Biomedical Engineering Reference
In-Depth Information
titanium alloys.
64-70
These are composed of non-toxic elements such as nb,
Ta, zr, Mo and sn that stabilise the bcc b-phase preserving the light weight
character of Ti alloys. as can be seen in Table 6.5, the Young's modulus
of b type titanium alloys is much smaller than that of a + b type titanium
alloys. Two promising biomedical alloys are Ti-35nb-7zr-5Ta (TnzT)
71
and Ti-12Mo-6zr-2Fe (TMzF)
72
which offer an elastic modulus of about 55
and 80 GPa, respectively. These materials do not reveal any toxicity versus
osteoblastic cells and have a high corrosion resistance.
68, 69
In addition to
solid solution, mechanical strength can be enhanced by cold working or by
precipitation of secondary phases (a and w), although in this case the Young's
moduli tend to increase with increasing precipitation. Volume fractions of
precipitates can be controlled by proper thermomechanical processing.
73, 74
on the other hand, the strength of b type alloys is comparable or superior
to those of aged a+b alloys.
64
a further decrease in Young's modulus can be achieved by the use of
foams as porous implants
75
or implants with a porous surface layer. an
open porosity would also permit good attachment of tissues to the surface
allowing the tissue ingrowth (see later section on metallic scaffolds). a
decrease in the Young's modulus of the metal will favour the load transfer
to the ingrown bone, stimulating new bone formation.
6.5.2 Ni-free Fe-based alloys
austenitic stainless steels, such us 316L, and CoCr alloys are biocompatible
materials widely used to produce permanent implants or parts which merely
come into contact with the human body. They are widely used because of
the excellent compromise between their various characteristics, such as
intrinsic mechanical strength, and wear resistance. austenitic stainless steel
must contain a nickel content in the range 9-15 wt% to ensure an austenitic
Table 6.5
Mechanical properties of Ti, Ti-based alloys and some representative
b
type titanium alloys developed for biomedical applications
Alloy
Microstructure
Young's
Yield strength
Tensile
modulus
E
(MPa)
strength
(GPa)
(MPa)
cpTi
a
105
170-485
240-760
Ti-6Al-4V
a
+
b
110
760-895
900-970
Ti-6Al-7Nb
a
+
b
105
921
1024
Ti-13Nb-13Zr
b
79
900
1030
Ti-12Mo-6Zr-2Fe
b
74-85
1000-1060
1060-1100
(TMZF)
Ti-15Mo-5Zr-3Al
b
82
771
812
Aged
b
+
a
100
1215
1310
Ti-35Nb-7Zr-5Ta
b
55
530
590
(TNZT)
