Resolution of lattice defects in metals by positron annihilation.

摘要

Positron annihilation has now become a much-used technique for the study of defects in solids, particularly in metals and alloys. The work in this thesis describes measurements made on a Doppler-broadening spectroscopy system designed to study the annihilation radiation emerging from metal samples as a consequence of electron-positron interaction. Defects can be produced in metals by thermal, mechanical or irradiation treatments. For the metals Cu, Al, Ni, Fe and Ti, an assessment has been made of the nature and number of defect types encountered in the mechanically deformed state by analysis of the annihilation lineshapes. Defects in molybdenum produced by neutron-irradiation have also been studied. The defect environment is seen to be dependent on the irradiation temperature.;The nature of the electron environment and of the positron behaviour at a specific defect site has been assessed by fitting a model to the observed lineshapes to account for the electron and positron momentum distributions. Such analysis has yielded estimates of the positron zero-point energy and local Fermi electron energies associated with individual defect-types. In the case of copper and aluminium, the calculated zero-point energy of ~ 6 eV associated with positron trapping at vacancies is found to be identical to that derived for dislocation trapping. It is concluded that positrons undergo point-like trapping at dislocations, either by trapping at jogs or at other irregularities along the dislocation line.;Trapping model fits to data from mechanically deformed samples reveal the nature of the stress-strain, work-hardening relationship associated with the mechanical deformation process.;Running integrals of the difference between the annihilation lineshapes from defective and reference (annealed) samples have provided defect-specific parameters characterising the shape of the centre-of-mass momentum distributions resulting from annihilations at defect traps. In order to assess the defect species in the mechanically deformed state, these parameters have been used to monitor positron annihilation behaviour during isochronal annealing. Under favourable circumstances, individual defect-types have been characterised.