Studies of defects and defect agglomerates by positron annihilation spectroscopy

A brief introduction to positron annihilation spectroscopy (PAS), and in particular lo its use for defect studies in metals is given. Positrons injected into a metal may become trapped in defects such as vacancies, vacancy clusters, voids, bubbles and dislocations and subsequently annihilate from the trapped state iri the defect. The annihilation characteristics (e.g., the lifetime of the positron) can be measured and provide information about the nature of the defect (e.g., size, density, morphology). The technique is sensitive to both defect size (in the range from monovacancies up to cavities containing 50-100 vacancies) and density in metals. Monovacancies can typically be detected in concentrations higher than a tenth of a part per million. For three dimensional vacancy clusters the sensitivity increases with increasing cluster size. The combination of PAS with theoretical calculations and with other experimental methods (in particular transmission electron microscopy) forms the basis for the use of PAS to quantitatively characterize defects and defect complexes, both visible and invisible is transmission electron microscopes: this is illustrated by some examples. Finally, the advantages of the use of PAS are pointed out. (C) 1997 Elsevier Science B.V.