The Mechanism of Sintering of α-Iron

Abstract

For the sintering of wire spool models of pure iron in the α-range, a rate law x = k • t 1/7 is observed, suggesting surface self-diffusion as the prevalent transport mechanism. The interpretation of this rate law in terms of surface diffusion was confirmed by measurements of the surface self-diffusivity of α-iron by independent methods, viz., by observations of thermal grooving at grain boundaries and of the leveling out of scratch profiles on plane iron surfaces. The techniques mentioned were also used to study the effect of oxygen in a hydrogen atmosphere on mass transport on iron surfaces. The velocity of mass transport is determined by the product of surface self-diffusivity D s and surface free energy γ s . Both properties were found to decrease with increasing oxygen partial pressure, as shown by measurements of grain boundary groove angles (for γs) and of scratch healing (for D s ) at varying dewpoints. The product D s γ s also determines the rate of bonding between particles in the early stages of sintering of powder compacts. The tensile strength of iron powder compacts sintered for short times in hydrogen atmospheres of varying dewpoint reveals a change in sintering kinetics which reflects the changes of γ s and D s . Such an influence of the surrounding atmosphere would be difficult to explain if sintering proceeded by volume diffusion, but it is easy to understand if surface diffusion is the mechanism of mass transport. The above results refer only to the initial stages of sintering. The final stages, where the decrease of porosity is the main feature of sintering, proceed by volume diffusion.