The Institute of Materials Science and Engineering, National Taiwan University

Abstract

The addition of fine titania powders increases the sintered density of carbonyl iron compacts from 6·91 to 7·28 g cm^-3 when the compacts were sintered at 1200°C for 1 h in hydrogen. Analysis of dilatometric curves and microstructures indicated that titania inhibited the early stage sintering owing to the blocking of the diffusion path of iron atoms. However, the titania powders helped prevent the exaggerated grain growth that usually occurs during the α → γ phase transformation. As the compact reached 950°C, a small part of the titania was reduced by hydrogen, and the reduced titanium dissolved into the iron matrix and enhanced sintering. Meanwhile, the remaining titania coarsened, and its effect on sintering inhibition diminished. The retardation of the grain growth, coarsening of the titania at high temperatures, and the diffusion rates enhanced by the reduced titanium resulted in improved final sintered densities of carbonyl iron compacts.