Investigation Of Point Defects In Intermetallic Componds By Positron Annihilation

This dissertation mainly covers two parts: the development of positron annihilation technique and the investigation of point defects in intermetallic componds. Fist of all, an advanced positron annihilation coincidence Doppler broadening system (CDB) was designed and developed and a peak to background ratio of 3.6×10~6 was obtained. It can be used to identify elements and to analyze the chemical environment in the positron annihilation site. The work mainly includes CDB system development and its optimization as well as performance enhancement.The emphasis of this paper is to investigate point defects in intermetallic componds by using positron annihilation technique. Furthermore, a preliminary investigation on the effects of electron structure and defects of intermetallic componds has been done by alloying. They are as follows:1. The point defects of B2-NiAl alloy have been investigated as a function of Ni compositions ranging from 47 to 55 at.% by using of positron lifetime and coincidence Doppler broadening measurements. Only single-component lifetimes have been measured. The results indicate that there exists only one major kind of vacancy defects. CDB measurements provide direct evidences of the dominant lattice defect types: the mono-vacancy on the Ni sublattices. Furthermore, the positron lifetimes remain unchanged on the Al-rich side, whereas the lifetimes decrease with the increase of Ni content on the Ni-rich side.2. Positron annihilation lifetime and coincidence Doppler broadening measurements have also been performed on alloying TiAl. The experiments results indicate that the effect on the electron structure of TiAl alloy is different when different amounts of Nb are doped. At the same time, other alloying elements were also doped into NiAl and TIAl alloys, which can greatly affect the crystal structure of intermetallic compound, respectively.3. The recovery behaviors of defects, induced by neutron radiation, were studied by positron annihilation system. CDB measurements provided that the dominant lattice defects types in TiAl alloy are on the Al sublattices. The lifetime measurements also identify the temperature range for defects recovery. The identical works were also performed on TiAl alloys doped by Nb and Si elements. The experiment results indicate the doped elements can influence the structure of defects and the temperature range of defects recovery.4. Single detector and coincidence Doppler broadening measurements using slow positron beam were carried out to study Ti/Al multilayer films. The main goal is to study the evolution of vacancy-type defects and inter-diffusion of elements in films subjected to different annealing conditions. The CDB measurements showed that Ti is the dominant diffusion species during the alloying process of Ti/Al by high temperature annealing.