Biomedical Engineering Reference
In-Depth Information
Fig. 3.2.9-9 Microstructure of Co-based MP35N, ASTM F562,
Biophase. (Photo courtesy of Smith & Nephew Richards, Inc.,
Memphis, TN.)
Fig. 3.2.9-8 Microstructure of the Co-Cr-Mo ASTM F75 alloy
made via HIP, showing the much smaller grain size relative to that
in Fig. 6. (Photo courtesy of Zimmer USA, Warsaw, IN.)
Titanium-based alloys
Fig. 3.2.9-9
) and the HCP platelets further impede
dislocation motion, the resulting structure is significantly
strengthened (
Table 3.2.9-2
). It can be strengthened
even further (as in the case of Richards Biophase) by an
aging treatment at 430-650
C. This produces Co
3
Mo
precipitates on the HCP platelets. Hence, the alloy
is truly multiphasic and derives strength from the com-
bination of a cold-worked matrix phase, solid solu-
tion strengthening, and precipitation hardening. The
resulting mechanical properties make the family of
MP35N alloys among the strongest available for implant
applications.
Composition
CP titanium (ASTM F67) and extra-low interstitial (ELI)
Ti-6Al-4Valloy (ASTM F136) are the two most common
titanium-based implant biomaterials. The F67 CP Ti is
98.9-99.6% Ti (
Table 3.2.9-4
). The oxygen content of CP
Ti (and other interstitial elements such as C and N) affects
its yield and its tensile and fatigue strengths significantly,
as discussed shortly.
With Ti-6Al-4V ELI alloy, the individual Ti-Al and
Ti-V phase diagrams suggest the effects of the alloying
Table 3.2.9-4 Chemical compositions of Ti-based alloys for implants
Material
ASTM
designation
Common/
trade names
Composition
(wt.%)
Notes
Pure Ti, grade 4
F67
CP Ti
Balance Ti
max 0.10 C
max 0.5 Fe
max 0.0125-0.015 H
max 0.05 N
max 0.40 O
CP Ti comes in four grades
according to oxygen content-
Grade 1 has 0.18% max O
Grade 2 has 0.25% max O
Grade 3 has 0.35% max O
Grade 4 has 0.40% max O
Ti-6Al-4V ELI
*
F136
Ti-6Al-4V
88.3-90.8 Ti
5.5-6.5 Al
3.5-4.5 V
max 0.08 C
max 0.0125 H
max 0.25 Fe
max 0.05 N
max 0.13 O
*
A more recent specification can be found from ASTM, the American Society for Testing and Materials, under F136-98e1 Standard Specification for Wrought Titanium-6
Aluminium-4 ELI (Extra Low Intersitial) Alloy (R56401) for Surgical Implant Applications.
