Sintering nickel and iron: Are there indications for defect-activated sintering?

Defects in powder metallurgical samples may enhance diffusion processes during sintering. Samples of high purity nickel reduction powder and pure carbonyl iron are prepared by interrupted sintering, i.e. removing the furnace at the target temperature and rapidly cooling the samples, which are then investigated by analytical methods like positron annihilation lifetime spectroscopy (PALS). Thus, we can follow the sintering process by changes in the microstructure. These results are compared to heavily deformed reference samples (iron taken from literature) to determine the recovery temperature (0.4 of the melting temperature), i.e. annealing of dislocations. We estimate an effective powder particle size, which can be used to model the shrinkage by a modified two-particle model. The results for the two systems will be compared. For larger samples (10-30mm in diameter) we follow the shrinkage by thermo-optical measurements to determine influences of the pressing tools by monitoring the shrinkage at different height positions during one run.