Biomedical Engineering Reference
In-Depth Information
nonequilibrium plate precipitates is an important factor for the remarkable increase in
σ
s
below 820 K. The maximum value of
σ
s
, 260 MPa, was obtained in the specimen heat-
treated at 687 K for 6.4 ks (open circle in Figure 9.32a).
The maximum recoverable strain
ε
max
for each heat treatment condition was obtained
by the thermal cycling test and plotted as a function of
T
h
in Figure 9.32b. As shown in the
figure,
ε
max
increases with decreasing
T
h
or increasing
σ
s
until
T
h
reaches down to 773 K.
Below 773 K, however,
ε
max
decreases with further decreasing
T
h
. The
ε
max
decreases at
687 K, because the crystallization does not complete for this heat treatment. Heating at this
temperature for 6.4 ks caused
ε
max
to increase as shown in Figure 9.32b because further
precipitation of the plate precipitates and further crystallization occur.
TiNiX Ternary Alloy Thin Films
TiNiCu Films
Applications of microactuators require high frequency and fast response (narrow transfor-
mation temperature range and hysteresis). One of the challenges for the successful applica-
tion of TiNi films is effective reduction of hysteresis and increase in operating frequency.
The binary TiNi alloy films have a large temperature hysteresis of about 30°C, and TiNi
films with small hysteresis are preferred for faster actuation. Addition of Cu in TiNi films
is effective in reducing the hysteresis (Du and Fu, 2004). Compared with TiNi binary alloy,
TiNiCu alloys also show less composition sensitivity in transformation temperatures,
lower martensitic yield stress, and superior fatigue property, and so on, which make them
more suitable for microactuator application (Chang and Grummon, 1997).
Figure 9.33 shows DSC curves measured in the TiNiCu thin films (Miyazaki and Ishida,
1999). The solid lines indicate the transformation upon cooling, whereas the dashed lines
indicate the reverse transformation upon heating. The Ti-48.7 at.% Ni binary and the
Ti-42.6Ni-5.0Cu (at.%) ternary alloys show single peak associated with the transformation
from B2 (parent phase) to M (monoclinic martensite) upon cooling and associated with the
reverse transformation from M to B2 upon heating, respectively. The sharpness of these
peaks indicates that the distribution of alloy composition is uniform. The DSC curve of
Ti-37.0Ni-9.5Cu (at.%) film shows two peaks. The first peak is very sharp and high, but the
second one is very diffuse and almost invisible. However, when the ordinate of the curve
is magnified, the second peak becomes visible. Based on the x-ray diffraction results, it
is confirmed that the former transformation is associated with the transformation from
B2 to O (orthorhombic martensite) upon cooling and the reverse transformation from O
to B2 upon heating. The lattice parameters of these three phases (B2, O, M) are shown
in Table 9.3. The Ti-26.6Ni-18.0Cu (at.%) shows only single-stage transformation which
is associated with the transformation from B2 to O upon cooling and the corresponding
reverse transformation from O to B2 upon heating, respectively. The transformation tem-
perature from O to M is supposed to decrease significantly with an addition of Cu, so that
it becomes unmeasurable.
Figure 9.34 shows the transformation temperatures of as a function of Cu content,
M*
and
O*
being the transformation peak temperatures of the B2 to M (or O to M) and B2 to O,
respectively, while
MA*
and
OA*
the transformation peak temperatures of the correspond-
ing reverse transformations, respectively. These temperatures are as high as those of bulk
specimens, implying that the thin films contain few impurities. The
M*
decreases slightly
with increasing Cu content until 9.5 at.%. The 9.5 at.% Cu alloy shows a two-stage transfor-
mation, the first transformation temperature
O*
being 314 K and the second one
M*
being
270 K. By adding Cu furthermore,
M*
decreases drastically, whereas
O*
increases slightly.
