Biomedical Engineering Reference
In-Depth Information
annealing below the b transus temperature between 480 and 650ºC. The
resulting microstructure, so-called mild annealed, strongly depends upon the
previous working. Figure 6.5 illustrates the typical biphasic microstructure
consisting of bright (b phase) and dark (a phase) zones.
Bimodal microstructure consists of isolated primary a grains in a transformed
b matrix. This microstructure may be developed by solution treatment below
the b transus, typically between 900 and 950ºC, followed by air cooling and
ageing below 700ºC.
6.3 Other metallic materials
6.3.1 NiTi shape memory alloys
In recent years the largest commercial successes of shape memory alloys
are linked to medical applications owing to their good corrosion resistance
and unique capacity to return to a previous shape when subjected to an
appropriate thermal procedure.
5,6
a widely known niTi alloy is nitinol (an
acronym for niTi naval ordnance Laboratory) that contains about 50 at%
ni and small amounts of Co, Cr, Mn and Fe. The shape memory effect is
related to the diffusionless reversible martensitic transformation induced
by changes in the temperature, as illustrated in Fig. 6.6. It is characterized
by its transition temperatures
M
s
and
M
f
which refer to the start and finish
of the formation and growth of the low-temperature phase martensite on
cooling, and
A
s
and
A
f
, which refer to the start and finish of the formation
and growth of the high-temperature phase austenite on heating. The overall
10
m
m
6.5
Secondary electron image revealing the biphasic microstructure
in an etched section of Ti-6Al-4V.
