Biomedical Engineering Reference
In-Depth Information
increases. Because of the significant phase transformation effect, thermal and intrinsic
stresses in the films are drastically relieved, resulting in a significant decrease in residual
stress. With the further increase in film thickness, more and more grain boundaries form
in the films. The grain boundaries are the weak points for generation of large distortion
and twinning processes. Therefore, as the film thickness increases, the constraining effect
from the neighboring grains becomes more and more significant, causing decreases in
recovery stress.
Temperature Memory Effect
A new phenomenon, temperature memory effect (TME), has also been reported in the
TiNi-based films (Wang et al., 2005). An incomplete thermal cycle upon heating in a SMA
(arrested at a temperature between austenite transformation start and finish temperatures,
A
s
and
A
f
) induced a kinetic stop in the next complete thermal cycle (Zeng et al., 2004),
and the kinetic stop temperature is a “memory” of the previous arrested temperature (see
Figure 9.46) (Wang and Zu, 2005). If a number
N
of incomplete heating processes with
different arrested temperatures are performed in a decreasing order,
N
temperatures can
be memorized. TME can be fully eliminated by following a complete transformation heat
cycle to above
A
f
. During the partial reverse transformation, only part of the martensite
transforms into the parent phase, with the rest of the martensite, M1, remaining. With a
further decrease in the temperature to below martensitic transformation finish tempera-
ture, the parent phase transforms back to martensite, and the newly formed martensite
is called M2. M1 and M2 are different martensite variants with different elastic energies,
which cause different transformation temperatures of M1 and M2 during the next heating
process. On the contrary, if a partial austenite to martensite transformation is performed
by an incomplete cycle on cooling, the next complete austenite to martensite transforma-
tion does not show any evidence of kinetic interruptions. TME can be found in both ther-
mally and external stress-induced transformations.
(a)
75.2˚C
(b)
Temperature (
º
C)
FIGURE 9.46
DSC results of (a) TiNiCu4 film and (b) temperature memory effect of same sample with a single incomplete
cycle on heating at 75.2°C. (Fu, 2009, with permission from Interscience Publisher.)
