot be reached through
cooling of the austenite phase. However, there are SMAs which remember
both states, as sketched in Fig 4.46 (b), and such compositions are called
two-way shape memory alloys. In MEMS applications, the SMA layers that
are currently being used as actuators/sensors are mainly capable of reacting
through the one-way SME.

Figure 4.46 SMA effects: (a) one-way SME; (b) two-way SME

The load-deformation (or equivalently, stress-strain) characteristics of
the martensite and austenite are schematically shown in Fig. 4.47 when the
loading increases gradually about the directions indicated by the arrows. The