Biomedical Engineering Reference
In-Depth Information
and precipitation of carbides (M
23
C
6
, M
7
C
3
, M
6
C, where M= Co, Cr or
Mo). There can also be interdendritic Co- and Mo-rich sigma intermetallics.
overall, the volume fractions of alpha and carbide phases are about 85%
and 15%, respectively. In the as-cast products, however, segregation during
cooling and a coarse grain size (~ 4 mm) structure that can strongly influence
implant properties, often negatively, cannot be avoided. Powder metallurgical
techniques, such as hot isostatic pressing (HIP), are used to enhance the
mechanical properties by improving the microstructure of the alloy (grain
size of about ~ 8 mm). The associated finer distribution of carbides has a
hardening effect as well.
Wrought alloys have a lower Cr content (19-21 wt%) and higher levels
of ni, Fe or Mn to stabilize the fcc phase in the annealed condition. Further
mechanical deformation (about 50% cold working) causes the transformation
to a hexagonal close-packed (hcp) phase that forms via a shear-induced
martensitic-type transformation. In this condition the microstructure consists
of a fcc matrix with fine hcp platelets, which causes an improvement in the
mechanical properties. additional hardening may be obtained by precipitation
of Co
3
Mo carbides on the hcp platelets during ageing between 500 and 600ºC.
The superior mechanical properties of the wrought ConiCrMo alloy make
it suitable for the fabrication of stems for hip joint prostheses.
6.2.3 Ti and Ti-base alloys
attempts to use titanium for implant fabrication date back to the late 1930s.
Titanium has a low density (4.5 g cm
-3
) and can be greatly strengthened by
alloying and thermomechanical processing techniques. owing to their high
reactivity in the presence of oxygen, an inert atmosphere or vacuum is required
during high temperature processing. Commercially pure titanium (CP-Ti) is
essentially a titanium (hcp crystal structure) and is available in four grades.
The primary difference between grades is the oxygen and iron content. oxygen
in particular has a great influence on ductility and strength. These materials
also contain hydrogen (0.015 wt%) and carbon (0.08 wt%). CP-Ti is selected
for its excellent corrosion resistance, especially in applications where high
strength is not required. Depending on the cold working conditions, grain
diameters in the range 10-150 mm are obtained.
Pure titanium undergoes a phase transformation, changing with increasing
temperature from an hexagonal close packed (hcp) structure, called the a
phase, to a body-centered cubic (bcc) structure, referred to as the b phase.
The transition temperature takes place at around 885ºC. Below the b-transus
temperature, diffusion processes are substantially slower. The completion of
transformation on heating to b is strongly influenced by elements stabilizing
the a phase (like o, al, n, C), raising the transition temperature, or the b
phase (like V, nb, Mo, Ta), lowering the transition temperature. at high
