Biomedical Engineering Reference
In-Depth Information
100
m
m
6.2
Backscattered electron image revealing the grain size of a non
etched section of 316 L (courtesy of M. Multigner, CENIM).
cannot be cold worked without intermediate heat treatments. However, such
heat treatments should not induce the formation of chromium carbide at the
grain boundaries which may decrease the corrosion resistance.
6.2.2 Co-based alloys
There are two types of CoCr alloys extensively used in implant fabrications
such as the knee and hip. The castable CoCrMo alloy known as 'Vitallium'
was formerly introduced by Venable and stucke in 1936.
4
This alloy has
been used for many decades in dentistry too. The second group of alloys
is processed by hot forging (wrought alloys) and contains tungsten and a
higher nickel content.
The two basic elements of the CoCr alloys form a
solid solution of up to 65% Co. Although a number of specifications exist
for cobalt-base alloys, the main alloys used are Co-28Cr-6Mo cast alloy
(asTM F 75), Co-20Cr-15W-10ni wrought alloy (asTM F 90) and Co-
35ni-20Cr-10Mo (asTM F562).
The cast alloys contain up to 0.5 wt% carbon to improve the castability
by lowering the melting temperature to ~1350ºC. The normal fabrication
involves a lost wax method by using a wax pattern of the desired component.
The chromium enhances the corrosion resistance as well as the strength of
the alloy. The molybdenum is added to produce finer grains, which results in
higher strengths for both cast and wrought components. The microstructure
of the cast products consist of a cobalt-rich matrix (a phase) with an fcc
structure. strengthening is produced by a solid solution of alloying elements
