Page 229 - Materials Chemistry, Second Edition

216 3 Metals

Figure 3.36. Deformation pathways for (a) shape-memory alloys, showing the reversible movement of
twin boundaries. Shown in (b) is the irreversible slip deformation of other alloys, such as carbon steels.

When austenite is cooled in the absence of applied stress, the material transforms
into a twinned form of martensite (Figure 3.36a). Since both austenite and twinned
martensite have the same macroscopic shape/size, reheating the material will not
result in any observable shape change. However, if the material is plastically
deformed through bending, etc. at low temperature, it will become detwinned and
the new shape will prevail. The Ni–Ti alloys are preferred since they have a greater
range of deformation (up to 8%), relative to other Cu-based alloys (4–5%). When the
material is reheated, the deformed martensite structure will be converted to the
original austenite phase with a different macroscopic structure. For comparative
purposes, Figure 3.36b illustrates the irreversible slip deformation that other types of
metals such as steel undergo as a result of the same stresses. Since these latter
materials do not have suitable twin planes, shape-memory transitions are not
possible resulting in a permanent shape alteration of the metal.
It is also possible to apply a stress to the material in its high-temperature austenitic
phase. However, since the temperature is above A f , the original shape will be
reformed immediately after the load is removed. Such an immediate shape change
is referred to as pseudoelasticity (or superelasticity), and is the active principle
underlying cellular phone antennae that may be greatly distorted only to immedi-
ately return to their original shapes.
In addition to temperature- or stress-induced transitions, there are now a number
of ferromagnetic shape-memory alloys that alter their shapes in response to a
magnetic field. Examples of these systems include Fe–Pd, Fe–Pt, Co–Ni–Al, Co–
Ni–Ga, and Ni–Mn–Ga. These materials are of great interest since the magnetic
response time is faster and more reliable than temperature-based transitions.
Whereas traditional alloys alter their structures as a result of the martensite–austen-
ite transition, magnetic analogues exhibit a change in structure while remaining in
the martensite phase. The change in shape is a result of the detwinning of preferred
planes based on their orientations with the applied magnetic field.