CONSTITUENTS OF ALLOYS With so many alloys, each having its special properties and internal structure, and with countless possible combinations, the need for scientific as well as industrial classification is apparent. The study of the constitution and crystalline structure of alloys by means of the microscope (q.v.), X-ray spectrometer, thermal analysis, chemical analysis and various physical tests, has resulted in a classification of the constituents with reference to the manner of aggregation of the atoms. Three types of constituent are found: (1 ) Pure or substantially pure metals; (2) solid solutions; (3) chemical compounds.
Apure metal constituent is com posed substantially of one kind of atom. The atoms of different metallic elements vary somewhat in effective size but they are on the order of a hundred-millionth of an inch in diameter. X-ray wave lengths are the same order of magnitude as the diameter of an atom and hence the arrangement of atoms in a crystal can be determined by diffraction of X-rays of known wave length. Although solid metals are opaque- they are crystalline. The crystalline structure is described by the type and dimensions of a space lattice. A space lattice consists of a series of points in space such that every point is situated similarly to every other point. Space may be imagined as divided into cells by three sets of parallel planes, the intersections of these planes constituting the points of the space lattice. The angles which these planes make with one another and the distances apart determine the type and constants of the crystal. Most pure metals crystallize in the cubic and hexagonal systems. (See CRYSTALLOGRAPHY.) The microscope is a coarser grained tool than the X-ray spectrometer and can only resolve particles comprising about i,000,000,000 atoms and more. It reveals the presence of grains and grain boundaries in metals as shown in a number of the accompanying micrographs shown in Plates I. and II. Each grain in a pure metal is the same kind of a crystal but differs in orientation of the crystal axes from its neighbours. Grains may also differ markedly in size and shape.
Before making micrographs of a sample of alloy, a surface is highly polished and usually "etched" by slight chemical attack. In Plate I., figs. I and 10, and in Plate II., figs. 5 and 8 the dif ferent colours represent different constituents. In Plate I., figs. 9 and 12 and in Plate II., fig. so, only one constituent is present, the different colours being due to different orientation of the grains.
Metallicsolid solutions are also crystalline. Adopting the terminology of aqueous solutions, metallic solid solutions consist of "solvent" and "solute." If, for example, a small amount of copper is alloyed with nickel, the crystalline identity of copper is lost and the nature of the nickel is changed. Copper atoms are supposed to substitute for some of the nickel atoms in the space lattice. Nickel is the solvent and copper is the solute. It so happens that nickel and copper crystallize with the same type of space lattice, and that each is soluble in the other in the solid state to an unlimited extent. Therefore in the alloy containing 5o atomic per cent of each metal no attempt is made to differentiate between solvent and solute. In other alloys of these metals the element present in the higher atomic per cent is the solvent. Although these metals crystallize with the same type of space lattice, a side of the unit cube in copper is 3.6o by
and in nickel 3.54 by
centi metres. The dimensions of the unit cube of the solid solution bear an approximately linear relationship to the atomic concen tration, that is, the unit cube of an alloy containing 5o atomic per cent each of copper and nickel is about 3.57 by
centi metres.
Nickel and chromium form a complete series of solid solutions but the pure metals crystallize with different types of space lattice. The solvent metal in any such solid solution alloy is the one whose type of space lattice persists. Within a certain range of composi tion it appears that either type of space lattice may be stable at ordinary temperatures. Nickel is the only solvent up to about 65% chromium, whereas chromium persists as the sole solvent up to only about 5% nickel. Many pairs of metals do not form solid solutions throughout the whole composition range. In the aluminium-magnesium alloys, for example, even the systems of crystallization are different, the former being cubic and the latter hexagonal. Solid aluminium dissolves only a limited amount of magnesium and vice versa. The aluminium-rich solid solutions have the same type of space lattice as aluminium, and the mag nesium-rich solutions the same as magnesium. These are re ferred to as cases of "limited solid solubility." Not only is the solid solubility limited but the limit changes with change in tem perature, usually increasing with increase in temperature, but sometimes the reverse. If a solute is present in an amount less than the solubility limit, the solution is "under-saturated," and if equal to the solubility limit it is "saturated." A solid solution saturated or nearly so at a relatively high temperature may be cooled so quickly that a "super-saturated" solid solution obtains either temporarily or more or less permanently at some lower temperatures.
An alloy composed of two elements is termed binary, one of three elements ternary, and one of four elements quaternary. Solid solution alloys may be binary or they may contain a large number of elements. Although atom substitution seems to be the rule in solid solutions, it appears that carbon atoms dissolved in solid iron occupy spaces between iron atoms.
Compounds.—Certain combinations of metals pro duce materials with space lattices different from that of either of the constituent elements. These materials are called, in general, compounds. Compounds have definite proportions of the elements, such as
MR,Si, etc. Usually inter-metallic compounds possess lower crystalline symmetry than the elements from which they are formed. The constituent atoms of a com pound crystal are supposed to be so fixed in their positions that different kinds of atoms are not readily interchangeable in posi tion. It is not supposed that an oxygen atom, for example, in
would be interchangeable with a copper atom in the
space lattice. On the other hand, a metallic compound may be the solvent of a solid solution.
seems to be soluble in solid Fe3C. If so, the material would be a solid solution of man ganese carbide in iron carbide. Even in this case, it is suggested that the manganese substitutes only for the iron in the space lattice, the carbon atoms being interchangeable only with one another.
