ALLOY' ( Fr. aloi, from Lat. alligare. to bind to, from ad, to + ligare, to tie). A mixture of two or more metals, usually produced artificially by fusion, although sometimes found native. Al loys are characterized by certain definite prop erties, which, according to Sir William C. Rob erts-Austen, include: Liquation, which is shown by the separation of that constituent which has the lowest melting point when the alloy is heated; density, which seldom corresponds to the mean of those of the constituents of the alloy, being usually either more or less than that shown by the percentage composition; tenac ity, which is usually greater than that of the constituents of the alloy, although it is some times diminished; hardness, which is almost al ways increased; extensibility, which is almost always diminished; and fusibility, the melting point being generally lower than the mean of the melting points of the constituent metals. Mat thiessen, who studied the subject of alloys very thoroughly, divided the constituents of alloys in to two classes: Those metals which impart to their alloys their physical properties in the pro portion in which they themselves exist in the alloy, and those which do not impart to their alloys their physical properties in the proportion in which they themselves exist in the alloy. In the first class lie places lead, tin, zinc, and cad mium, and in the second class, in all probability, the rest of the metals. lie divided the physical properties of alloys into three classes: (1) Those which in all cases are imparted to the alloy ap proximately in the ratio in they are pos sessed by the component metals; (2) those which in all eases are not imparted to the alloy in the ratio in which they are possessed by the component metals: (3) those which in some cases are and in others are not, imparted to the alloy in the ratio in which they are possessed by the component metals. As types of the first class, specific gravity, specific heat, and expan sion due to heat, may be taken; as types of the second class. the fusing points and crystalline form; and as types of the third class, the con ducting power for heat and electricity, sound, elasticity, and tenacity. Very few of the metal lic elements are found pure in nature, but for the most part they are found alloyed with some other metallic element: thus gold usually con tains traces of silver; copper is alloyed with sil ver or bismuth; lead almost always contains sil ver and frequently antimony; platinum occurs as an alloy with iron, iridium, osmium, and other metals. The great value of alloys in com merce is due to the fact that certain properties which are desirable for practical purposes may be imparted to many metals by a suitable addi tion of other metals. For instance, gold and silver are too soft for use as coins when pure, but may be rendered sufficiently hard by the ad mixture of small proportions of copper. Simi larly the hardness of copper is greatly increased by the addition of zinc, yielding brass. In mak ing alloys, the least fusible metal is melted first, after which the others are added. When three metals are used to form the alloy, they are melted in pairs and afterward together. The fused mass must be kept well stirred until the mixture is complete, otherwise the tendency would be for the heavier metal to sink to the bottom and the alloy would not be of uniform composition. The more important alloys, which are included among the materials of constructive engineering. are the bronzes, the brasses, the coin
alloys, and a few alloys of tin, lead, zinc, anti mony, and bismuth. All the other alloys are of use for a few special purposes only. The fol lowing are the principal alloys, their composi tion amf uses: Bronze is an alloy of copper and tin. The knowledge of bronze is very old, it being used by the ancients for making coins, weapons, tools, and ornaments. :Many of these ancient peoples were very skillful bronze founders. The principal bronzes are those used in coinage, in ordnance, in statuary, in bells and musical in struments, and in mirrors and the specula of tel escopes. rain bronze as made by the Greeks and Romans consisted of from 90% copper and 4% tin, to 95% copper and 2% tin. Modern inves tigations have shown the range of good alloys for this purpose to be quite large. varying from 96% copper and 4% tin to S0% copper and 14% tin. the best falling near the middle of this range. Gun. bronze has different compositions in different countries, but the most common pro portion would seem to he 90% copper and 10% tin, or 89% copper and 11% tin. When well made it is solid, yellowish in color. denser than the mean of its constituents, and much harder, stronger, and more fusible than commercial cop per: it is somewhat malleable when hot and much less so when cold. Statuary bronze is nearly the same composition as gun bronze. It should be rapidly melted, poured at a high temperature, and quickly cooled to get the best results. Bell metal is richer in tin than the pre ceding, and varies in composition somewhat with the size of the bell, the proportion of tin being the larger in the ease of small hells. The range of good practice in bell metal is from 18`Y, to 30% tin and from 82% to 70% copper. Chinese gongs arc made from 75% to SO% copper and 22% to 20% tin, and are beaten into shape with the hammer. being tempered at intervals during the process. (See ANNEALING.) Bell metal is dense and homogeneous, fine ground, malleable if quickly cooled in the mold, rather more fusible than gun bronze, but otherwise similar. excelling, however, in hardness, elasticity, and sonority.. Speculum, metal contains often as much as tin; it is almost silvery white, extremely hard and brittle, and capable of taking a very perfect polish. Bronze for bearings and other friction surfaces in machinery is made of many propor tions, varying from 88% to 96% copper, as more or less hardness is required. Phosphor bronze is a triple alloy of copper, zinc, and tin, which has been given exceptional purity by fluxing with phosphorus. It is very tough and hard, and is. used for piston rings and valve covers, pinions, cog wheels, screw propellers, etc. Tobin bronze. is an alloy of copper and zinc in the proportion of about 59 to 38, with small percentages of tin, iron, and lead. It has great tensile strength, and corrodes with great difficulty. Aluminum bronze consists of 90% copper and 10% aluminum, and is an exceedingly tenacious material. Mangan ese bronze is as alloy consisting of about 88 % copper, ly,% tin, 8.7% zinc, and smaller per centages of iron, lead, and phosphorus; it is much_ used for making screw propellers. Both Tobin bronze and manganese bronze are in reality more nearly brasses, since the zinc percentage is greater than the tin percentage.