PhD Thesis_RuiMSMartins.pdf - RUN UNL

In-situ XRD studies during growth of Ni-Ti SMA films and their complementary ex-situ characterization
grains. This nucleation process occurs in the interior of the film (without a link to the surfacenucleated
columnar grains), resulting in multi-directional laminar microstructures, which may
account for the observation of multiple martensitic transformation events.
Vestel et al., in a later work [117], studied the details of the crystallization process in
Ti-rich Ni-Ti films (390-450°C), including the precipitate formation. The nucleation in high
concentrations of Ti-rich particles (Ti 2 Ni) at the columnar grain interface was notorious for
films with a complete crystalline structure (which is not the case for partially crystallized
films). A higher density of precipitates was detected at the columnar/plate grain interface. The
authors propose that during the crystallization process, excess Ti is driven into the amorphous
region ahead of the crystal/amorphous interface, leading to Ti-rich precipitates preferentially
at the columnar/plate grain interface.
Although for Ti-rich compositions higher transformation temperatures can be achieved
compared to equiatomic or Ni-rich compositions, it is necessary to have in mind that a noncontrolled
growth of Ti 2 Ni precipitates can disturb or even impede the growth of martensite
plates [118]. On the other hand, it has been shown that Ti-rich Ni-Ti thin films crystallized
from the amorphous state exhibit fine flake-like microstructures that have never been reported
in bulk materials [118]. Guinier-Preston (GP) zones (submicrometric plate precipitates) and
homogeneously distributed Ti 2 Ni precipitates inside a grain cause different shape memory
behaviour from those of bulk Ni-Ti. The GP zones can be deformed elastically during
martensitic transformation or mechanical deformation, and, thus, they do not affect the growth
of martensite plates during martensitic transformations or mechanical twinning shear in the
martensite. This assures that the film can obtain a large shape recovery strain. The relation
between composition, heat-treatment temperature and resulting microstructures are shown in
Fig. 1.31 for a Ni-Ti film (Ti-48.2at.%Ni) heat-treated for the period of 1 h [9]. During heattreatment
of the amorphous film, the microstructure in the B2 matrix changes in the sequence
of plate precipitates along {100} planes of the matrix [Fig. 1.31(e) and (d)], plate precipitates
and spherical Ti 2 Ni precipitates with moiré fringes [Fig. 1.31(c)], and spherical Ti 2 Ni
precipitates [Fig. 1.31(b)] with increasing processing temperature.
Chapter 1 - Background and Literature Review 55