Abstract

The sintering process in compacts of nickel reduction powder is investigated by positron lifetime spectroscopy. Additionally, the lifetime data obtained are compared to the data on the recovery and recrystallization after plastic deformation caused by cold rolling or pressing, and on the annealing out of vacancy clusters caused by low-temperature electron irradiation. Due to the small grain sizes inside the powder particles at lower temperature, positrons annihilate at grain boundaries, leading to vacancy-cluster-like signals. At intermediate temperature, a surface signal is detected . This is in agreement with an effective powder-particle size of estimated from scanning electron microscopy. When sintering starts, i.e. above , the only detected defect signal, besides a small surface signal, stems from large-angle grain boundaries. At the intensive-shrinkage stage, there are contributions from different shrinkage mechanisms. The experimentally observed shrinkage rates can be explained by Coble creep which seems to dominate the material transport. Nabarro-Herring and dislocation creep play only minor roles in this system.

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