Biomedical Engineering Reference
In-Depth Information
Another recent work [204] is devoted to enhancement of strength and
ductility of the Ti-6Al-7Nb alloy. h is alloy is regarded as being less harm-
ful to humans from the medical point of view in comparison to Ti-6Al-4V.
It has been demonstrated that formation of an ultrai ne-grained structure
in this alloy via equal channel angular pressing in combination with heat
and deformation treatments results in high strength (UTS=1400 MPa) and
ductility (elongation 10%). h ese levels of properties are very attractive
for new applications and fabrication of medical implants with enhanced
service properties.
Titanium alloys consisting of mainly the β phase have drawn substantial
attention because they exhibit Young's moduli ranging between 55 GPa
and 90 GPa, and thus result in less stress shielding [195, 205-208]. In addi-
tion, these Ti alloys contain only non-toxic elements such as Nb, Zr, and
Ta. Unlike Ti64 where V can leach out from the surface passive oxide i lm
into the human body, the addition of Nb stabilizes the i lm, thus improv-
ing the passivation and corrosion resistance of titanium alloys in the body.
However, high hardness and low Young's modulus are desirable but hardly
coexist in this group of materials. h is is because the single phase β-Ti
alloys, which exhibit the lowest Young's modulus, are generally obtained
at er solution treatment, and so are relatively sot . Substantial strengthen-
ing can be achieved by ageing treatments that induce a i ne and uniform
precipitation of ω and α phase components, but this inevitably increases
the Young's modulus of the alloy [195, 205, 209, 210]. Consequently, there
is a critical need to devise strategies to produce β-Ti alloys with low Young's
modulus, and high strength, making them more suitable for use in dental
and orthopedic applications.
h e results of the recent study suggest that it is possible to design nano-
crystalline β-Ti alloys that meet the simultaneous requirements of high
strength, low modulus of elasticity and excellent biocompatibility. Notably,
all of the alloying elements (Nb, Ta, Zr, and O) in the β-Ti alloy are non-
toxic and non-allergenic [195]. h e nano-grain nature of the material leads
to improved bulk mechanical properties, plus the nanotopography on the
surface contributes to improved biological responses. Higher strength
is evident by the superior hardness which arises from grain rei nement
[211]. Lower rigidity was achieved, which is attributed to the nanocrystal-
line structure and the complete elimination of the ω phase. In addition
to these desirable mechanical properties, the nanocrystalline β-Ti alloy
also displays excellent
in vitro
biocompatibility, indicated by enhanced cell
attachment and proliferation. h is novel nanocrystalline β-Ti alloy has a
signii cant potential as a new generation of implant material with signii -
cant promise in load bearing biomedical applications.
