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

We consider the specific problem of the influence of an inhomogeneous distr ibution of defects in solids on positron annihilation characteristics. In d etail, we investigate the effect of micro-structure, i.e. dislocations, vac ancies, vacancy clusters, grain and subgrain boundaries, pores or inner sur faces, on positron lifetime spectroscopy. Only few materials show such smal l grain sizes that positron annihilation is affected. One example are powde r compacts, made out of small and fine-grained powder, during different sta ges of the sintering process. All samples generically show positron trappin g at grain boundaries (tau(GB) approximate to 300 ps) and at surfaces (tau( surf) = 500-600 ps). tau(GB) = 300 ps corresponds to small voids consisting of roughly eight vacancies. Different defects can lead to similar annihila tion parameters. Hence, we compare the lifetime data obtained from porous a nd fine-grained samples to the kinetics of defect annealing after irradiati on and plastic deformation, e.g. the thermal stability of dislocations or v acancy clusters. We conclude that tau(GB) approximate to 300 ps is apparent ly not related to vacancy clusters in the matrix, but is due to positron tr apping at large-angle grain boundaries. This large open volume is in contra st to common grain boundary models. The change of porosity and grain size w ith temperature, i.e. during sintering, has been determined in a correlated study by metallography and X-ray line-profile analysis. The effective powd er particle size ranges from approximate to 0.5 to approximate to 15 mu m, while the grain sizes are always smaller. The only detectable lattice defec ts in all samples above recrystallization temperature are grain boundaries, besides a surface component in very fine powders. (C) 1999 Kluwer Academic Publishers.