Introduction

Ultrafine or nanocrystalline powder has either particle size or grain size less than about 100 nm (0.1 Z'm). The synthesis techniques are described in the article "Ultrafine and Nanophase Powders" in this Volume. Ultrafine powder forms are of two types. The first is particulate, generally a single crystal, whose size is less than about 100 nm (Fig. 1). Synthesis routes include evaporation/condensation, sol-gel, and other precipitation techniques. The other powder form has conventional powder size but is composed of a fine, nanocrystalline grain structure. This structure is obtained through a deformation processing technique, such as high-energy, attrition ball milling (Ref 2), or severe plastic deformation consolidation.

Consolidation of these materials into bulk form with retention of the fine structure is the subject of this article. Particle curvature is the dominant feature affecting consolidation. It produces a large driving force for densification and enables surface/boundary diffusion mechanisms to operate at substantial rates. Densification takes place at temperatures consistently below those of larger grained powders by up to hundreds of degrees, and the activation energies for sintering are lower. Low sintering temperature may result in retention of fine grain size, elimination of sintering aids, avoidance of undesirable phase transformations, and absence of deleterious interfacial-reaction decomposition.

Understanding and controlling grain growth and particle coarsening are critical to successful consolidation of nanocrystalline powder. Pressureless sintering has been used to produce bulk materials with nanocrystalline structure, but control of particle agglomeration is critical to its success. Other manufacturing processes involve applied pressure of the order of the curvature stresses or greater: sinter forging, HIP, hot/warm pressing, extrusion, and high-pressure consolidation. Molecular dynamic simulations enhance our understanding of the fundamentals of sintering, including particle rotation and neck formation as sintering mechanisms.

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