Chapter 160 Mechanical alloying and mechanically induced chemical reactions

Abstract

Studies of mechanical alloying and mechanochemical processing of the rare earths and their alloys have shown that a wide variety of structures, ranging from metastable nanocrystalline to amorphous phases, can be synthesized. The ability to alloy and form phases at low temperatures, without melting or high-temperature processing, is a unique feature of mechanically activated processing. The low-temperature nature of the process enables disordering and amorphization to occur, with the resulting structures being determined by kinetic rather than thermodynamic considerations. Low-temperature heat treatment of mechanically alloyed structures can also result in the formation of new non-equilibrium phases due to the kinetic constraints of diffusion. The application of mechanical alloying to chemical refining also enables the direct synthesis of metals and alloys, without the need for heating. While in principle mechanochemical processing allows three processes, refining, alloying and powder manufacture, to be carried out in a single low-temperature process, attention must be given to the handling, further processing of highly reactive as-milled powders, and the removal of reaction by-products. Mechanical alloying provides significant advantages in the processing of rare-earth permanent magnets. The production of alloy powder by low-temperature processing in a sealed, inert environment minimises the sources of oxidation inherent in conventional processes. The structures obtained by mechanical alloying and heat treatment are characterized by excellent hard magnetic properties. In particular, the nanocrystalline microstructures inherently developed by the mechanical alloying process, appear to be ideally suited to the synthesis of remanence-enhanced magnetic materials. Since with remanence enhancement there is no need for an alignment process, significant potential exists for achieving cost savings and simplification of the manufacturing process. As shown by the studies on rare-earth permanent magnets, the non-equilibrium structures developed by mechanical alloying, with or without subsequent heat treatment, exhibit unique properties. There is thus much potential for the development of new mechanically allowed materials for applications such as catalysts, electrodes, hydrogen-storage containers and other applications, where property improvements are associated with the amorphous and nanocrystalline structures developed by mechanical alloying.