Biomedical Engineering Reference
In-Depth Information
(a) Ti−48.7at.%Ni
(c) Ti−37.0Ni−9.5Cu (at.%)
A
s
180 MPa
180 MPa
MA
s
MA
f
OA
s
120 MPa
120 MPa
ε
A
ε
M
60 MPa
A
f
Heating
60 MPa
OA
f
M
f
M
s
O
f
ε
P
M
f
Heating
Cooling
M
s
O
s
Cooling
250
300
350
400
450
500
150
200
250
300
350
400
Temperature (K)
Temperature (K)
(b) Ti−42.6Ni−5.0Cu (at.%)
180 MPa
(d) Ti−26.6Ni−18.0Cu (at.%)
A
s
Heating
Cooling
Heating
OA
s
120 MPa
180 MPa
120 MPa
60 MPa
OA
f
60 MPa
A
f
M
f
O
f
O
s
Cooling
M
s
250
300
350
400
450
500
200
250
300
350
400
Temperature (K)
Temperature (K)
FIGURE 9.35
Strain versus temperature curves measured during cooling and heating under a variety of constant stresses in
TiNiCu thin films. (From Miyazaki, S., Ishida, A.,
Mater. Sci. Eng.
, 273-275, 106-133, 1999, with permission from
Elsevier.)
9.5 at.% to 18.0 at.% in the Cu-rich region where the transformation only occurs from B2
to O. This Cu dependence of the
ε
max
in the thin films is similar to that in bulk specimens.
However, the
ε
max
is a little smaller than that of the bulk specimens in the Cu-poor region.
This is supposed to come from the grain size effect; that is, the grain size of thin films is
smaller than that of bulk specimens.
The permanent strain
ε
P
due to slip deformation appears when the specimen is subjected
to thermal cycling under a constant stress which is higher than the critical stress for slip.
The critical stress for slip
σ
s
can be estimated by extrapolating the data of
σ
P
to zero strain
in a diagram showing the
σ
p
versus constant applied stress relationship. Values of
σ
s
esti-
mated in this way are shown in Figure 9.36b. It is found that
σ
s
increases with increasing
Cu content. For example,
σ
s
of the 0 at.% Cu specimen is only 55 MPa and that of the 18
at.% Cu specimen increases to 350 MPa, showing that the addition of Cu is also effective to
increase the stress for slip.
