Shape Memory Alloys                                                                                                                 Deutsch | English

Shape Memory Alloy (SMA) materials have received increasing attention due to the unique ability to return to a predetermined shape when heated. They are also called smart metals. Within particular load limits the changing of the shape is completely reversible. This effect is observable especially on metallic alloys (e.g. NiTi) but also on polymers.

Schematic of the Shape Memory Effect Strain against temperature

This inherent property arises as a result of the reversible crystalline phase transformation that occurs between the low temperature martensite and high temperature austenite phases. Both of them exhibit a well-regulated atomic structure.

We distinguish between three different kinds of Shape Memory Effects:

One-way effect   

Two-way effect 

Pseudo elasticity

When a shape memory alloy is in
its cold stay, the metal can be bent
or stretched into a variety of new
shapes and will hold that shape until
it is heated above the transition temp-
erature. Upon heating, the shape
changes back to its original shape,
regardless of the shape it was when
cold. When the metal cools again it
will remain in the hot shape, until
deformed again.

The two-way shape memory effect
is the effect that the material remem-
bers two different shapes: one at low
temperatures, and one at the high
temperature shape. This can also be
obtained without the application of an
external force. The reason the mate-
rial behaves so differently in these situ-
ations lies in training. Training implies
that a shape memory can "learn" to
behave in a certain way.

One of the commercial uses of shape
memory alloy involves using the
pseudo-elastic properties of the metal
during the high temperature (austenitic)
phase. The frames of reading glasses
have been made of shape memory alloy
as they can undergo large deformations
in their high temperature state and then
instantly revert back to their original
shape when the stress is removed. This
kind of frames are "nearly indestructable".