Improved densification of carbonyl iron compacts by the addition of fine alumina powders

An investigation of the effect of alumina particles on the sintering behavi or of a carbonyl iron powder compact was carried out in this study. Two dif ferent-sized alumina, 0.05 and 0.4 mu m, were added to the iron compact at amounts up to 1.2 wt pet. When 0.4 mu m alumina particles were added, no si ntering enhancement was observed. But, in contrast to previous results repo rted in literature, the addition of 0.1 to 0.2 wt pet of 0.05 mu m alumina particles was found to improve the densification. With 0.1 wt pet, the sint ered density increased from 7.25 to 7.40 g/cm(3) after the compact was sint ered at 1350 degrees C for 1 hour in hydrogen. Dilatometric curves showed t hat alumina impeded the early-stage sintering of iron in the alpha phase, b ut improved densification in the gamma phase at high temperatures. These re sults, along with microstructural analysis, suggested that alumina particle s exhibit dual roles; their physical presence blocks the diffusion of iron atoms, thus causing inhibition of sintering, while their grain-boundary pin ning effect prevents exaggerated grain growth of iron and helps densificati on. It follows that, depending upon the amount and size of the alumina powd ers, either an increase or decrease in the final sintered density can be ob tained.