Shape Memory NiTi Materials - Bionanomaterials for Dental Applications - page 191

In the case shown in Fig. 8.5, the sample was cut along the
[100] direction and in the same direction, stress σ xx was applied
during cooling from the austenite to the martensite phase. For the
phenomenon, occurring σ xx stress magnitude has to be larger than
σ sv , which is the minimum value of stress necessary to obtain single
martensite variant, and smaller than σ b blocking stress level that is
a relatively low 6−10 MPa, which suppressed the reorientation of
martensite variants.
In the irst stage where the stress is applied, we observe a
small shortening of the sample, as shown in Fig. 8.5, resulted in
single martensite variant 1, in the whole sample. At the microscale,
we can also distinguish several magnetic domains resulting from the
variant structure — the existing easy axis of magnetization, which
in the discussed case, lies along the shorter edge of the tetragonal
structure.
Furthermore, external magnetic ield H y is applied orthogonal
to the direction of the applied stress and also in direction of the
easy magnetization axis of martensite variant 1. The subsequent
magnetization of the sample begins nucleation and growth of
martensite variant 2, whose easy magnetization axis is parallel to it.
This process is realized through the magnetic domain wall motion,
reorientation, and migration of the twins. The growth of martensite
variant 2 goes on at the cost of martensite variant 1 and results in
strain reorientation of the sample. Finish stadium of magnetization,
where the value of external ield accedes H y > H f(1-2) , results in
microscale by obtaining single martensite variant 2, with preferable
direction of magnetic domains by the inluence of external ield,
together with elongation of sample.
When the value of acting compressive stress is higher than σ b ,
the process of martensite variant reorientation (Fig. 8.6) cannot
be realized as it was mentioned earlier. The polarization of single
martensite variant by external magnetic ield is possible, but can be
done only at the same axis with working stress, because the small
amount of energy is necessary for reversible rotation of magnetic
domain walls.
For the orthogonal direction of applied stress to external
magnetic ield, the rotation of magnetic domain walls is not available
because it requires work against the magnetocrystaline anisotropy.
Therefore, the domain walls stay perpendicular to the direction of
the easy axis of magnetization.