Smaller crystallites in sintered materials? A discussion of the possible mechanisms of crystallite size refinement during pulsed electric current-assisted sintering

• Crystallite size refinement in Spark Plasma Sintered copper was studied by XRD/SEM.
• Porous compacts show finer crystallites than dense compacts.
• Finer crystallites are due to melting–solidification and reduction of surface oxide.
• In broader context, refinement is due to phase transitions or chemical reactions.

One of the most well-known and technologically attractive advantages of pulsed electric current-assisted sintering is its capability of fast and efficient consolidation of nanostructured powders into nanostructured or submicron-grained compacts. But can the application of pulsed electric current produce crystallites in the sintered material smaller than those that were present in the powder? Microscopic observations of this effect have been reported for inter-particle contacts formed during Spark Plasma Sintering and high-voltage consolidation. Our results obtained on Spark Plasma Sintered porous compacts consolidated from electrolytic copper powder suggest that finer crystallites in the sintered material than those in the raw powder can form in areas other than necks between the particles. Based on the electron microscopy and X-ray diffraction evidence as well as results obtained by other authors, we consider two possible mechanisms of crystallite size refinement during pulsed electric current-assisted sintering: rapid solidification of locally formed melt and reduction of oxide films present on the surface of powder particles in a reducing environment of the sintering die/chamber causing the in situ formation of metal particles. The possibilities of crystallite size refinement are further examined in a broader context as related to phase transitions and chemical reactions in the sintered materials.

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