Constitutional and/or Thermal Vacancies in some Intermetallic Compounds

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Nonstoichiometry of intermetallics may give rise to the existence of constitutional defects such as atoms occupying atomic sites of a sublattice other than its own sublattice (antisite atoms) or vacancies of deficient atomic species (constitutional vacancies). Such structural defects and their properties in intermetallics, which depend on nonstoichiometry, have been studied by positron annihilation methods.Positron annihilation methods are very sensitive to vacancies and small vacancy clusters which are quite difficult to be studied by other techniques. Positron studies of constitutional defects in some intermetallics have been carried out in this work. In-situ measurements of thermal equilibrium defects and studies of non-equilibrium defects on some intermetallics have been also done. The vacancy formation and migration properties in some important intermetallic componds have been directly observed by the positron-annihilation lifetime method.The conventional positron-lifetime measurement, however, is difficult to be carried out at high temperatures. In order to achieve in-situ studies of given materials at high temperatures more easily, a new beta^+_<-gamma> coincidence lifetime spectrometer using energy-selected fast-positron beam has been constructed by the author's group. The new positron apparatus will become a very powerful tool to study lattice defects at high temperatures in refractory materials.