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

REVIEW ARTICLE: Glassy metals

The minimum solute concentration in a binary alloy system necessary to obtain a stable amorphous phase by rapid quenching, C B min , collected from the published reports on glass formation of 66 systems, was found to be inversely correlated with the atomic volume mismatch, | (v b −v A v A | , where v A is the atomic volume of the matrix and v B is the atomic volume of the solute. The atomic scale elasticity theory was developed to calculate the atomic level stresses in solid solution, which led to the stress criteria for the topological instability of solid solution. It was found that C B min is closely correlated with the critical solute concentration for the topological instability of solid solution. This result indicates that the atomic size ratio between the constituent elements is the most important factor in the determination of value of C B min , and the amorphous alloys are stabilized partly because the solid solutions of the corresponding compositions are topologically unstable.

Atomic size effect on the formability of metallic glasses

Amorphous, nanoquasicrystalline and nanocrystalline alloys in Al-based systems

The Al-Si (Aluminum-Silicon) system

Rapid solidification experiments of Al-based Al-Fe-Si ternary alloys have been performed both by the gun and single roller methods. Glass forming tendency is found to be largest near theβ-phase (Al9Fe2Si2) composition, where the amorphous state is obtained both by the gun and single roller methods. Crystallization behaviour of amorphous AlFe13.0Si17.4 alloy, nearβ-phase composition, followed the course of (i) the appearance of fineα-Al particles and (ii) the transformation of the matrix intoβ-phase. The ratio of crystallization temperature and the melting point,T
x/T
m, for AlFe13.0Si217.4 amorphous alloy is smaller than 0.5 indicating the difficulty of glass formation of these alloys.

Formation and crystallization of Al-Fe-Si amorphous alloys

The effect of undercooling on grain structure was investigated in pure copper and alloys up to Cu ∼ 0.39wt% 0 (eutectic composition), in which grain refinement does not occur at any degree of bath undercooled when the oxygen content is less than 300 p.p.m. Grain refinement occurs in these alloys when the oxygen content exceeds about 300 p.p.m. and the undercooling prior to nucleation exceeds 100 K without quenching. Fragmentation affects primary, secondary and tertiary dendrite arms during and after recalescence. Quenching after recalescence at various solidification times retains transient grain structures. When the sample, which should have achieved complete grain refinement by furnace cooling, is quenched immediately after nucleation, the structure shows a trace of radiating fan-shaped grains originating from a single point of nucleation.

The solidification process of highly undercooled bulk Cu-O melts

An amorphous phase in a splat-cooled Fe-3.8 wt%C alloy

Studies on the crystallographic growth habit of primary silicon crystals in an Al-16 wt% Si alloy were carried out by X-ray micro focus Laue analysis and ECP (electron channelling pattern) analysis.

Crystal growth of the primary silicon in an Al-16 wt % Si alloy

Mechanical alloying of elemental aluminium and molybdenum powders was performed by conventional low-energy ball milling. Seven compositions (Al-3, 10, 17, 20, 27, 50 and 75 at% Mo), as well as elemental aluminium and molybdenum powders, were milled up to 1000 h. The structural changes during milling were followed by X-ray diffraction. In all cases milling produced refinement of the microstructure, and prepared powders were microcrystalline with grain sizes of the order of nanometres. A supersaturated solid solution of molybdenum in aluminium was formed. Only for the Al-75 at% Mo powders was a solution of aluminium in molybdenum observed.

K. Kobayashi

Papers

Formation and crystallization of Al-Fe-Si amorphous alloys

The solidification process of highly undercooled bulk Cu-O melts

An amorphous phase in a splat-cooled Fe-3.8 wt%C alloy

Crystal growth of the primary silicon in an Al-16 wt % Si alloy

Structural changes during mechanical alloying of elemental aluminium and molybdenum powders