PhD Thesis_RuiMSMartins.pdf - RUN UNL

In-situ XRD studies during growth of Ni-Ti SMA films and their complementary ex-situ characterization
Nishida et al. [11] have studied the precipitation processes in Ni-rich Ni-Ti alloys and
reported the TTT (time-temperature-transformation) diagrams for several composition alloys.
At lower aging temperature and shorter aging time, Ni 4 Ti 3 phase appears, while at higher
aging temperature and longer aging time Ni 3 Ti phase appears and at intermediate temperature
and time Ni 3 Ti 2 phase appears (Fig. 1.3). Further studies [12] were conducted in an aged Ti-
52 at% Ni alloy in order to clarify details of the microstructural aspects of the Ni 3 Ti 2 phase
using optical and electron microscopy and electron diffraction. It has been found that the
Ni 3 Ti 2 precipitate exhibits a phase transformation. The sequence of transformation events was
described as occurring in two-step transformations: parent phase (tetragonal) ⇔ intermediate
phase (orthorhombic) characterized by antiphase-like domain morphology ⇔ low temperature
phase characterized by antiphase and needle-like domains morphology (monoclinic). Later,
Hara et al. [13] have performed a study to determine the crystal structure of the Ni 3 Ti 2
precipitate and to clarify the characteristics of the transformation from the crystallographic
point of view. The crystal structure of the high and low temperature phases, and twinning in
the low-temperature phase were studied in detail. The crystal structure of the hightemperature
phase is tetragonal with Pd 3 Ti 2 type (space group: I4/mmm). The values
determined by the Rietveld method for the lattice parameters were: a = 0.3095(38) and
c = 1.3585(169) nm. The low-temperature phase exhibits an orthorhombic structure with
Al 3 Os 2 type (Bbmm). The lattice parameters obtained were: a = 0.4398(49), b = 0.4370(48),
and c = 1.3544(150) nm.
Fig. 1.3: TTT diagram describing aging behaviour for Ti-52 at% Ni [11].
Chapter 1 - Background and Literature Review 10