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eturned to its original state when heated. Metals that possess this property are referred to as shape
memory alloys (SMAs) [12].
According to Smartlab Shape Memory Alloys can be defined as “a unique class of metal alloys that
can recover apparent permanent strains when they are heated above a certain temperature. The
SMAs have two stable phases - the high-temperature phase, called austenite and the low-temperature
phase, called martensite” [186].
The phase in which the alloy is situated depends upon the temperature and the pressure exercised on
the object.
Shape memory alloys, such as nickel-titanium NiTi, have been developed to provide increased
protection against sources of heat, even extreme heat. A shape memory alloy possesses different
properties below and above the temperature at which it is activated. Below this temperature, the alloy
is easily deformed. At the activation temperature, the alloy exerts a force to return to a previously
adopted shape and becomes much stiffer. The temperature of activation can be chosen by altering the
ratio of nickel to titanium in the alloy.
For explanation of the working principle, we consider a NiTi-alloy, whose structures are illustrated in
the following drawing.
Fig. 100 Martensite and austenite structure [186]
The martensite and the austenite structure are clearly distinct. The a, b and c sides in the martensite
grid all have different lengths. When pressure is exercised on the grid, these lengths will adapt
themselves to compensate the pressure. The angle γ can also change as a function of the pressure
exercised. Thanks to these properties, a shape memory alloy in the martensite structure can easily be
transformed. Care should be taken, however, that the pressure exercised is not too big, as once the
bonds are broken, the material will not be able to fully regain its original shape.
A thermal cycle looks as follows. The dividing line between martensite and austenite is determined by
pressure and temperature. The straight lines on the graph below represent the limits of these
transitions.
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