The influence of microstructure on the sintering process in crystalline metal powders investigated by positron lifetime spectroscopy: II. Tungsten powders with different powder-particle sizes

Compacts of tungsten powder with five different powder-particle sizes (from 0.4 mu m to 75 mu m) are subjected to pressureless sintering. We investiga te the change in microstructure during the sintering process by positron li fetime spectroscopy. So as to be able to distinguish between defects having the same positron lifetime,we investigate their kinetics when the sample i s annealed. In particular, we consider the annealing out of vacancy cluster s after low-temperature electron irradiation, as well as recovery and recry stallization of a tungsten sheet, in as-manufactured form. Making measureme nts on uncompacted powder, we find an increasing fraction of positrons anni hilating in surface states with decreasing powder-particle size. The powder -particle and grain sizes (influencing the x-ray domain size) are monitored additionally by means of metallography and x-ray diffraction. We find that all of the methods give results in agreement with each other. The small gr ain sizes at lower temperature, about one fifth of the powder-particle size , cause positrons to annihilate at grain boundaries, leading to vacancy-clu ster-like signals. At the intensive-shrinkage stage, there are certainly co ntributions from different shrinkage mechanisms. The observed shrinkage rat es can be explained by Coble creep. It is possible that dislocations also p lay a role as vacancy sources and sinks, since the intensive-shrinkage stag e occurs in a temperature region wherein recrystallization takes place.