Review positron annihilation in fine-grained materials and fine powders - an application to the sintering of metal powders

Abstract

We consider the specific problem of the influence of an inhomogeneous distribution of defects in solids on positron annihilation characteristics. In detail, we investigate the effect of micro-structure, i.e. dislocations, vacancies, vacancy clusters, grain and subgrain boundaries, pores or inner surfaces, on positron lifetime spectroscopy. Only few materials show such small grain sizes that positron annihilation is affected. One example are powder compacts, made out of small and fine-grained powder, during different stages of the sintering process. All samples generically show positron trapping at grain boundaries (τGB ≈ 300 ps) and at surfaces (τsurf = 500–600 ps). τGB = 300 ps corresponds to small voids consisting of roughly eight vacancies. Different defects can lead to similar annihilation parameters. Hence, we compare the lifetime data obtained from porous and fine-grained samples to the kinetics of defect annealing after irradiation and plastic deformation, e.g. the thermal stability of dislocations or vacancy clusters. We conclude that τGB ≈ 300 ps is apparently not related to vacancy clusters in the matrix, but is due to positron trapping at large-angle grain boundaries. This large open volume is in contrast to common grain boundary models. The change of porosity and grain size with temperature, i.e. during sintering, has been determined in a correlated study by metallography and X-ray line-profile analysis. The effective powder particle size ranges from ≈0.5 to ≈15 μm, while the grain sizes are always smaller. The only detectable lattice defects in all samples above recrystallization temperature are grain boundaries, besides a surface component in very fine powders.