Biomedical Engineering Reference
In-Depth Information
TiNi single phase region terminates at 903 K in Figure 9.5; however, the region seems to
extend to around room temperature in a narrow Ni content width according to empirical
information.
Crystallography of Martensitic Transformation
The parent phase of the TiNi has a CsCl-type B2 superlattice, whereas the martensite phase
is three-dimensionally close packed (monoclinic or B19'), as shown in Figure 9.6. The TiNi
alloy also shows another phase transformation before the martensitic transformation
according to heat-treatment and alloy composition. This transformation (rhombohedral
phase or R-phase transformation) can be formed by elongating along any one of the
→
111
→
directions of the B2 structure as shown in Figure 9.7 and characterized by a small lattice
distortion when compared with that of the martensitic transformation. The R-phase trans-
formation usually appears before the martensitic transformation when the martensitic
transformation start temperature
M
s
is lowered by some means than the R-phase transfor-
mation start temperature
T
R
. There are many factors that effectively depress
M
s
as follows
(Miyazaki et al., 1986):
(1) Increasing Ni content
(2) Aging at intermediate temperatures
(3) Annealing at temperatures below the recrystallization temperature after cold
working
(4) Thermal cycling
(5) Substitution of a third element
Among these factors, items (2)-(5) are effective to realize the R-phase transformation.
The martensitic transformation occurs in such a way that the interface between the mar-
tensite variant and parent phase becomes an undistorted and unrotated plane (invariant
plane or habit plane) to minimize the strain energy. To form such a martensite variant
(habit-plane variant), it is necessary to introduce a lattice invariant shear, such as twins,
dislocations or stacking faults. The lattice invariant shear is generally twinning, which is
reversible, in the SMAs.
b
a
c
c
´
c
´
´
b
´
b
β
a
´
a
0
a
´
B2
Lattice correspondence
M-phase
FIGURE 9.6
Crystal structures of parent (B2) and martensite (B19') phases and lattice correspondence between two phases.
(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.)
