The influence of microstructure on the sintering process in crystalline metal powders investigated by positron lifetime spectroscopy: I. Electrolytic and spherical copper powders

Abstract

We investigate the influence of microstructure (dislocations, and grain and subgrain boundaries) on the sintering process in compacts of electrolytic and spherical copper powders by means of positron lifetime spectroscopy. We compare the lifetime data obtained to the kinetics of the annealing out of vacancy clusters after low-temperature electron irradiation, and the kinetics of recovery and recrystallization after plastic deformation. The change of powder-particle and grain sizes with temperature is determined in a complementary study by metallography and x-ray line-profile analysis. At the intensive-shrinkage stage, the effective powder-particle size in electrolytic copper powder is and the grain size is . Due to the dendritic morphology of the powder, the effective powder-particle size is much smaller than that determined by particle-size analysis . Because of the small powder-particle and grain sizes, a measurable fraction of positrons annihilate at grain boundaries and in surface states, i.e. at inner pore surfaces. At higher temperatures , grain boundaries are, besides a small surface component for compacts of electrolytic powder, the only detectable lattice defects in both powders. We find that the observed shrinkage rates can be explained - at least qualitatively - by Coble creep, while Nabarro-Herring and Kosevic (dislocation) creep seem to play only a minor role in the systems investigated.