Biomedical Engineering Reference
In-Depth Information
luids due to the loosening of implant, infection, wear or fretting
corrosion, and metabolism [11]. Unfortunately, TiO
2
ilm is sensitive
for the luoride ions, always present in oral environments [12, 13,
30], as well as often in tooth gels.
Because the vanadium is a toxic element, vanadium-free Ti alloys
such as Ti-6Al-7Nb and Ti-13Nb-13Zr have been developed and
replaced vanadium in Ti-6Al-4V [35]. The Ti-6Al-7Nb alloy can be
a better alternative to Ti-6Al-4V because of its corrosion resistance
and resistance to loss of mechanical properties with changes in pH
in simulated body luid environment [3, 24].
The microstructures of the Ti-6Al-4V consist of two phases α
and β (Fig. 5.6). Due to a two-phase microstructure, Ti-6Al-4V is
more susceptible to corrosion across the grain boundaries because
of galvanic cell formation. Choubey
et al
. [3] investigated corrosion
resistance (in simulated body luids — Hank's solution) of the
Ti-6Al-4Nb, Ti-6Al-4Fe and Ti-5Al-2.5Fe alloys and compared
their properties with the parent Ti-6Al-4V. All the alloys were
α-β-type alloys. Vanadium, niobium, and iron are β-stabilizers, while
aluminum is an α-stabilizer; α was the dominant phase in all these
alloys. The addition of niobium increases the grain size considerably
and results in Widmanstätten-type structure. These new V-free
alloys passivates immediately after immersion in the corrosive
solution [3]. Addition of alloying elements like niobium or iron does
not signiicantly affect corrosion current density in the passive range.
The passive range for Ti-6Al-4V, Ti-6Al-4Fe and Ti-6Al-4Nb is
comparable, whereas for Ti-6Al-4Nb is lower. The corrosion rate of
Ti-5Al-2.5Fe, Ti-6Al-4V, Ti-6Al-4Fe, and Ti-6Al-4Nb is comparable
and is not drastically deteriorated by the Fe substitution [3].
(a)
(b)
Figure 5.6
Optical micrographs of the Ti-6Al-4V surface [3].
