V. V. Boldyrev

The science and technology of mechanical alloying

Abstract Mechanical alloying (MA) is a powder metallurgy processing technique involving cold welding, fracturing, and rewelding of powder particles in a high-energy ball mill, and has now become an established commercial technique to produce oxide dispersion strengthened (ODS) nickel- and iron-based materials. MA is also capable of synthesizing a variety of metastable phases, and in this respect, the capabilities of MA are similar to those of another important non-equilibrium processing technique, viz., rapid solidification processing (RSP). However, the “science” of MA is being investigated only during the past 10 years or so. The technique of mechanochemistry, on the other hand, has had a long history and the materials produced in this way have found a number of technological applications, e.g., in areas such as hydrogen storage materials, heaters, gas absorbers, fertilizers, catalysts, cosmetics, and waste management. The present paper discusses the basic mechanisms of formation of metastable phases (specifically supersaturated solid solutions and amorphous phases) by the technique of MA and these aspects are compared with those of RSP. Additionally, the variety of technological applications of mechanically alloyed products are highlighted.

Mechanochemistry of Solids: Past, Present, and Prospects

A historical retrospective is presented beginning from the early observations by alchemists to the establishment of mechanochemistry as a branch of science. The changes in structure and chemical properties of solids under three-axes loading and by combined action of pressure and shift are demonstrated. The peculiarities of the phenomena taking place upon stressing of particle assemblies in various types of energy-intensive grinding mills are discussed. Based on the contemporary concepts, the mechanism of stress field formation and relaxation is analyzed. Among decisive factors influencing the mechanochemical synthesis, the formation and renewal of contact area between reacting compounds, the explosive evolution of heat, and the feedback phenomenon are emphasized. The perspective directions of practical application in the area known as mechanical alloying, as well as in preparation of functional ceramics and catalysts, and in pharmacy are discussed. The main directions for improving research, construction of milling devices, training of specialists, and exchange of knowledge are proposed.

Mechanism and kinetics of mechanochemical processes in comminuting devices: 1. Theory

For the models built up on the basis of the nonlinear elastoplastic theory describing the collisions of solids, parameters of impact interaction between the working bodies and the material to be treated have been calculated for a series of comminuting devices, i.e., different types of ball mills, disintegrator, etc. Pressure and temperature impulses at the contacts of the treated particles have been calculated. Generalised equations have been deduced for the kinetics rate constants of mechanochemical reactions and for the mechanical activation of the substances in comminuting devices.

Magnesium mechanical alloys for hydrogen storage

Abstract Mechanical alloying is a promising new way to fabricate hydrogen storage materials consisting of unusual pairs of metals. Mechanical alloying can be used more easily for fabricating Mg2Ni, other MgNi and intermetallic systems than melting and sintering techniques. Systems such as MgFe, MgCo, MgNb and MgTi, that cannot be prepared by conventional techniques, can be fabricated by mechanical alloying, in addition to any other pairs or more complex mixtures of metals and non-metals. We have pointed out that intimate contact of metals can be achieved in high energetic ball mills. This contact plays an important role in hydriding and dehydriding processes. In this study we have used two groups of catalytic additives. The first group are “hydrogen pumps” such as CeHx. In the second group the added metal does not form any hydride in the conditions under study, but is known as an H2 → HH reaction catalyst, e.g., cobalt, nickel and probably iron.

The hydriding properties of a mechanical alloy with composition Mg-25%Fe☆

A method of mechanical alloying was used to produce an Mg-25%Fe composite hydrogen storage material. The presence of a pure metallic iron surface in contact with the magnesium leads to the promotion of the dissociative adsorption of hydrogen and the hydrogenation of Mg-25%Fe begins at the maximum rate even in the first cycle of hydriding. The activation of the mechanical alloy is completed by the third cycle of hydriding. The process of formation of Mg2FeHx is also studied.

Mechanism and kinetics of mechanochemical processes in comminuting devices: 2. Applications of the theory. Experiment

Abstract The equations describing the kinetics of contact melting and crystallization of the substances have been deduced on the basis of the obtained distributions of temperature and pressure impulses during impact–friction interaction of the particles treated in comminuting devices. Possible mechanisms of the formation of nanocrystal particles and of chemical reactions during the crystallization of thin films of a melt at high rates of local temperature changes in the near-contact regions have been studied. Various examples are presented that deal with the application of the generalized kinetic equation to the calculations of ab initio rate constants of specific mechanochemical processes in comminuting devices. The obtained theoretical values have been compared with experimental results.

Mechanochemistry and mechanical activation of solids

Abstract The paper reviews investigations on mechanochemistry and mechanical activation of solids and discusses the most urgent problems which concern this field.

On the mechanism of the thermal decomposition of ammonium perchlorate

The experimental results confirming the proton-transfer mechanism of the ammonium perchlorate (AP) thermal decomposition over a wide temperature range are presented. The proton conductivity of AP was detected. It is shown by use of a time-of-flight mass spectrometer that the primary products of the thermal decomposition of AP are ammonia and perchloric acid. An endothermal stage, in a condensed phase, during the thermal decomposition of AP is discovered by a calorimetric investigation. The heat absorbed at this stage coincides with the NH3-HClO4 dissociation energy. The features of the catalytic effect of homogeneous and heterogeneous additives on AP thermal decomposition and on some kinetic regularities of chlorine dioxide formation at this decomposition are in good agreement with the proton-transfer mechanism of AP thermal decomposition.

Hydrothermal reactions under mechanochemical action

Abstract The possibility for hydrothermal processes to take place under the action of mechanical pulses that arise in mechanical activators is considered for the system: a solid substance—water. Experimental data are presented on the mechanochemical synthesis of gidenbergite, tobermorite and other hydrosilicates that are usually produced according to the autoclave technology.

Tribochemical equilibrium in mechanical alloying of metals

The structure of the product in mechanical alloying depends both on the elemental composition and the milling conditions. An increase of ball energy led to more pronounced crystallinity of the product. Mechanical alloying at small ball energy leads to the formation of amorphous alloys for Zr-Co and Cu-Ti systems. Demixing of Ti3Cu4 into crystalline TiCu and TiCu4 and demixing of Zr50Co50 into Zr3Co and ZrCo2 was found. The results are explained on the basis of the concept of tribochemical equilibrium.