Characterization of Powders

The product of mechanical alloying (MA) or mechanical milling (MM) is in powder form. Successful application of these materials, either in the as-synthesized powder condition or after consolidation to bulk shape, requires that the powders and/or the products of consolidation be fully characterized. Consolidation of the mechanically alloyed/milled powders requires their exposure to high temperatures and/or pressures. This can be done by a number of techniques such as hot pressing, hot isostatic pressing, hot extrusion, plasma-activated sintering, or shock consolidation. Consolidation of the ‘‘nonequilibrium’’ powders synthesized by MA/MM techniques (containing supersaturated solid solutions; metastable intermediate phases; quasicrystalline, amorphous, or nanocrystalline phases) to full density is a nontrivial problem. Achievement of full density requires that the powders be exposed to high temperatures and/or high pressures for extended periods. This could result in the crystallization of amorphous phases, formation of equilibrium phases from the metastable phases produced, and coarsening of nanometer-sized grains. However, if one consolidates the powders at low temperatures and/or pressures to retain ‘‘nonequilibrium’’ features, sufficient bonding between particles may not exist; consequently, the material will be porous and not fully dense. Therefore, innovative methods must be adopted to achieve full densification while retaining the ‘‘nonequilibrium’’ effects in these mechanically alloyed/milled powders. Sufficient literature is available on this aspect, and the reader is referred to a recent review for additional details [1].