Biomedical Engineering Reference
In-Depth Information
precipitates grow by aging they lose the suppressing force against the growth of the pref-
erentially oriented martensite variants.
The aging effect was also investigated in the Ti-43.9 at.% Ni thin films. There is no aging
effect observed in the transformation temperatures as shown by the DSC results in Figure
9.25a. Two-stage transformation behaviors are observed both upon cooling and heating.
The appearance of the R-phase is usually attributed to a fine internal structure consist-
ing of dislocations and/or precipitates in bulk specimens. However, the R-phase cannot
be attributed to such internal structure in the Ti-43.9 at.% Ni thin film because there are
neither dislocations nor Ni-rich Ti
3
Ni
4
precipitates. The cause for the appearance of the
R-phase is considered to be another fine internal structure consisting of Ti
2
Ti2Ni compounds
and small grains. The grain size of sputter-deposited TiNi thin films generally ranges from
0.5
μ
m to several microns, whereas the grain size of solution-treated bulk TiNi alloys is up
to several tens of microns. This fine internal structure in the thin films also suppresses the
martensitic transformation more effectively than the R-phase transformation.
Figure 9.25b shows the shape memory behavior in the thin films. Again no aging effect
is observed in the shape memory behavior. The transformation temperatures and trans-
formation strains do not change with aging time. The reason why there is no aging effect
in the Ti-43.9 at.% Ni thin film is that in Ni-poor TiNi alloy, the Ti
3
Ni
4
precipitates cannot
be created, so that there is no variation in Ni content of the matrix. The strain-versus-
temperature curves clearly show two-stage deformation behavior both upon cooling and
heating, corresponding to the two-stage transformation behavior in Figure 9.25b.
M
s
of both the Ti-51.9 at.% Ni and Ti-43.9 at.% Ni thin films are replotted against aging
time in Figure 9.26, where
M
s
measured by both DSC and mechanical tests are included.
Controlling transformation temperature is one of the important techniques to fabricate
(a)
Ti−43.9 at.% Ni thin film
973 K 3.6 ks W.Q. − 773 K X ks W.Q.
(b)
Ti−43.9 at.% Ni thin film
973 K 3.6 ks W.Q. − 773 K X ks W.Q.
R*
Cooling
Heating
X
= 0
σ
= 240 MPa
M
*
X
= 3.6
X
= 36
X
= 360
X
= 360
X
= 36
Cooling
X
= 3.6
X
= 0
X
= 3.6
X
= 36
X
= 360
X
= 0
RA
*
A
*
M
s
Heating
R
s
200
250
300
350
200
250
300
350
Temperature (K)
Temperature (K)
FIGURE 9.25
(a) Effect of aging time on transformation temperatures measured by DSC in the Ti-43.9 at.% Ni thin film.
(From Miyazaki, S., Ishida, A.,
Mater. Sci. Eng.
, 273-275, 106-133, 1999, with permission from Elsevier.) (b) Effect
of aging time on shape memory behavior in Ti-43.9 at.% Ni thin film. (From Miyazaki, S., Ishida, A.,
Mater. Sci.
Eng.
, 273-275, 106-133, 1999, with permission from Elsevier.)
