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

Particle reinforced metal matrix composites are now being produced commerically, and in this paper the current status of these materials is reviewed. The different types of reinforcement being used, together with the alternative processing methods, are discussed. Depending on the initial processing method, different factors have to be taken into consideration to produce a high quality billet. With powder metallurgy processing, the composition of the matrix and the type of reinforcement are independent of one another. However, in molten metal processing they are intimately linked in terms of the different reactivities which occur between reinforcement and matrix in the molten state. The factors controlling the distribution of reinforcement are also dependent on the initial processing method. Secondary fabrication methods, such as extrusion and rolling, are essential in processing composites produced by powder metallurgy, since they are required to consolidate the composite fully. Other methods, suc...

Particle reinforced aluminium and magnesium matrix composites

During the past five years, the authors have developed in their laboratory a new type of solar cell that is based on a photoelectrochemical process. The light absorption is performed by a monolayer of dye (i.e., a Ruthenium complex) that is adsorbed chemically at the surface of a semiconductor (i.e., titanium oxide (TiO{sub 2})). When excited by a photon, the dye has the ability to transfer an electron to the semiconductor. The electric field that is inside the material allows extraction of the electron, and the positive charge is transferred from the dye to a redox mediator that is present in solution. A respectable photovoltaic efficiency (i.e., 10%) is obtained by the use of mesoporous, nanostructured films of anatase particles. The authors show how the TiO{sub 2} electrode microstructure influences the photovoltaic response of the cell. More specifically, they focus on how processing parameters such as precursor chemistry, temperature for hydrothermal growth, binder addition, and sintering conditions influence the film porosity, pore-size distribution, light scattering, and electron percolation and consequently affect the solar-cell efficiency.

Nanocrystalline titanium oxide electrodes for photovoltaic applications

http://nanomechanics.mit.edu/sites/default/files/documents/2007_Acta_nc_Overview.143.pdf

Toward a quantitative understanding of mechanical behavior of nanocrystalline metals

Amorphous, nanoquasicrystalline and nanocrystalline alloys in Al-based systems

The development of refractory metal alloys for aerospace applications is reviewed. Refractory metals are prime candidates for many high temperature aerospace components because of their high meltin...

Recent advances in aerospace refractory metal alloys

Superplastic behavior of a 20% Al2O3/YTZ ceramic composite

Superplasticity in ceramics and ceramic composites is reviewed. Deformation characteristics, and in particular the strain rate sensitivities in these materials, are discussed. From the strain rate sensitivity and grain size dependence, deformation mechanisms in these superplastic materials have been addressed. It appears that grain boundary sliding is the predominant deformation mechanism. Constitutive equations governing the superplastic flow in superplastic ceramics and ceramic composites are presented. Microstructural features in these materials, e.g. grain size and morphology, and glassy phases at grain boundaries, are described. Microstructure stability, and particularly, concurrent grain growth during superplastic deformation and its effect on superplastic elongation, is discussed. In this review, the superplastic properties and microstructures of some other advanced materials, e.g. intermetallics, metal matrix composites, and mechanically alloyed alloys, are also briefly reviewed and compar...

Recent advances in superplastic ceramics and ceramic composites

The discovery of very-high-strain-rate superplasticity in a mechanically alloyed 15 vol pct SiCp/IN9021 aluminum composite is reported. Tensile elongations of more than 500 pct were obtained at strain rates between 5 to 100/s at 823 K. The maximum elongation of 610 pct was recorded at 6/s and 823 K. The potential for high-strain-rate superplasticity in this mechanically alloyed composite is found to be superior to a number of other reinforced aluminum composites. In particular, the superplastic strain rates were many orders of magnitude higher than those for typical superplastic alloys. It is demonstrated that mechanically alloyed composites can exhibit superplasticity in the very high strain rate over 1/s. 13 refs.

Superplastic behavior in a mechanically alloyed aluminum composite reinforced with SiC particulates

Superplasticity has been observed in several whisker-reinforced aluminum composites at extremely high strain rates of approximately 0.1–1 s−1. These composites include β-SiC-2124Al, β-Si3N4-2124Al, α-Si3N4-7064Al and β-Si3N4-6061Al. The materials generally exhibited a strain rate sensitivity value of about 0.3 and a maximum elongation of about 300%. The exact deformation mechanism leading to superplasticity is apparently a unique one. Experimental results indicated that the ability of a whisker-reinforced composite to undergo high-strain-rate superplasticity depends upon the morphology, crystal structure and chemistry of the whiskers, as well as on the chemical composition at the whisker-matrix interfaces. It is proposed that the presence of a low melting point region, or in some cases a liquid phase, at the whisker-matrix interfaces is responsible for the observed phenomenon of superplasticity at very high rates. These regions are present at the deformation temperature as a result of solute segregation. It is noted that the phenomenon is not observed in all whisker-reinforced composites despite tha fact that they contain fine grain sizes. Thus a fine matrix grain size is a necessary but insufficient condition for the observation of high-strain-rate superplasticity in whisker-reinforced composites.

J. Wadsworth

Papers

Recent advances in aerospace refractory metal alloys

Superplastic behavior of a 20% Al2O3/YTZ ceramic composite

Recent advances in superplastic ceramics and ceramic composites

Superplastic behavior in a mechanically alloyed aluminum composite reinforced with SiC particulates

High-strain-rate superplasticity in aluminum matrix composites