PhD Thesis_RuiMSMartins.pdf - RUN UNL

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In-situ XRD studies during growth of Ni-Ti SMA films and their complementary ex-situ characterization 1.1.1. Phase diagram and crystal structures The Ni-Ti alloy system is very attractive, since the alloy can be improved by utilizing various phase transformations, both diffusional and diffusionless. The SME and SE occur in near equiatomic Ni-Ti alloys with compositions ranging from ≈ 45-55 at.% Ni. Observing the Ni-Ti binary phase diagram in Fig. 1.2 [1], it is visible that the solubility in Ni-Ti changes gradually with temperature on the Ni-rich side, while it has a steep solvus boundary on the Tirich side. A small deviation from stoichiometry in Ni-Ti alloys can give rise to a significant precipitation of second phases. It was found that in Ni-rich Ni-Ti alloys, diffusional transformations take place during aging or slow cooling from high temperature. The precipitation sequence can be written as B2→Ni 4 Ti 3 →Ni 3 Ti 2 →Ni 3 Ti (stable) [11]. The Ni 4 Ti 3 metastable phase (rhombohedral until cell) appears at lower temperatures and at shorter aging times. The presence of Ni 4 Ti 3 is especially important for improving shape-memory characteristics and strength, since it precipitates on a very fine scale. The inset of Fig. 1.2 shows the metastable equilibrium between Ni-Ti and Ni 4 Ti 3 . Fig. 1.2: Phase diagram of a Ni-Ti alloy to which the phase equilibrium between the B2 and Ni 4 Ti 3 phases are added. [1]. Chapter 1 - Background and Literature Review 9
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
Page 1 and 2: RUI MIGUEL DOS SANTOS MARTINS IN-SI
Page 3 and 4: ACKNOWLEDGEMENTS The research work
Page 5 and 6: At CENIMAT, Luís Pereira also carr
Page 7 and 8: SUMÁRIO O fabrico de películas fi
Page 9 and 10: LIST OF SYMBOLS AND ABREVIATIONS α
Page 11 and 12: TABLE OF CONTENTS Introduction ....
Page 13 and 14: LIST OF FIGURES Fig. 1.1: Power/wei
Page 15 and 16: Fig. 2.3: Calculated integrated pho
Page 17 and 18: Fig. 3.25: Pole figures of Ni-Ti B2
Page 19 and 20: Fig. 3.66: SAED pattern obtained on
Page 21 and 22: LIST OF TABLES Tab. 1.1: Factors af
Page 23 and 24: In-situ XRD studies during growth o
Page 29: In-situ XRD studies during growth o
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