ALLOYS - CONSTITUTION DIAGRAMS The number, composition and proportions of the constituents in equilibrium at any given temperature, and changes due to change in temperature may be represented graphically in a con stitution diagram. Such diagrams present only equilibrium con ditions and are not concerned with the arrangement of the con stituents. A binary alloy diagram may be represented on a two dimensional chart, as in fig. I, but ternary alloy systems require three dimensions for suitable graphic representation. Complete graphic representation of the constitution becomes difficult if not hopeless in alloys containing four or more components. Plane charts, however, within restricted composition limits, particularly when only two of the components are allowed to vary at one time, are exceedingly useful even with complex alloys. In alloy systems each physically distinct and homogeneous part is called a "phase," and the chemical elements or compounds which par ticipate in the equilibria are called "components." In fig. I, the liquid alloy is one phase, the solid aluminium or aluminium-rich solid solution another, and the solid compound another. Aluminium and are the components. In the whole system other compounds are formed and the elements, aluminium and copper, may be regarded as the components. The liquidus is the line or curve representing the beginning of solidifi cation, and the solidus represents the end of solidification on cool ing. In some alloy systems the liquidus passes through a mini mum temperature, coinciding with the solidus at a certain com position. If the solidified alloy having such a minimum freezing point shows the presence of at least two phases under the microscope it is called a eutectic alloy. Eutectics may be binary, ternary, etc., in accordance with the number of phases present. Eutectic alloys freeze and melt at a constant temperature rather than through a temperature range, and the eutectic composition does not, except by coincidence, represent simple whole number proportions of the elements.
The properties and structures of alloys may be profoundly changed by mechanical working and by heat treatment. Certain of these changes in structures are indicated in the accompanying micrographs and the corresponding changes in properties are indicated in the titles.
BIBLIOGRAPHY.-C. H. Desch, Metallography; E. F. Law, Alloys; Bibliography.-C. H. Desch, Metallography; E. F. Law, Alloys; C. H. Gulliver, Metallic Alloys; W. M. Guertler, Metallographie; C. Vickers, Metals and Their Alloys; Z. Jeffries and R. S. Archer, The Science of Metals; W. Rosenhain, Introduction to Physical Metallurgy; S. L. Hoyt, Metallography, Principles, vol. i., Metals and Common Alloys, vol. ii.; A. Sauveur, Metallography and Heat Treatment of Iron and Steel; International Critical Tables; Kent's Mechanical Engineers Hand Book; The transactions, proceedings or journals of many societies including Iron and Steel Institute, Institute of Metals, and Faraday Society, of Great Britain ; American Institute of Mining and Metallurgical Engineers, American Society for Testing Materials, and American Society for Steel Treating. Periodicals: The Metallur gist; Supplement to the Engineer (London) ; Zeitschrift fiir Metall kunde; Revue de Metallurgie; The Metal Industry (New York).
(Z. J.)