Metals - Orthopaedic Biomaterials in Research and Practice - page 188

TrIP steels
There is a continuing interest in alternative alloys for orthopaedic
applications. The principal driving forces are the designers' wishes
for improved properties, especially in fatigue; the manufacturers' con-
cerns over limiting costs, both of raw materials and of manufacturing
processes; and the researchers' concerns over the biologic activity of
components of present alloys. TRIP (transformation-induced plasticity)
steels are a class of steels that may be cold worked after heat treatment to
produce significantly higher strengths for a given degree of work hard-
ening while retaining greater ductilities than can be achieved in austen-
itic stainless steels. Unfortunately, they have somewhat higher corrosion
rates than stainless steels; additional alloy improvements will be neces-
sary before clinical use.
Fatigue strength
The typical orthopaedic application may be characterized as involv-
ing low-stress, high-cycle fatigue loading. Thus, the fatigue behavior of
implant alloys has taken on great importance. The endurance limit is
frequently defined as the residual strength (alternate term for fatigue
strength) after 10 7 load-unload cycles. Since a typical lower extremity
device experiences 1 to 2 million cycles per year, this reflects 5-10 years
of service and has come to be a routine comparative measure of fatigue
behavior. Table 7.9 is a tabulation of the fatigue strength limits for some
of the alloys previously discussed.
These values should be interpreted with some caution as they are
somewhat test dependent and are all measured in air. The fatigue resis-
tance superiority of titanium and its alloys becomes clearer in corrosive
environments: under these conditions, as in vivo , the ratio of σ f /σ u tends
to fall for stainless steels and cobalt-base alloys but is essentially unaf-
fected for titanium-base alloys.
Trade names
Table 7.10 gives a compilation of past and current trade names for ortho-
paedic alloys, their manufacturer's names, and the ASTM standards that
they meet. Although two alloys that meet the same standard may have
very similar compositions, a fairly wide range of properties is possible
since different starting material purities, manufacturing techniques, and
finishing techniques, especially different degrees of cold work, different
annealing cycles, and different surface treatment methods, all contribute
to a broad range of possible mechanical properties in any one system.
However, there are no clear “winners”; each manufacturer has had to
develop his material types to suit their designs and vice versa, so that
performance is satisfactory and failure rates are acceptably low.