NANOCRYSTALLINE IRON SINTERING BEHAVIOR AND MICROSTRUCTURAL DEVELOPMENT

Nanocrystalline (20 nm) iron powder was closed-die sintered in a hydro gen atmosphere at a stress of 10.1 MPa and at temperatures between 670 and 1270 K. The maximum densification rate was approximately 6 X 10(- 4) s-1. Density greater than 90 pct was obtained at sintering temperat ures greater than 990 K. Densification was marked microstructurally by local gradients which appeared after initial cold compaction. Oxygen content in the starting powder was high but was effectively a monolaye r of surface adsorbed oxygen. Despite the reducing sintering atmospher e, oxide was present in dense specimens as a fine dispersion of order 0.1 to 1 mum. The extent of oxide formation can be controlled by close d-die sintering to a stable structure of interconnected porosity follo wed by open-die resintering in the reducing atmosphere. Final grain si ze in material sintered 1 hour at 1080 K was generally less than 200 n m, although scattered coarsening to approximately 5 mum was observed.