The cold isostatic pressing of composite powders—I. Experimental investigations using model powders

Metallic contact formation and growth by dislocation glide

An investigation of the effect of alumina particles on the sintering behavior of a carbonyl iron powder compact was carried out in this study Two different-sized alumina, 005 and 04 µm, were added to the iron compact at amounts up to 12 wt pct When 04 µm alumina particles were added, no sintering enhancement was observed But, in contrast to previous results reported in literature, the addition of 01 to 02 wt pct of 005 µm alumina particles was found to improve the densification With 01 wt pct, the sintered density increased from 725 to 740 g/cm3 after the compact was sintered at 1350 °C for 1 hour in hydrogen Dilatometric curves showed that alumina impeded the early-stage sintering of iron in the α phase, but improved densification in the γ phase at high temperatures These results, along with microstructural analysis, suggested that alumina particles exhibit dual roles; their physical presence blocks the diffusion of iron atoms, thus causing inhibition of sintering, while their grainboundary pinning effect prevents exaggerated grain growth of iron and helps densification It follows that, depending upon the amount and size of the alumina powders, either an increase or decrease in the final sintered density can be obtained

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

In compacts containing metal and oxide particles, metal/oxide surface contacts modify the diffusional processes. Different distributions of such dissimilar-surface contacts and their influence on the sintering characteristics of some metal-oxide compacts are considered.By selecting appropriate volume fractions and diameters of matrix and second-phase particles, structures containing different types of dissimilar surface contact distribution have been produced. Isochronal sintering of these structures clearly demonstrates that (a) the presence of chemically inert second-phase particles always hinders densification of the composite, (b) the sintering densification (for a given volume fraction of second-phase particles) is a strong function of (dm/ds), where dm and ds are the diameters of the matrix and second-phase particles, respectively, and (c) the nature of dissimilar-surface contact distribution has a significant effect on the densification of the composite.

B. N. Singh

Papers

Dissimilar-surface contacts and their influence on sintering characteristics of powder composite compacts*