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

Interpretation of X-ray Powder Diffraction Patterns . By H. Lipson and H. Steeple. Pp. viii + 335 + 3 plates. (Mac-millan: London; St Martins Press: New York, May 1970.) £4.

X-Ray Diffraction

Recent developments of phosphate glasses for a variety of technological applications, from rare-earth ion hosts for solid state lasers to low temperature sealing glasses, have led to renewed interest in understanding the structures of these unusual materials. In this review, spectroscopic and diffraction studies of simple phosphate glasses, including v-P2O5 and binary phosphate compositions, are described. Special attention is given to the structures of anhydrous ultraphosphate glasses, which have received close attention from the glass community only in the past six years.

Review: the structure of simple phosphate glasses

The Bond-Valence Method: An Empirical Approach to Chemical Structure and Bonding

Using the specialization of the extended RISM equation to infinitely dilute systems, we have calculated correlation functions and interionic potentials of mean force for a set of models corresponding to the first few alkali halides in water. From the results obtained at infinite dilution we calculate the lowest order corrections to the solution properties of the ions. Higher concentrations are explored by using the interionic potentials of mean force at infinite dilution as effective solvent mediated pair potentials. Our results indicate that certain thermodynamic properties, such as the mean activity coefficients and osmotic pressures, are quite sensitive to the details of both the theory and the potential models.

Alkali halides in water: Ion–solvent correlations and ion–ion potentials of mean force at infinite dilution

The structure of several CaCl2 aqueous solutions was studied by means of x‐ray diffraction. It is demonstrated that the solutions can be treated as being composed of independent hydrated ions, and for this reason the structural problem is limited to the study of close hydration. This description allows the calculation of structure functions in very good agreement with experimental data when using coordination number six and octahedral geometry for both the ionic species present.

X‐ray diffraction study of the average solute species in CaCl2 aqueous solutions

X-ray diffraction data for three aqueous solutions of MgCl2 were analyzed. It is shown that in a very concentrated solution the structure and correlation functions can be reproduced almost completely by using a nearest-neighbor model of ionic hydration, whereas in the more dilute solutions a certain number of interactions between the hydrated cation and external water molecules must also be considered. The mean coordination geometry for the close hydration was found to be octahedral both for cation and anion with mean cation–water distances (2.10–2.12 A) and mean anion–water distances (3.11–3.14 A) in accordance with results found in other cases.

X‐ray diffraction study of MgCl2 aqueous solutions

Two concentrated (5 and 7 mol l−1) aqueous solutions of NaNO3 were investigated by x‐ray diffraction. The features of the total radial functions calculated from experimental intensity data show that the positional correlation is not very strong in the systems investigated. However, it is possible to account for experimental data only by using models in which hexahydrated Na+ ions have definite correlations with NO3− ions, even if not through formation of isolated ion pairs. The compatibility of our observations with spectroscopic results is also discussed.

Interactions and structure in aqueous NaNO3 solutions

Amorphous metal-metalloid powders have been prepared by chemical reduction of Fe2+, Co2+, and Ni2+ ions with potassium borohydride in aqueous solution. The chemical composition of the samples, TM-B-O, varies with the metal and with the preparation steps. The as-prepared Co-B-O and Ni-B-O samples are definitely amorphous, while the Fe-B-O sample mainly consists of bcc I±-Fe. Crystallization of the samples through thermal treatment has been investigated by X-ray diffraction. The Co-B-O sample evolves toward fee metallic cobalt accompanied first by metastable o-Co3B and, at high temperature, by t-Co2B. The Ni-B-O sample gives rise to fee metallic Ni and o-Ni3B, which decomposes at higher temperatures. Iron borides are not detected in the thermal treatment of the Fe-B-O sample. No evidence of metal oxides is present in all the samples, thus suggesting the prevalent bonding of oxygen to boron.

Amorphous metallic powders prepared by chemical reduction of metal ions with potassium borohydride in aqueous solution

The scattering of x rays from a concentrated NH4NO3 aqueous solution has been measured and analyzed at 25°C. It is shown that the sample may be considered as a solution of nitrate ions in water. In this way the existence of discrete NO−3–H2O interactions is demonstrated unequivocally. Good agreement with experimental data is achieved through a model in which each O atom in the nitrate ion gives rise to three weak hydrogen bonds with water molecules.

Giovanni Licheri

Papers

X‐ray diffraction study of the average solute species in CaCl2 aqueous solutions

X‐ray diffraction study of MgCl2 aqueous solutions

Interactions and structure in aqueous NaNO3 solutions

Amorphous metallic powders prepared by chemical reduction of metal ions with potassium borohydride in aqueous solution

On NO−3–H2O interactions in aqueous solutions