• PDF / 5,449,770 Bytes
• 4 Pages / 576 x 777.6 pts Page_size
• 108 Downloads / 343 Views

DOWNLOAD

REPORT

---

26

others, although not in tonnage production, are key factors in devices with billion-dollar markets. This issue of MRS Bulletin focuses on the full range of applications for intermetallic compounds. It should serve as an introduction to the subject for all engineers who need materials with unique characteristics in order for their devices or processes to perform in desired ways. It should equally be of interest to materials scientists who seek to understand the fundamental nature of these materials and why they behave so well in their known applications. Finally collaboration of both groups should be stimulated as they seek to adapt known IMCs to new uses or to synthesize new IMCs for other applications. The history of IMC application can be divided into four periods. In the first period—prehistoric times—humans used the ordered alloys Nature provided (e.g., meteoritic Fe-Ni for tools, and native Au-Cu [tumbaga] for jewelry and fish hooks). The next period, from ancient to near-modern times, was one of serendipitous discovery when alloy compositions and processing techniques were optimized for particular applications without the realization that IMCs were responsible for the successes achieved. Here we may cite examples: SbSn to harden tin and other low-melting alloys for tableware, printing type, and bearings; 5-CuSn for mirrors; Ag2Hg3 and Sn8Hg for dental fillings; and (Cu,Fe)Zn for ship sheathing and hardware. This was followed by the modern period of "enlightened empiricism." In this period, with a good metallurgical understanding of the relations between composition, structure, and properties, as well as between processing and structure, simple systems could be selected having IMCs with desirable properties. Their composition and processing could

be modified in purposeful ways to enhance the combination of properties relevant to the application at hand. Examples include Ni3(Al,Ti)-strengthened superalloys, Fei 4 Nd 2 B-based permanent magnets, (Nb,Ti)3Sn multifilament, highfield superconducting solenoids, and shape-memory devices from NiTi. We are just now entering a period of true alloy design where, given the combination of properties desired for an application, we should be able to use the available compilations of fundamental data on phase diagrams, crystal structures, thermodynamic quantities, and atomic parameters, together with first-principle quantum-mechanical approaches, to define the particular IMCbased system and structure that will achieve the goal sought. This is not merely a current opportunity. We have no other choice. Villars' has estimated that about 11,000 distinct binary IMCs exist, most of which are known only through phase diagrams and crystallographic studies. Yet even here, we have knowledge of the properties for only a few score. Villars further calculates that there are about 500,000 true ternary intermetallics, only 3% of which are known to exist, and for only an infinitesimally tiny fraction of these do we have any knowledge of their properties. For the likely 10 X 106 quaternary