Biomedical Engineering Reference
In-Depth Information
rolling, rod rolling, sheet rolling, extrusion, and swaging. Typically,
the temperature range for the process for the NiTi system is 600
o
C
to 900
o
C approximately (0.55−0.75)
T
m
.
For the inal product shape with the inished surface, understood
as a reined microstructure with optimum mechanical properties, a
combination of cold working and annealing is required. For Nitinol
wires in the as-drawn conditions, material microstructure consists
of deformed martensite with the high density of dislocation.
As shown in Fig. 8.22, to achieve the best stress-strain behavior
— maximum superelasticity — the material should be heat treated
in the temperature range of 400-500°C. To obtain the best shape
settings, a combination of temperature, time, and strain has to
be considered during the designing and manufacturing process
selection (see Fig. 8.23).
Figure 8.22
Stress-strain behavior of NiTi alloy in various treatment
conditions [40].
Figure 8.23
Temperature time transformation diagram for Ni
50.8
Ti
49.2
wire form [31].