Biomedical Engineering Reference
In-Depth Information
Transformation Temperatures
The martensitic transformation temperatures are conventionally determined by the elec-
trical resistivity measurement or by the differential scanning calorimetry (DSC). Figure
9.9a and b shows example results of such measurements applied to an equiatomic TiNi
alloy, which was solution-treated at 1273 K for 3.6 ks. When the specimen is cooled from
the parent B2 phase, the martensitic transformation starts at
M
s
(the martensitic transfor-
mation start temperature) by evolving transformation heat, that is, the change in chemical
enthalpy
Δ
H
is negative and the reaction is exothermic as shown in the DSC curve upon
cooling. The electrical resistivity shows a normal decrease upon cooling in the B2 phase
region and increase of the decreasing rate at the onset of the transformation at
M
s
because
of the crystal structural change. Upon further cooling, the martensitic transformation fin-
ishes at
M
f
(the martensitic transformation finish temperature).
Upon heating the specimen from the martensite phase, the martensite phase starts to
reverse transform to the B2 phase at
A
s
(the reverse martensitic transformation start tem-
perature) and finish at
A
f
(the reverse martensitic transformation finish temperature). The
DSC curve upon heating shows an endothermic reaction, that is,
Δ
H
is positive and trans-
formation heat is absorbed. The
M
s
is shown in Figure 9.10 as a function of Ni content. In
the composition range of the TiNi, the
M
s
decreases with increasing Ni content above 49.7
at.% Ni, whereas they are constant below 49.7 at.% Ni.
A
f
is about 30 K higher than
M
s
in
all composition region. The reason for the constant
M
s
in the Ni-content region less than
49.7 at.% can be ascribed to the constant Ni content in the TiNi phase, because the Ti
2
Ni
appears in the Ni-content region less than 49.7 at.%, keeping the Ni content of the TiNi to
be 49.7 at.%.
Shape Memory and SE
The mechanisms of SME and SE are explained using a two-dimensional crystal model
shown in Figure 9.11. The crystal structure of the parent phase is shown in Figure 9.11a.
(a)
(b)
M+B2
M
B2
M
*
M
s
Cooling
M
f
M
f
M
A
s
A
f
A
f
A
s
Heating
M+B2
B2
M
A
*
200
300
200
300
400
Temperature (K)
Temperature (K)
FIGURE 9.9
(a) Electrical resistance vs. temperature curve showing transformation temperatures of Ti50.0 at.% Ni alloy;
(b) DSC curves showing transformation temperatures of Ti50.0 at.% Ni alloy. (From Miyazaki, S., in Miyazaki
et al., eds.,
Thin Film Shape Memory Alloys: Fundamentals and Device Applications
, Cambridge University Press, UK,
2009, reproduced with permission from Cambridge University Press.)
