Biomedical Engineering Reference
In-Depth Information
structure, precipitation, defects, film/substrate interface) and external parameters (ther-
momechanical treatment, applied maximum stress, stress and strain rate, the amplitude of
temperature cycling frequency) after long-term thermomechanical cycles.
Figure 9.43 shows the strain versus temperature curves as a function of the number of
cyclic deformation associated with the R-phase transformation under 50 MPa in a Ti-43.9
at.% Ni solution-treated thin film. In the cooling process of the initial cycle (
N
= 1), the
R-phase transformation starts at 333 K and finishes at 316 K, resulting in a shape change of
0.13% strain. In the heating process, the reverse R-phase transformation starts at 323 K and
finishes at 336 K, resulting in a perfect shape recovery and small temperature hysteresis
(
H
R
) of 4 K. Because of the small hysteresis, a quick response is expected in microactua-
tors using such R-phase transformation. After cycling for 100 times, no significant change
appears in the shape of the curves. The reason for the stability can be explained by the fact
that the R-phase transformation strain is so small that slip deformation hardly occurs.
Figure 9.44 shows the effect of thermally induced cyclic deformation on the strain-
temperature curve under a stress of 250 MPa in a solution-treated Ti-43.9 at.% Ni thin film.
The curves of the cyclic deformation show a two-stage deformation; upon cooling, a shape
change appears at
R
s
due to the R-phase transformation, and upon further cooling, a sec-
ond shape change occurs at
M
s
because of the martensitic transformation as shown in the
initial cyclic deformation curve. The strains
ε
R
and
ε
M
are 0.28% and 1.12%, respectively.
Upon heating, the original shape of the specimen is almost recovered due to a two-stage
deformation associated with the reverse-transformations occurring at
A
s
for the first stage
and at
RA
s
for the second stage. The first stage is associated with the reverse martensitic
transformation from the martensitic phase to the R-phase, whereas the second with the
Ti−43.9 at.% Ni thin film
973 K 3.6 ks W.Q.
σ
= 50 MPa
Heating
N
= 100
N
= 50
N
= 10
N
= 1
Cooling
350
Temperature (K)
300
FIGURE 9.43
Effect of thermal cyclic deformation on strain vs. temperature curves associated with R-phase transformation
for a solution-treated TiNi thin film;
N
indicates number of cycle. (From Miyazaki, S., Ishida, A.,
Mater. Sci. Eng.
,
273-275, 106-133, 1999, with permission from Elsevier.)
