Practical metallurgical application of intermetallic compounds (IMCs) occurred more than 3,000 years before they were recognized as distinct entities in alloys. Pliny, The Elder (A.D. 23–79) recorded in his encyclopedia a then old practice: the use of mercury both to recover gold from sands and other dispersed sources, and to gild less-noble metal objects. In both cases, the key factor is the formation of a Au-Hg intermetallic compound (amalgam) stable at room temperature but readily decomposable on heating to produce solid gold. An illustration of the Au-recovery process, reproduced from Ercker (1574) appears in Figure 1. Use of the amalgam processes for silver apparently occurred later. Bronze mirrors were silver-coated with the amalgam process by the Chinese in the second century B.C., and silver was recovered from crushed sulfide ores using mercury in the famous Potosi process (1566, but probably known much earlier).
The key properties of intermetallics that make possible their diverse applications in chemical and metallurgical processes are their high melting points relative to one or all of their constituent elements, their often sharply defined composition, their brittleness, and their controllable reactivity/stability–that is, systems can be chosen such that a stable intermetallic forms easily at room or low temperatures that is nonetheless readily decomposable at a higher temperature. Once the intermetallic forms however, a useful physical property (e.g., hardness or conductivity) or chemical property (oxidation, sulfidation, corrosion resistance, nonsticking quality, etc.) may be that which is ultimately exploited in use.
We will review these two classes of applications using representative examples from both process metallurgy and chemistry.