Biomedical Engineering Reference
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probably relates to further phase changes in the non-
equilibrium cored microstructure in the original cast F75
alloy. However, it has been found that a modified
sintering treatment can return the fatigue strength back
up to about 200 MPa ( Table 3.2.9-2 ).
Beyond these metallurgical issues, a related concern
with porous-coated devices is the potential for decreased
fatigue performance due to stress concentrations in-
herent in the geometrical features where particles are
joined to the substrate (e.g., Fig. 3.2.9-2 ).
ASTM F799
The F799 alloy is basically a modified F75 alloy that has
been mechanically processed by hot forging (at about
800 C) after casting. It is sometimes known as thermo-
mechanical Co-Cr-Mo alloy and has a composition
slightly different from that of ASTM F75. The micro-
structure reveals a more worked grain structure than as-
cast F75 and a hexagonal close-packed (HCP) phase that
forms via a shear-induced transformation of FCC matrix
to HCP platelets. This microstructure is not unlike that
which occurs in MP35N (see ASTM F562).
The fatigue, yield, and ultimate tensile strengths of
this alloy are approximately twice those of as-cast F75
( Table 3.2.9-2 ).
Fig. 3.2.9-6 Microstructure of as-cast Co-Cr-Mo ASTM F75
alloy, showing a large grain size plus grain boundary and matrix
carbides. (Photo courtesy of Zimmer USA, Warsaw, IN.)
from metal shrinkage upon solidification of castings.
Figures 3.2.9-7C and 3.2.9-7D exemplify a markedly
dendritic microstructure, large grain size, and evidence of
microporosity at the fracture surface of a ASTM F75
dental device fabricated by investment casting.
To avoid problems such as the above with cast F75, and
to improve the alloy's microstructure and mechanical
properties, powder metallurgical techniques have been
used. For example, in HIP, a fine powder of F75 alloy is
compacted and sintered together under appropriate
pressure and temperature conditions (about 100 MPa at
1100 C for 1 hour) and then forged to final shape. The
typical microstructure ( Fig. 3.2.9-8 ) shows a much
smaller grain size (w8 m m) than the as-cast material.
Again, according to a Hall-Petch relationship, this mi-
crostructure gives the alloy higher yield strength and
better ultimate and fatigue properties than the as-cast
alloy ( Table 3.2.9-2 ). Generally speaking, the improved
properties of the HIP versus cast F75 result from both the
finer grain size and a finer distribution of carbides, which
has a hardening effect as well.
In porous-coated prosthetic devices based on F75
alloy, the microstructure will depend on the prior
manufacturing history of the beads and substrate metal
as well as on the sintering process used to join the beads
together and to the underlying bulk substrate. With Co-
Cr-Mo alloys, for instance, sintering can be difficult,
requiring temperatures near the melting point (1225 C).
Unfortunately, these high temperatures can decrease the
fatigue strength of the substrate alloy. For example, cast-
solution-treated F75 has a fatigue strength of about 200-
250 MPa, but it can decrease to about 150 MPa after
porous coating treatments. The reason for this decrease
ASTM F90
Also known as Haynes Stellite 25 (HS-25), F90 alloy is
based on Co-Cr-W-Ni. Tungsten and nickel are added to
improve machinability and fabrication. In the annealed
state, its mechanical properties are about the same as
those of F75 alloy, but when cold worked to 44%, the
properties more than double ( Table 3.2.9-2 ).
ASTM F562
Known as MP35N, F562 alloy is primarily Co (29-38.8%)
and Ni (33-37%), with significant amounts of Cr and Mo.
The ''MP'' in the name refers to the multiple phases in its
microstructure. The alloy can be processed by thermal
treatments and cold working to produce a controlled
microstructure and a high-strength alloy, as follows.
To start with, under equilibrium conditions pure solid
cobalt has an FCC Bravais lattice above 419 C and
a HCP structure below 419 C. However, the solid-state
transformation from FCC to HCP is sluggish and occurs
by a martensitic-type shear reaction in which the HCP
phase forms with its basal planes (0001) parallel to the
close-packed (111) planes in FCC. The ease of this
transformation is affected by the stability of the FCC
phase, which in turn is affected by both plastic de-
formation and alloying additions. Now, when cobalt is
alloyed to make MP35N, the processing includes 50%
cold work, which increases the driving force for the
transformation of the FCC to the HCP phase. The HCP
phase
emerges
as
fine platelets within
FCC
grains.
Because
the
FCC
grains
are
small
(0.01-0.1 m m,
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