Page 228 - Materials Chemistry, Second Edition

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3.2. Metallic Structures and Properties

Figure 3.34. Hysteresis loop associated with the phase transitions of shape-memory alloys.

Figure 3.35. Unit cell representations of (a) the CsCl structure of austenitic TiNi and (b) the monoclinic
structure of martensitic TiNi.

a final temperature, A f , is reached (Figure 3.34). Since the martensite phase is a
highly distorted structure relative to fcc austenite (Figure 3.35), the alloy is highly
soft and ductile and may be easily deformed while in its low-temperature phase.
You may be thinking that this is the opposite of the Fe–C system that was previously
discussed. That is, the martensite phase that was generated through fast quenching
austenite resulted in an extremely hard material – much stronger than the native
austenite phase. However, in that system, the martensite is associated with intersti-
tially dissolved carbon that adds strength through solute hardening, but brittleness
through the introduction of additional grain boundaries. It should be noted that there
are 24 possible ways of accomplishing the austenite–martensite transformation.
That is, austenite has six equivalent facial planes, and each of these may shear
along two perpendicular axes.