Mechanical alloying (MA) is a solid-state powder processng technique involving repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill. Originally developed to produce oxide-dispersion strengthened (ODS) nickel- and iron-base superalloys for applications in the aerospace industry, MA has now been shown to be capable of synthesizing a variety of equilibrium and non-equilibrium alloy phases starting from blended elemental or prealloyed powders. The non-equilibrium phases synthesized include supersaturated solid solutions, metastable crystalline and quasicrystalline phases, nanostructures, and amorphous alloys. Recent advances in these areas and also on disordering of ordered intermetallics and mechanochemical synthesis of materials have been critically reviewed after discussing the process and process variables involved in MA. The often vexing problem of powder contamination has been analyzed and methods have been suggested to avoid/minimize it. The present understanding of the modeling of the MA process has also been discussed. The present and potential applications of MA are described. Wherever possible, comparisons have been made on the product phases obtained by MA with those of rapid solidification processing, another non-equilibrium processing technique.

Mechanical Alloying And Milling

Thermodynamic and kinetic aspects of the crystal to glass transformation in metallic materials

An account is given of the research that has been carried out on mechanical alloying/milling (MA/MM) during the past 25 years. Mechanical alloying, a high energy ball milling process, has established itself as a viable solid state processing route for the synthesis of a variety of equilibrium and non-equilibrium phases and phase mixtures. The process was initially invented for the production of oxide dispersion strengthened (ODS) Ni-base superalloys and later extended to other ODS alloys. The success of MA in producing ODS alloys with better high temperature capabilities in comparison with other processing routes is highlighted. Mechanical alloying has also been successfully used for extending terminal solid solubilities in many commercially important metallic systems. Many high melting intermetallics that are difficult to prepare by conventional processing techniques could be easily synthesised with homogeneous structure and composition by MA. It has also, over the years, proved itself to be superior to rapid solidification processing as a non-equilibrium processing tool. The considerable literature on the synthesis of amorphous, quasicrystalline, and nanocrystalline materials by MA is critically reviewed. The possibility of achieving solid solubility in liquid immiscible systems has made MA a unique process. Reactive milling has opened new avenues for the solid state metallothermic reduction and for the synthesis of nanocrystalline intermetallics and intermetallic matrix composites. Despite numerous efforts, understanding of the process of MA, being far from equilibrium, is far from complete, leaving large scope for further research in this exciting field.

Novel materials synthesis by mechanical alloying/milling

A new method for the preparation of amorphous alloys is the mechanical alloying process. So far, three amorphization reaction types were observed when this technique is used. Which reaction type occurs depends on the local conditions during the milling, such as impact energy and local temperature just after an impact. Amorphous alloys prepared by milling following different reaction types are compared to each other and to the corresponding melt-spun alloys. Furthermore, criteria for amorphization and a method to determine the glass forming range of TM-TM systems is given.

Amorphization by ball milling. A review

We report on high‐energy ball milling of Ru and AlRu. The deformation results in a drastic decrease of the crystal size to a nanometer scale and in an increase of atomic‐level strain. This is accompanied by a disordering of the crystal lattice as is shown by means of the long‐range‐order parameter in AlRu. The specific heat increases by more than 15%–20%, indicating large changes in the vibrational and configurational part of the entropy. The stored energy of cold work is up to 6 kJ/mol for AlRu and 10 kJ/mol for Ru. This is almost 40% of the heat of fusion of Ru and exceeds by far the energies stored by other deformation processes.

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Structural and thermodynamic properties of heavily mechanically deformed Ru and AlRu

Amorphous alloy powders of the types NiZr, CoZr, FeZr, and CuZr are produced by mechanical alloying from crystalline elemental powders. The alloying and amorphization process is monitored by microstructural investigations, x‐ray diffraction, and, where applicable, by magnetization measurements. The crystallization temperatures, determined by differential scanning calorimetry, are comparable to those measured for rapidly quenched and solid state reacted amorphous metals of the same compositions.

L. Schultz

Papers

Amorphization of transition metal Zr alloys by mechanical alloying