The Anisotropy of Dimensional Change on Sintering of Iron

The anisotropy of dimensional change on the sintering of iron was investigated by dilatometry. Dimensional changes are different along the longitudinal and transversal directions, and shrinkage is more pronounced parallel to the compaction direction. This phenomenon is particularly pronounced during the early stage of sintering, in the alpha field below the Curie temperature. The results were elaborated according to the kinetics model for shrinkage to calculate an effective diffusion coefficient along the two directions. Such an effective diffusion coefficient is higher parallel to the compaction direction than perpendicular to it, and both are larger than the diffusion coefficient calculated on the basis of the activation energy reported in the literature for pure iron. This discrepancy is attributed to the defectiveness introduced by cold compaction in the particle contact regions, which may enhance diffusivity owing to the dislocation pipe mechanism, which, in turn, is particularly intense below the Curie temperature. This interpretation may also justify anisotropy of shrinkage because the powder particles are inhomogeneously deformed by uniaxial cold compaction. The enhanced diffusion coefficient increases with time and shows a maximum at temperatures below the Curie point. This trend was discussed with reference to the self-activated sintering mechanism.