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

Recently a significant figure-of-merit (ZT) improvement in the most-studied existing thermoelectric materials has been achieved by creating nanograins and nanostructures in the grains using the combination of high-energy ball milling and a direct-current-induced hot-press process. Thermoelectric transport measurements, coupled with microstructure studies and theoretical modeling, show that the ZT improvement is the result of low lattice thermal conductivity due to the increased phonon scattering by grain boundaries and structural defects. In this article, the synthesis process and the relationship between the microstructures and the thermoelectric properties of the nanostructured thermoelectric bulk materials with an enhanced ZT value are reviewed. It is expected that the nanostructured materials described here will be useful for a variety of applications such as waste heat recovery, solar energy conversion, and environmentally friendly refrigeration.

Enhancement of Thermoelectric Figure-of-Merit by a Bulk Nanostructuring Approach

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

Ball-milling experiments of the ordered intermetallic compound ${\mathrm{Fe}}_{50}$${\mathrm{Al}}_{50}$ are carried out in a calibrated vertical vibrating frame grinder at various temperatures T and milling intensities I, defined as the momentum transferred by the ball to the unit mass of powder per unit time. The long-range order (LRO) parameter, as quantified by x-ray diffraction, decreases as a function of milling time and reaches a steady-state value which is a function of T and I. The time evolution of the LRO parameter can be fitted to a simple model of order-disorder transition under external forcing, in which the atomic exchanges occur because of two mechanisms: the forced atomic jumps, the frequency of which is found to be proportional to I and the thermally activated jumps, the frequency of which is found to be a linear function of I. The forcing parameter, defined as the ratio of the latter two jump frequencies, is found to be the control parameter of the steady degree of LRO achieved under ball milling. Moreover, the kinetic path of the LRO parameter depends on the milling intensity and milling temperature and is in semiquantitative agreement with the model.

Order-disorder transformation in Fe-Al under ball milling.

Structural properties of Fe50Co50 nanostructured powder prepared by mechanical alloying

Structural changes in stoichiometric CoGa during mechanical attrition in a high-energy ball mill were monitored by measurements of magnetic properties and lattice parameter. The high-field magnetization increases, whereas the lattice parameter decreases as a function of milling time. For long milling times, values of both parameters saturate. The results are interpreted by the generation of atomic triple-defect disorder, also characteristic of this compound in equilibrium at high temperatures: antisite Co atoms in combination with vacancies on the Co sublattice in a 1:2 ratio. By comparison to the magnetic properties of ${\mathrm{Co}}_{\mathit{x}}$${\mathrm{Ga}}_{100\mathrm{\ensuremath{-}}\mathit{x}}$ compounds with excess Co, the fractions of antisite Co atoms and vacancies are derived. On the basis of these values, the decrease of the lattice parameter can be calculated and compared to the actual measurement. The good agreement proves that, by the apparently crude technique of mechanical attrition, well-defined nonequilbrium states in intermetallic compounds can be generated.

/pdf/mechanical-attrition-and-magnetic-properties-of-cscl-1zewewdwno.pdf

Mechanical attrition and magnetic properties of CsCl-structure Co-Ga.

The stoichiometric intermetallic compound CoAl with the CsCl-structure was subjected to ball milling. Structural changes during ball milling were studied by measurements of the magnetization and the lattice parameter. The behaviour of Coal upon milling turns out to be quite similar to that of CoGa. The increase of the magnetization and the decrease of the lattice parameter with the milling time show that, similar to CoGa, triple-defect disorder is generated by ball milling. By comparing the measured magnetization to the magnetization of CoxAl100−x compounds with excess Co, the concentration of defects is derived for milled samples. In order to interpret our results, the existing experimental data of lattice parameters, X-ray densities calculated from the lattice parameters and macroscopic densities for CoxAl100−x compounds were analyzed according to Edelin's equations. The defect volumes for vacancies and anti-site atoms obtained by fitting these experimental data have reasonable values. By means of the defect concentrations obtained from our magnetization measurements and the defect volumes by fitting the experimental data, the change of the lattice parameter was calculated by means of Edelin's equation. The agreement between the calculated lattice parameter and the measured lattice parameter is quite satisfactory.

Atomic disorder induced by mechanical milling in the intermetallic compound CoAl

Atomic Disorder and Phase Transitions in Intermetallic Compounds by High-Energy Ball Milling

The behavior of the intermetallic compound ${\mathrm{Co}}_{3}$${\mathrm{Sn}}_{2}$ upon ball milling was studied by x-ray diffraction, high-field-magnetization measurements, and subsequently by differential scanning calorimetry. It turns out that starting from the stoichiometric-ordered compound, mechanical attrition of ${\mathrm{Co}}_{3}$${\mathrm{Sn}}_{2}$ generates atomic disorder in the early stage of milling. The nonequilibrium phase transformation from the low-temperature phase with orthorhombic structure to the high-temperature phase with a hexagonal structure was observed in the intermediate stage of milling. It was accompanied by the creation of increasing atomic disorder. After long milling periods, the phase transformation was completed and the atomic disordering became saturated. All the physical parameters measured in the present work remained constant during this period. The above outcome was confirmed by comparison with the high-temperature phase thermally induced by quenching. The good agreement of the results obtained by different techniques proves that the ball milling generates well-defined metastable states in ${\mathrm{Co}}_{3}$${\mathrm{Sn}}_{2}$.

/pdf/atomic-disorder-phase-transformation-and-phase-restoration-3cl2z0hpxn.pdf

Atomic disorder, phase transformation, and phase restoration in Co3Sn2.

Mechanical Attrition of Intermetallic Compounds

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