Biomedical Engineering Reference
In-Depth Information
370
350
330
310
290
270
250
230
210
190
170
48.5
Ms
49.0
49.5
50.0
50.5
51.0
51.5
Ni content (at.%)
FIGURE 9.10
Ni-content dependence of
M
s
temperature. (From Miyazaki, S., in Miyazaki et al., eds.,
Thin Film Shape Memory
Alloys: Fundamentals and Device Applications
, 2009, reproduced with permission from Cambridge University
Press.)
It perfectly transforms to the martensite upon cooling below
M
f
as shown in Figure 9.11b,
where two martensite variants labeled A and B with the same crystal structure but differ-
ent orientations are shown. Thermally induced martensite should be a habit plane variant
because it will be formed in the parent phase and is necessary to connect the parent phase
along the habit plane. However, the lattice invariant shear is not shown in A and B variants
for simplicity. The lattices of both martensite variants are made by distorting the parent
phase lattice upon the transformation, creating the same shear strain with opposite senses
to each other. Therefore, the martensite morphology in Figure 9.11b is self-accommodated
(a)
Cooling (
T
<
M
f
)
Heating (
T
>
A
f
)
(c)
(b)
A
B
Unloading
(
T
>
A
f
)
Loading
(
T
>
A
f
)
Loading (
T
<
M
f
)
Shear
FIGURE 9.11
Schematic figure showing specimen shapes and crystal structures upon cooling, heating, loading, or unloading
during shape memory and superelastic behavior.
