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

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 kinet ics of recovery and recrystallization after plastic deformation. The change of powder-particle and grain sizes with temperature is determined in a com plementary study by metallography and x-ray line-profile analysis. At the i ntensive-shrinkage stage, the effective powder-particle size in electrolyti c copper powder is approximate to 5 mu m and the grain size is approximate to 2 mu m. Due to the dendritic morphology of the powder, the effective pow der-particle size is much smaller than that determined by particle-size ana lysis (approximate to 34 mu m). Because of the small powder-particle and gr ain sizes, a measurable fraction of positrons annihilate at grain boundarie s and in surface states, i.e. at inner pore surfaces. At higher temperature s (T > 550 degrees C), grain boundaries are, besides a small surface compon ent for compacts of electrolytic powder, the only detectable lattice defect s in both powders. We find that the observed shrinkage rates can be explain ed-at least qualitatively-by Coble creep, while Nabarro-Herring and Kosevic (dislocation) creep seem to play only a minor role in the systems investig ated.