Classes of materials used in medicine - Biomedical Engineering Desk Reference - page 238

Table 3.2.9-3 Chemical compositions of co-based alloys for implants

Material ASTM

designation

Common
trade names

Composition
(wt.%)

Notes

Co-Cr-Mo F75 Vitallium

Haynes-Stellite 21
Protasul-2
Micrograin-Zimaloy

58.9-69.5 Co
27.0-30.0 Cr
5.0-7.0 Mo
max 1.0 Mn
max 1.0 Si
max 2.5 Ni
max 0.75 Fe
max 0.35 C

Vitallium is a trade mark of
Howmedica, Inc.
Hayness-Stellite 21 (HS 21) is
a trademark of Cabot Corp.
Protasul-2 is a trademark of
Sulzer AG, Switzerland.
Zimaloy is a trademark of
Zimmer USA.

Co-Cr-Mo F799 Forged Co-Cr-Mo

Thermomechanical Co-Cr-Mo
FHS

58-59 Co
26.0-30.0 Cr
5.0-7.00 Mo
max 1.00 Mn
max 1.00 Si
max 1.00 Ni
max 1.5 Fe
max 0.35 C
max 0.25 N

FHS means, ''forged high
strength'' and is a trademark of
Howmedica, Inc.

Co-Cr-W-Ni F90 Haynes-Stellite 25

Wrought Co-Cr

45.5-56.2 Co
19.0-21.0 Cr
14.0-16.0 W
9.0-11.0 Ni
max 3.00 Fe
1.00-2.00 Mn
0.05-0.15 C
max 0.04 P
max 0.40 Si
max 0.03 S

Haynes-Stellite 25 (HS25) is
a trademark of Cabot Corp.

Co-Ni-Cr-Mo-Ti F562 MP 35N

Biophase
Protasul-1( )

29-38.8 Co
33.0-37.0 Ni
19.0-21.0 Cr
9.0-10.5 Mo
max 1.0 Ti
max 0.15 Si
max 0.010 S
max 1.0 Fe
max 0.15 Mn

MP35 N is a trademark of SPS
Technologies, Inc.
Biophase is a trademark of
Richards Medical Co.
Protasul-10 is a trademark of
Sulzer AG, Switzerland.

relative amounts of the alpha and carbide phases should
be approximately 85% and 15%, respectively. However,
because of nonequilibrium cooling, a ''cored'' micro-
structure can develop. In this situation, the interden-
dritic regions become solute (Cr, Mo, C) rich and contain
carbides, while the dendrites become depleted in Cr and
richer in Co. This is an unfavorable electrochemical sit-
uation, with the Cr-depleted regions being anodic with
respect to the rest of the microstructure. (This is also an
unfavorable situation if a porous coating will sub-
sequently be applied by sintering to this bulk metal.)
Subsequent solution-anneal heat treatments at 1225 C
for 1 hour can help alleviate this situation.
Second, the solidification during the casting process
results not only in dendrite formation, but also in a rela-
tively

because it decreases the yield strength via a Hall-Petch
relationship between yield strength and grain diameter
(see Eq. 3.2.9.2 in the section on stainless steel).
The dendritic growth patterns and large grain diameter
(w4 mm) can be easily seen in Fig. 3.2.9-7A , which
shows a hip stem manufactured by investment casting.
Third, casting defects may arise. Figure 3.2.9-7B
shows an inclusion in the middle of a femoral hip stem.
The inclusion was a particle of the ceramic mold (in-
vestment) material, which presumably broke off and
became entrapped within the interior of the mold while
the metal was solidifying. This contributed to a fatigue
fracture of the implant device in vivo, most likely because
of stress concentrations and crack initiation sites associ-
ated with the ceramic inclusion. For similar reasons, it is
also desirable to avoid macro- and microporosity arising large grain size. This is generally undesirable