Biomedical Engineering Reference
In-Depth Information
the same moduli as Co-Cr-Mo alloys (210
-
230 GPa) [BRU 04]. To
enhance fabricability, Cr content is reduced while Ni and W are
added. To produce wrought Co-based alloys, the content of carbon is
reduced, leading to a decrease in carbide precipitation and therefore a
lower strength. However, in Co-Ni-Cr-Mo alloys, the high content of
nickel (33-37%) stabilizes the hexagonal close-packed phase (hcp)
and hcp bands emerge in the face-centered cubic grains, which results
in a strengthened structure. Moreover, to maintain a good corrosion
resistance and counteract the decrease in Cr content, the content of Mo
is enhanced. This also strengthens the material because of the
precipitation of Co
3
Mo within the hcp phase. Co-Ni-Cr-Mo alloys
have eventually a superior fatigue (500-793 MPa) and ultimate tensile
strength (1,206-1,795 MPa) than the Co-Cr-Mo alloys, and they are
the strongest alloys available for medical applications. They are
particularly suitable for long service life device such as the stems of
hip implants [BRU 04]. Co-Cr-W-Ni alloys are not as corrosion
resistant as alloys containing Mo. They are mainly used for fracture
fixation implants [PIL 09]. Due to the cost of tungsten and cobalt,
Co-Cr-W-Ni alloys are significantly more expensive than stainless
steel, which limits their usage [ONG 14]. Finally, some concerns arise
with these alloys due to the presence of poor biocompatible Ni.
However, the rate of nickel ion release was found to be the same for
Co-Ni-Cr-Mo and 316L stainless steel despite a higher Ni content in
the Co-based alloy [PAR 07].
3.2.4.
Shape-memory alloys
SMAs are materials that respond to stress or heating by undergoing
transition in their metallic crystal structure. Indeed, the martensitic
phase is a low-temperature stable phase which is easy to deform,
while the austenitic phase is stable at high temperature and is a rigid
body-centered cubic arrangement. The properties of SMA are
determined by the atomic composition and the processing methods.
One-way SMAs are materials that undergo deformations at low
temperature (in the martensitic phase) and return to their original
shape when heated to their austenitic phase. Two-way SMAs are more
complex since they are materials recovering a specific shape when
heated and finally returning to an alternate shape when cooled to the
