The influence of microstructure on the sintering process in crystalline metal powders investigated by positron lifetime spectroscopy: III. Nickel reduction powder

The sintering process in compacts of nickel reduction powder is investigate d by positron lifetime spectroscopy. Additionally, the lifetime data obtain ed are compared to the data on the recovery and recrystallization after pla stic deformation caused by cold rolling or pressing, and on the annealing o ut 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 s ignals. At intermediate temperature, a surface signal is detected (400-900 degrees C). This is in agreement with an effective powder-particle size of 3-5 mu m estimated from scanning electron microscopy. When sintering starts , i.e, above 900 degrees C, the only detected defect signal, besides a smal l surface signal, stems from large-angle grain boundaries. At the intensive -shrinkage stage, there are contributions from different shrinkage mechanis ms. The experimentally observed shrinkage rates can be explained by Coble c reep which seems to dominate the material transport. Nabarro-Herring and di slocation creep play only minor roles in this system.