Biomedical Engineering Reference
In-Depth Information
5.4
Alloys with a Future?
Scientists are a strange race of people and always on the run to unexplored corners
hoping to discover the ultimate philosopher's stone. Two other classes of materials
are introduced hoping that in the future a couple of neonates will emerge from these
classes.
Amorphous Zirconium
All biometals and bioalloys scrutinized thus far were polycrystalline, not perfect but
with a fair degree of long-range order. A particular class of materials, the atoms of
which have less predictable neighbors, are the
glasses
. Metallic glasses are not easy
to make but, since a few years, glassy ternary and quaternary alloys are gaining a
marketable status. Of interest are the recently developed nickel-free metallic glasses
Zr
48
Cu
43
Al
7
Ag
2
and these could be new biomat-candidates. Some can be produced
in bulk, meaning here rods of 20 mm diameter, YS of 1.9 GPa, a plasticity of 1.3%,
a yield strain of 2% and exhibiting negligible cytotoxicity. The stress-strain curve
has a linear part from 0 to
1,900 MPa followed by a short curved transit part and
ending after
1.9% strain in a linear quasihorizontal plastic zone.
Another opening is the design of metallic glass matrix composites as reported
by Hofmann et al. [225]. The inhomogeneous microstructure with isolated den-
trites in a bulk metallic glass (BMG) seems to stabilize the glass matrix against
the catastrophic failure. These authors report on 5 compositions but all contain toxic
beryllium and the unwelcome nickel. The list of mechanical properties is stupendous
but it is waiting on a similar development for Be- and Ni-free glasses.
The potential biomedical applications are self-expandable stents, blades for
surgical instruments and micrometer-sized gear for implanted devices (actuators,
pumps,...?) [226-228]. The authors, however, are not generous with information.
Notice that, irrespective of their biomedical use, glasses are an intriguing class of
materials (we mentioned in earlier chapters the topic of Elliott with the basic prin-
ciples of the amorphous state [229]). Metallic glasses are most often metastable
at ordinary temperatures and difficult to make in 'bulk'. The best known and
widespread application is the photoactive cylinder of a photocopy machine coated
with a film of amorphous selenium. Amorphous metals are part of an active research
community and an emerging field in the biomaterials world. It remains of course an
open question what the success niche for applications will be.
Al-Cr-Fe
The alloying elements in this new fabric are old faithful metals throughout the for-
mer chapters. We already formulated some reservation with respect to Al but the
long experience with Ti6Al7Nb attenuated for the time being the tune of this alarm
bell. Al-Cr-Fe alloys belong to the class of
complex metal alloys
(CMAs) with
