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The transmission electron microscope (TEM) is one of the most useful tools available to the materials scientist. Yet both the complexity and expense of the equipment, and the huge investment in time necessary to become proficient in specimen preparation and image acquisition and analysis, mean that it is difficult for most industrial institutions to maintain a state-of-the-art TEM facility. How can industry overcome this problem? One solution is to set up a collaboration with a university, an industrial partner, or a government research laboratory. Such collaborations can be extremely valuable to the company, which gains access to microscopes, specimen-preparation equipment and the expertise of professional microscopists, and to the research laboratory, which benefits from the industrial perspective and the private sector's proficiency in materials preparation and processing.
Such collaborations exist, and they can produce excellent results. In this article, we present three case studies in which successful collaboration has occurred between industry and one of the Department of Energy's scientific user facilities, the National Center for Electron Microscopy (NCEM-see sidebar). Our aim is not only to describe results that we hope will be of scientific interest but also to encourage industrial researchers to consider collaborations with institutes such as NCEM.
Just what is an intermetallic compound? An intermetallic compound is a true compound of two or more metals that has a distinctive structure in which the metallic constituents are in relatively fixed abundance ratios and are usually ordered on two or more sublattices, each with its own distinct population of atoms. Often substantial or complete disorder may be obtained as a result of a low ordering energy or the intervention of some external agency—for example, extreme cooling rates or radiation. Deviations from precise stoichiometry are common on one or both sides of the nominal atomic ratio, necessitating some misordering and/or the introduction of vacancies. Usually metal-metalloid compounds such as silicides, arsenides, or tellurides are also included in the category since the phenomenology of most such compounds is similar to that of metal-metal compounds. More than 25,000 distinct intermetallics are now known.
To many, intermetallic compounds (IMCs) are classed as “exotics” or “advanced materials.” Such categorization should apply only to the recent interest in their potential use as monolithic high-temperature structural materials. Actually IMCs have been used for thousands of years in an astonishing range of applications exploiting not only their mechanical properties but their chemical, electrical, magnetic, optical, and semiconducting properties as well. Most readers will be surprised to learn that they carry within their own bodies (dental fillings) or on their persons (jewelry or pocket lighters) IMCs for which selection and processing are uniquely appropriate to those applications. The economic impact of IMCs is as unappreciated as the diversity of their applications. Some are regularly produced in quantities of 1,000s of tons per year while others, although not in tonnage production, are key factors in devices with billion-dollar markets.
One of the greatest challenges currently facing the materials community is the need to develop a new generation of materials to replace Ni-based superalloys in the hot sections of gas-turbine engines for aircraft-propulsion systems. The present alloys, which have a Ni-based solid-solution matrix surrounding Ni3Al-based precipitates, are currently used at temperatures exceeding 1100°C, which is over 80% of the absolute melting temperature. Since Ni3Al melts at 1395°C and Ni at 1453°C, it is clear that significantly higher operating temperatures, with the attendant improvements in efficiency and thrust-to-weight ratio, can only be attained by the development of an entirely new materials system. This problem is a primary reason for the current high level of interest in high-temperature intermetallic compounds.
The development of such a material system has important implications for national defense and for spin-offs to civilian technology, as well as for the economy and balance of payments. Obviously it would be a boon to any economy to have these new materials developed domestically, as was the case in the United States for the currently used single-crystal technology applied to Ni-based superalloys. As an example, the aerospace industry is one area where the United States is still the undisputed world leader, with net exports of $29 billion in 1989, twice that of any other U.S. industry.
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.
Intermetallic compounds constitute a very important class of materials for electromagnetic applications. In this article, some important materials and applications are discussed in the following areas: (1) magnetic, magnetoresistive, and magnetostrictive applications; (2) superconductor applications; (3) semiconductor and optical applications; (4) magneto-optical applications; and (5) thermoelectric applications. Emphasis is placed on materials that are important in existing devices and applications or show promise for future applications. The interested reader should consult the reviews in Westbrook and Fleischer's book, and the many references contained therein, for further information.
So ubiquitous are intermetallic compounds (IMCs) in all areas of materials application that examples of their use in industrial, medical, consumer, and military products are almost limitless. We will in this brief discussion attempt to identify some of the more important of these applications, which we categorize as miscellaneous. These include shape-memory alloys (SMAs), gold alloys used in jewelry, dental amalgams, tribology applications, diffusion barriers in electronic devices, elevated-heat storage systems, nuclear applications, metal-matrix composites, and high damping alloys. In some cases, the IMC is present as a precipitate or dispersed particle that provides strengthening or other property modification while in other examples, the IMC is employed in bulk form.