Mechanical Alloying of Nanocrystalline Materials and Nanocomposites

Mechanical alloying is an effective method to synthesize nanocrystalline metal powders in the monolithic and composite states. The process involves repeated cold welding, fracturing and rewelding of powder particles in a high energy ball mill. Although originally developed to produce oxide dispersion strengthened superalloys, the technique has been later shown to be capable of producing a variety of metastable phases including nanocrystalline materials. The present article describes the synthesis of nanocrystalline materials and nanocomposites by mechanical alloying. The basic principles of the process, process parameters that affect the constitution and microstructure of the processed powders, and the mechanisms of alloying and grain refinement, including their consolidation to full density while retaining the nanostructures, are described. Methods to achieve the smallest possible grain size are then highlighted. Typical examples of the synthesis of nanocomposites containing a high volume fraction of nanometer-sized reinforcements in aluminum, and process optimization to achieve superplastic behavior in titanium-based nanocomposites are then discussed. The ubiquitous problem of powder contamination during milling and solutions to eliminate or minimize this are also mentioned.