Diffusion in intermetallics

Intermetallic compounds or ordered alloys (intermetallics) have recently attracted much attention as materials for high-temperature applications. Diffusion is fundamental and ubiquitous in the art and science dealing with solid materials at elevated temperatures.;A knowledge of the diffusion behaviour of intermetallics is therefore of interest for the production of these materials and for their use in technological applications. Whereas diffusion in many pure metals and dilute alloys is thoroughly investigated and reasonably well understood, systematic diffusion studies for intermetallic compounds are relatively scarce. An atomistic understanding of diffusion in intermetallics in terms of defect structure and diffusion mechanisms is obviously more complex than for metallic elements. This overview is devoted to binary intermetallics. The reader is reminded of the more frequent structures (B2-, L1(2)-, DO3-, DO19-, L1(0)- and cubic Laves-type structures) of intermetallics. Some structural implications for diffusion in these materials including the Cu3Au rule are considered. Various diffusion phenomena in binary systems like self-diffusion, diffusion of foreign atoms, single-phase and multiphase interdiffusion are described and illustrated by experimental examples from our laboratory. Some limitations of the Darken-Manning equation, which is sometimes used to correlate tracer diffusion of components and interdiffusion, are discussed. Self-diffusion of components-the most basic diffusion process in solids-is reviewed for intermetallics with the above mentioned structures. Relevant factors for self-diffusion like the crystal structure, the state of order and disorder, the temperature and composition dependence are illustrated. In a few cases diffusion of selected foreign elements will also be considered. Particular attention will be devoted to our present understanding of diffusion in terms of defect structure and atomic mechanisms. This review also includes a summary of recent investigations on diffusion in the DO3-type intermetallic compound Fe3Si, Ti self-diffusion in intermetallic compounds of the Ti-Al system and single-phase and multiphase interdiffusion in various intermetallics including the cubic Laves-phase Co2Nb.