Application of mechanical alloying/milling for synthesis of nanocrystalline and amorphous materials

Mechanical alloying (MA) and mechanical milling (MM) techniques have been widely utilised over the past two decades for synthesis of various alloys and composites with equilibrium or metastable structure at room temperature.One of the most interesting features of MA/MM is the ability to produce nanocrystalline and amorphous materials. Several mechanisms for formation of nanocrystalline and amorphous structures have been introduced based on experimental findings and similarity of MA/MM to other solid-state processing routes. In this paper, the recent experimental observations reported on development of nanocrystalline and amorphous structures using ball mill technique are selectively examined and critically reviewed to provide further insight into the key issues related to this solid-state technique. The review includes four major parts. It begins with a brief introduction to MA/MM and the principles of ball milling process for synthesis of materials. The second part is devoted to the formation of nanocrystalline structure by ball milling process. The earlier studies are summarised with special emphasis on the minimum grain size obtainable by ball milling, besides the recent progresses on the prediction of minimum grain size based on dislocation models are analysed and discussed in detail. The third part deals with the amorphisation reaction induced by ball milling. This section includes the thermodynamic and kinetic aspects, the criteria developed for amorphisation reaction and microstructural evolution of powders during MA leading to the amorphisation reaction. The last part is devoted to the bulk amorphous/nanocrystalline alloys produced from precursor powder.