Biomedical Engineering Reference
In-Depth Information
M
f
A
s
100
A
f
M
s
0
Temperature
6.6
Schematic representation of the martensite transformed during
cooling and heating periods under constant stress.
transformation describes a hysteresis of the order of 10-50ºC, although the
beginning and end of the transformation during heating and cooling extends
over a much larger temperature range.
6
The transformation temperature
strongly depends on the ni content which provides the ability to tailor the
properties to a specific application.
7
Titanium-nickel alloys show unusual properties, that is, if they are
deformed below the transformation temperature, they revert back to their
original shape as the temperature is raised. The great advantage is that the
device can be implanted in an optimal shape and after surgery the desired
functional shape is obtained
in situ
by the action of body heat.
a second important property is the ability of the material to recover an
apparently plastic strain on loading at a temperature above
A
f
, as illustrated
in Fig. 6.7. The amount of this reversible deformation is much greater (~
8-10%) than the conventional elastic strain (< 0.2 %). If deformed above
M
s
but below
A
f
, some strain-induced martensite will remain stable, leading
to an incomplete shape restoration. This pseudoelasticity or superelasticity
is also closely related to the martensitic phase transformation, which is also
thermoelastic in nature. It is worth remarking that once the critical stress
is reached, the sample will start to transform to martensite. During further
straining, the stress is almost constant until the austenite is fully transformed.
The temperature dependent character of the superelastic effect is of less
importance because of the stable temperature of the human body.
The martensitic transformation in nitinol causes an abrupt change in the
Young's modulus
E
and yield stress (see Table 6.4). When changing shape,
the alloys can recover substantial amounts of strain or generate a force as
a consequence of variations in the body temperature, which has triggered a
number of commercial applications. In the field of orthopaedics, applications
