PhD Thesis_RuiMSMartins.pdf - RUN UNL

More documents

Recommendations

Info

In-situ XRD studies during growth of Ni-Ti SMA films and their complementary ex-situ characterization composition can change the transformation temperatures by more than 100°C. In the Ni-Ti system, the Ni-rich side has some solubility at high temperature (Fig. 1.2) and the transformation temperatures are strongly dependent on the Ni concentration. Increasing Ni causes a drastic decrease in the transformation temperatures. The martensitic transformation temperature drops to below 100 K for a Ni content exceeding 51.5 at.% (Fig. 1.6). On the Tirich side the transformation temperatures do not exhibit such dependence with composition, probably due to the fact that the solubility limit of Ni-Ti phase on the Ti-rich side is almost vertical. Fig. 1.6: M s as a function of Ni content for binary Ni-Ti SMA. The different symbols represent data from different authors and the solid line is given by thermodynamic calculations [18]. The phase transformations can be investigated by measuring some physical properties such as electrical resistivity (ER) as a function of temperature. This technique was chosen in order to study the Ni-Ti films produced during this <strong>PhD</strong> work. Figure 1.7 shows the temperature dependence of the resistivity for a Ni-Ti bulk sample heat-treated at 380°C, obtained by Uchil et al. [19]. During cooling from B2-phase, the resistivity value decreases linearly with the temperature down to R s . Self-accommodated R-phase transformation starts at R s and the austenite parent phase (B2) transforms to the twinned R-phase with further decrease of the temperature. Twinning in an alloy matrix results in electron scattering that in turn leads to the increase of the ER [17]. The twinned structure density in B2 phase increases with the decrease of the temperature (due to the B2 ⇒ R-phase transformation), producing a higher ER as a result of the more pronounced increase of lattice imperfections. R-phase transformation ends once the temperature decreases to R f . An additional cooling promotes the R-phase distortion and, at M s , the R-phase starts transforming to monoclinic B19’ martensite, Chapter 1 - Background and Literature Review 13
In-situ XRD studies during growth of Ni-Ti SMA films and their complementary ex-situ characterization which exhibits less lattice distortion. The martensite transformation finishes when the temperature reaches M f , and, cooling below M f , does not result in more phase changes. On heating, the authors did not identify the R-phase reverse transformation. The transformation to cubic B2 austenite parent phase is completed at temperature A f and the microstructure completely reverses to the original microstructure consisting of single cubic B2 austenite. Fig. 1.7: Temperature dependence of electrical resistivity of a Ni-Ti sample, heat-treated at 380°C, in a complete thermal cycle [19]. 1.1.2. Shape memory effect (SME) and superelasticity effect (SE) The SME in Ni-Ti alloy was discovered in the 1960s at the Naval Ordnance Laboratory [20], hence the acronym Nitinol is commonly used in referring to the Ni-Ti system (Nickel Titanium Naval Ordnance Laboratory). The SME is a result of the change in crystalline structure as the material is heated and cooled. The martensitic transformation is a displacive type solid-state transformation and atoms are cooperatively rearranged or shifted into a new structure (without diffusion). The displacement of each atom is not large but since all of the atoms move in the same direction in a domain or variant, the transformation results in a macroscopic change in shape. This phenomenon may lead to unique properties such as the SME and SE. The martensitic transformation is described by a shear-like mechanism, as shown schematically in Fig. 1.8(a) and (b) [1]. When the B2 parent phase in Fig. 1.8(a) is cooled below M s , the structure changes into martensite, as in Fig. 1.8(b). The formation of martensite is normally referred to as athermal transformation, since the amount of new phase present is typically dependent only upon temperature, but not upon the length of time at the transformation temperature. In the case of Ni-Ti, twinning is the dominant accommodation process for martensite transformation. It can accommodate shape changes in a reversible way Chapter 1 - Background and Literature Review 14
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 33: In-situ XRD studies during growth o
Page 85 and 86:
In-situ XRD studies during growth o
Page 87 and 88:
Page 89 and 90:
Page 91 and 92:
Page 93 and 94:
Page 95 and 96:
Page 97 and 98:
Page 99 and 100:
Page 101 and 102:
Page 103 and 104:
Page 105 and 106:
Page 107 and 108:
Page 109 and 110:
Page 111 and 112:
Page 113 and 114:
Page 115 and 116:
Page 117 and 118:
Page 119 and 120:
Page 121 and 122:
Page 123 and 124:
Page 125 and 126:
Page 127 and 128:
Page 129 and 130:
Page 131 and 132:
Page 133 and 134:
Page 135 and 136:
Page 137 and 138:
Page 139 and 140:
Page 141 and 142:
Page 143 and 144:
Page 145 and 146:
Page 147 and 148:
Page 149 and 150:
Page 151 and 152:
Page 153 and 154:
Page 155 and 156:
Page 157 and 158:
Page 159 and 160:
Page 161 and 162:
Page 163 and 164:
Page 165 and 166:
Page 167 and 168:
Page 169 and 170:
Page 171 and 172:
Page 173 and 174:
Page 175 and 176:
Page 177 and 178:
Page 179 and 180:
Page 181 and 182:
Page 183 and 184:
Page 185 and 186:
Page 187 and 188:
Page 189 and 190:
Page 191 and 192:
Page 193 and 194:
Page 195 and 196:
Page 197 and 198:
Page 199 and 200:
Page 201 and 202:
Page 203 and 204:
Page 205 and 206:
Page 207 and 208:
Page 209 and 210:
Page 211 and 212:
Page 213 and 214:
Page 215 and 216:
Page 217 and 218:
Page 219 and 220:
Page 221 and 222:
Page 223 and 224:
Page 225 and 226:
Page 227 and 228:
Page 229 and 230:
Page 231 and 232:
Page 233 and 234:
Page 235 and 236:
Page 237 and 238:
Page 239 and 240:
Page 241 and 242:
Page 243 and 244:
Page 245 and 246:
Page 247 and 248:
Page 249 and 250:
Page 251 and 252:
Page 253 and 254:
Page 255 and 256:
Page 257 and 258:
Page 259:
show all

PhD Thesis_RuiMSMartins.pdf - RUN UNL

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?