Biomedical Engineering Reference
In-Depth Information
TABLE 9.4
Transformation and Shape Memory Characteristics of Ti-Ni, Ti-Ni-Cu, and Ti-Ni-Pd Alloy
Thin Films
Ti-Ni(M-Phase)
Ti-Ni-Cu(O-Phase)
Ti-Ni-Pd(O-Phase)
M*
(
O*
) (K)
332
313
385
A*
(
OA*
) (K)
359
324
401
Hys
(K)
27
11
16
ε
max
(%)
3.8
3.0
2.5
σ
S
(MPa)
90
173
200
Source:
Miyazaki, S., Ishida, A.,
Mater. Sci. Eng. A
, 273-275, 106-133, 1999, with permission from Elsevier.
solid-solution hardening effect by the third element of Pd. The summary of the transfor-
mation and shape memory characteristics of these three alloy thin films is shown in Table
9.4.
M*
(
O*
) and
A*
(
OA*
) are abbreviations of the transformation peak temperatures for
the martensitic (orthorhombic) and reverse-martensitic (reverse-orthorhombic) transfor-
mations, respectively, were measured by DSC.
Hys
is an abbreviation of the temperature
hysteresis, that is, the temperature difference between
M*
(
O*
) and
A*
(
OA*
).
ε
max
and
σ
s
stand for the maximum recoverable transformation strain and the critical stress for slip,
respectively.
Film Stress and Stress Evolution
Film stress and stress evolution in the films could pose potential problems in applications,
as it may influence not only adhesion between film and substrate, but also deformation
of MEMS structure, mechanics, and thermodynamics of transformation and SE effects,
and so on (Craciunescu et al., 2003). Large residual stress could lead to either film crack-
ing or decohesion under tension, or film delamination and buckling under compression.
Deposition conditions, post-deposition thermomechanical treatment, and composition of
the TiNi films could have important consequences with respect to the development of
residual stress. These have been studied in detail and reported in Fu et al. (2003). In the
crystalline TiNi films, large tensile stress is generated during heating due to the phase
transformation from martensite to austenite, whereas during cooling, the martensitic
transformation occurs and the tensile stress drops significantly because of the formation
and alignment of twins. The stress generation and relaxation behaviors upon phase trans-
formation are significantly affected by film composition, deposition, and/or annealing
temperatures, which strongly control the formation and evolution of intrinsic stress, ther-
mal stress, and phase transformation behaviors (Fu and Du, 2003).
Using the curvature method, stress change as a function of temperature can be mea-
sured in situ with change in temperature. The martensitic transformation temperatures
and hysteresis, multistage transformation, and magnitude of shape recovery can be eas-
ily obtained from the stress-temperature curves (Fu and Du, 2002, 2003; Fu et al., 2003).
Figure 9.39a shows a typical curve of the measured stress of a Ti
50
Ni
46
Cu
4
film as a function
of temperature up to 100°C. The stress-versus-temperature plot shows a closed hysteresis
loop shape. At room temperature, the stress is tensile with a low value. During heating,
the tensile stress increases significantly because of the phase transformation from mar-
tensite to austenite. Above austenite transition start temperature (
A
s
), the stress increases
linearly until the temperature reaches to austenite transition finish temperature (
A
f
). With
