Biomedical Engineering Reference
In-Depth Information
Ti−43.9 at.% Ni thin film
973 K -3.6 ks W.Q.
σ
= 250 MPa
N
= 100
N
= 50
N
= 10
N
= 1
M
s
R
s
250
300
350
Temperature (K)
FIGURE 9.44
Effect of thermal cyclic deformation on strain vs. temperature curves associated with martensitic trans-
formation for a solution-treated TiNi thin film;
N
indicates number of cycle; ks after solution-treatment at
973 K for 3.6 ks. (From Miyazaki, S., Ishida, A.,
Mater. Sci. Eng.
, 273-275, 106-133, 1999, with permission from
Elsevier.)
reverse R-phase transformation from the R-phase to the parent phase. Although there is an
unrecoverable strain which is caused by slip deformation, it is only 0.03%.
With increasing the number of cycles, the R-phase transformation characteristics such
as
R
s
,
ε
R
, and
H
R
are kept almost constant, whereas the martensitic transformation char-
acteristics change apparently. For example,
M
s
rises and hence the temperature difference
between
R
s
and
M
s
decreases. Besides, temperature hysteresis (
H
M
) decreases and mar-
tensitic transformation strain increases gradually. Such changes in the martensitic trans-
formation can be considered to be caused by the internal stress field which is formed by
the introduction of dislocations during cyclic deformation. The internal stress field over-
laps with the external applied stress so that the martensitic transformation temperatures
increase. However, the R-phase transformation characteristics show few changes during
cyclic deformation because the R-phase transformation involving a small strain is not so
sensitive to applied stress.
Fu et al. (2002) studied the fatigue of the constrained TiNi films using the changes of
recovery stress during cycling and showed that the recovery stress of TiNi films from cur-
vature measurement decreased dramatically in the first tens of cycles and became stable
after thousands of cycles.
This reduction of the recovery stress is believed to be resulted
from the dislocation movement, grain boundary sliding, void formation, or partial debond-
ing at the film/substrate interfaces, nonrecoverable plastic deformation, changes in stress,
and so on. Transformation temperatures also changed dramatically during cycling. The
repeated phase changes will alter the microstructure and hysteresis of the transformation
and in turn lead to changes in transformation temperatures, stresses, and strains. All these
changes in the martensitic transformation behavior become insensitive to thermal cycling
after the number of cycles exceeds 50, indicating that training is effective in stabilizing
the shape memory behavior. Such a steady state has been achieved by the work harden-
ing during cyclic deformation. Therefore, it is concluded that the stabilization of the shape
memory characteristics against thermal cycles under stress can be improved by training.
