Biomedical Engineering Reference
In-Depth Information
8.2
Overview of Shape Memory Materials
As we can see from the above considerations, shape memory
behavior can occur in different types of materials, for a different
kind of external stimuli inluence. Materials from the group of
shape memory alloys may retain their original shape depending on
the prevailing temperature and pressure circumstances. This gives
rise to the possibility of using them as a source of displacement or
force. Temperature is used as the external stimulus for the shape
memory effect control.
These crucial facts led to the idea of looking for more precise
systems, and the result was magnetic shape memory alloys. Different
expression brings a novel class of the smart materials — shape
memory polymers, which featured highly recoverable strains (near
400%), ease of processing, and low density and costs [21].
For a better understanding, magnetic shape memory alloys
(MSMA) and shape memory polymers (SMP) will be discussed
briely now.
8.2.1
MSMAs — Magnetic Shape Memory Alloys
The unique ability of magnetic shape memory materials is that they
can recover from up to 6%, and for a single crystal up to 10%, of
strains in a moderate magnetic ield [16]. These materials can work
at higher frequency range, which is not attainable by conventional
SMAs. Also, transformation control mechanism is more precise
when we use magnetic ield. Ullakko [47] was the irst to observe
the reversible transformation due to magnetic ield inluence. In
general assumption the material should possess shape memory
effect and strong magnetocrystalline anisotropy. These two issues
allow the control of reorientation structure processes.
To further explain the existing mechanism, we will use the
most examined example of MSM: NiMnGa alloy. We also simplify
the structure representation. Figure 8.4 shows a non-deformed
NiMnGa austenite body center cubic cell with lattice parameter
of
a
o
and non-deformed, possible tetragonal martensite cell with
lattice parameters
a
and
c
, in which we can distinguish the local
magnetization vector in each ferromagnetic variant, which is oriented
in one crystallographic direction called easy axis of magnetization.
