Biomedical Engineering Reference
In-Depth Information
[9]. For example, martensite is softer and less stiff, while its elastic
modulus reaches 31−35 GPa compared with austenite's 84−98 GPa
[4, 18].
The austenite phase, also called the mother phase, reveals
the B2 type CsCl structure with
a
o
= 3.015 Å. The martensite phase
appears to be the most common in monoclinic structure where
a
= 2.889 Å,
b
= 4.120 Å,
c
= 4.622 Å and
γ
= 96.8° [21].
From the crystallographic point of view, it is important to esta-
blish the vision of structure deformation that shows diffusionless
mechanism driving shape memory effect. Transformation from the
austenite to the martensite phase in 3-D crystallographic structure
is shown in Fig. 8.11. We may distinguish 24 possible martensite
variants that may appear as an effect of twinning — the mirror
symmetry of atoms displacements along a speciic plane. This speciic
material situation results from the capability of twin boundaries that
can be easily moved in the structure, without the creation of defects
such as dislocation, contrary to most metals, in which deformation
directly uses the slip or the dislocation movement [41].
Figure 8.11
Lattice cell of NiTi illustrating the six-face diagonal planes;
each of them can shift in two directions or shear in two
directions, as shown by the arrows, to produce a total of
24 martensite variants [41].
For the plane consideration of uniaxial loading, we may
distinguish only two variants of deformation, which in detwinned
form of martensite enter the positive or the negative strain to the
structure as shown in Fig. 8.12.
The thermodynamic stability behavior of austenitic and
martensitic phases in SMA provides the ability to “remember”
different kinds of shape designed in the austenite phase.
