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

Magnetic Relaxation in Fine‐Particle Systems

Alloys created between immiscible elements

Hydrogen is recognized as a possible future energy carrier, which can be produced from renewable energy and water. A major challenge in a future ‘hydrogen economy’ is the development of safe, compact, robust, and efficient means of hydrogen storage, in particular for mobile applications. The present review focuses on light metal boron based hydrides, for which the general interest has expanded significantly during the past few years. Synthesis methods, physical, chemical and structural properties of novel boron based hydrides are reviewed along with new approaches for improving kinetic and thermodynamic properties: (i) anion substitution, (ii) reactive hydride composites and (iii) nanoconfinement of hydrides and chemical reactions. The light metal borohydrides reveal a fascinating structural chemistry and have the potential for storing large amounts of hydrogen. A combination of the different approaches may provide a new route to a wide range of interesting energy storage materials in the future.

Tailoring properties of borohydrides for hydrogen storage: A review

A breakthrough in hydrogen storage technology was achieved by preparing nanocrystalline hydrides using high-energy ball milling and the use of suitable catalysts/additives. These new materials show fast or in case of Mg-based hydrides very fast absorption and desorption kinetics within minutes, thus qualifying lightweight Mg- or Al-based hydrides for storage applications. This article summarizes our current understanding of the kinetics of Mg-based light metal hydrides, describes an approach for a cost-effective processing technology and highlights some promising new developments in lightweight metal hydride research.

Tailoring hydrogen storage materials towards application

Cu-Co has been chosen as a model system to study phase formation during mechanical alloying in systems with components exhibiting a positive heat of mixing. Phase formation is monitored by scanning electron microscopy, x-ray diffraction, and calorimetric measurements. Although the occurrence of a single fcc Bragg pattern cannot definitely prove the alloy formation on an atomic scale, the quantitative agreement of the heat release upon decomposition of the alloy with the heat of mixing, calculated from the available thermodynamic data of the system, is regarded as clear evidence for the formation of an fcc solid solution. As a mechanism of alloy formation the dissolution of Cu and Co particles during milling is discussed. It is proposed that the formation of a homogeneous solid solution is energetically favored if the particle size is reduced below a critical value, which amounts to about 1\char21{}2 nm, depending on the composition of the alloy and the temperature. The driving force is provided by the chemical contribution of the Cu/Co interface enthalpy and by the high configurational entropy of the randomly distributed atoms in the solid solution.

C. Gente

Papers

Formation of thermodynamically unstable solid solutions in the Cu-Co system by mechanical alloying.