Metals - Orthopaedic Biomaterials in Research and Practice - page 181

Table 7.6 titanium-base alloy compositions

Alloy
ASTM F 67 F 136

ISO 5832-3

Other Pure Ti Ti6Al4V Ti-5Al-2.5Fe

Elements (w/o)
Al — 5.5-6.5 4.5-5.5

C <0.1 <0.08 <0.08

Fe <0.5 <0.25 2-3

H <0.015 <0.015 <0.0125

N <0.05 <0.05 <0.05

O <0.4 <0.2 <0.2

Ti >99 Balance Balance

V — 3.5-4.5 —

the ability to manipulate their microstructure by chemical additions and
heat treatment permit the generation of a very wide range of microstruc-
tures and mechanical properties. Because of concerns with the toxicity
of vanadium and aluminum, alternative alloys of Ti6Al7Nb (ASTM F
1295) and Ti-5Al-2.5Fe have been developed with a similar metallurgi-
cal structure.
In a continued search for a Ti alloy with high corrosion resistance,
good toxicity, low modulus (to minimize stress shielding), high strength,
and good notched fatigue strength, alternative β-Ti alloys are being devel-
oped. β-Ti does not alloy with Al or Va, thus reducing concerns with
cytotoxicity. The principal alloying elements in β-Ti include tantalum,
iron, niobium, zirconium, and molybdenum. The most notable of the
β alloys include Ti-13Nb-13Zr, Ti-12Mo-6Zr-2Fe (ASTM F 1813), and
Ti-15 Mo. All except Ti-6Al-7Nb have a low elastic modulus designed to
combat stress shielding.
Titanium in a porous form has been developed to reduce the stiff-
ness mismatch between the alloy and its supporting bone and to facili-
tate ingrowth of new bone tissue. Porous titanium has been prepared
via slurry foaming, plasma spray coating, sintering of titanium fibers or
powder, and the space holder method. Each fabrication technique has
unique advantages and disadvantages. Material properties for Ti in its
porous form are presented in Chapter 13.

Mechanical
properties

Titanium is lighter than stainless steel and cobalt chromium alloys,
with a density of 4.5 g/cm 3 . Titanium and its alloys have moduli that are
roughly half those of stainless steel and cobalt-base alloys, combined
with relatively high strengths (a higher strength-to-density ratio than any
other metal) and low ductilities. In fact, F 136 is a relatively brittle mate-
rial (Table 7.7). Another of the attractions of alternative Ti alloys is the
potential for greater ductility, producing easier formability, thus making
Ti suitable for a greater range of applications. Because of the formation