Novel Synthesis Techniques

Abstract

Intermetallic compound thin films have been synthesized by methods employing diffusion across interfaces of solid-solid or solid-liquid reactants, ion implantation of constituents followed by annealing, or molecular beam epitaxy (see chapters by Ng and Moustakas and by Ramanath et al. in this volume). Bulk intermetallics have been produced by fusion of elemental constituents, by consolidation of pre-alloyed powders (see chapter by Seetharaman and Semiatin in this volume), or through combustion (reaction) synthesis during (or prior to) consolidation of reactants. The high heat of reaction associated with intermetallic formation aids the diffusion kinetics and promotes reaction synthesis of intermetallic compounds of high purity and fine grain size. Large volume changes accompanying in-situ formation of intermetallic alloys during reaction synthesis can, however, lead to retained porosity and other defects that deteriorate the overall properties of bulk solids. Mechanochemical synthesis, involving mechanical activation of precursor powders, has also been used to enhance their chemical reactivity and synthesize intermetallics via controlled reaction mechanisms at lower temperatures and in significantly shorter time scales. Consequently, intermetallic alloys with highly refined and even nanocrystalline microstructures, as well as free from defects associated with uncontrolled combustion reactions, have been formed. Mechanochemical synthesis also permits the formation of non-equilibrium phases and compounds with a wide range of solid solubilities. Ball-milling using vibratory and attritor mills and shock-compression employing high-velocity impact with a gas-gun or explosive devices have been used for mechanochemical synthesis. Approaches involving the use of plasmas, electric-fields, and microwaves, have also been employed for field-activated reaction synthesis of ultrafine-grained intermetallic alloys. In their overview of synthesis and processing of intermetallics, Martin and Hardwick in Chapter 27, Vol. 1, provided a succinct description of solid-state reactions, and general characteristics of combustion synthesis, mechanical alloying, shock compression, ion implantation and ion-beam mixing processes. They also described the advantages and disadvantages of these processes, and applications to specific intermetallic systems documented in the literature until the early nineties. Significant advances in the development of new synthesis and processing technologies have been made since that time. For example, time-resolved diagnostics have been used to further the understanding of reaction mechanisms by Mukasyan et al. (1999) during combustion synthesis, by Charlot et al. (1999a) during mechanical alloying, and by Thadhani et al. (1997) during shock-induced reaction synthesis. Combining of various processes, such as devitrification of mechanically-amorphized powder compacts prepared by He and Ma (1996) using pulsed-electrodischarge, or by Counihan et al. (1999) using shock-consolidation techniques, and development of novel variations of reaction processes, e.g. micropyretic synthesis by Dey and Sekhar (1999) and electric-field-assisted reaction synthesis by Munir (2000), has enabled control and optimization of reaction processes for fabricating intermetallics with unique microstructural characteristics.