ALLOY, the name given to a combination obtained by fusing metals with each other. Few metals are employed in the pure state, with the exception of iron, copper, lead, tin, zinc, platinum, aluminium; metals are more frequently used in the forms of alloys for technical purposes. Every industrial application necessitates special qualities that may not occur in any isolated metal, but which may be produced by the proper mixture of two or more of these substances. Thus gold and silver, which in their pure state are too soft and flexible for the manufacture of plate, coin, trinkets, &c., are hardened by the addition of a tenth part of copper, while the colour and other valuable qualities are not materially impaired. Similarly copper is rendered hard by mixture with zinc, when we obtain brass, an alloy of a beautiful yellow colour, easier to work than the pure metal. If brass has to be used in turning operations it is found to tear under the action of the chisel, unless a small quantity of lead has been added. These examples are sufficient to show that an alloy is really an industrial metal, often of greater importance than the metals which compose it....
Alloys are equally interesting from a purely scientific point of view. They are not only mixtures of metals having certain particular qualities, but in reality are true chemical compounds, generally dissolved in an excess of one of the constituent metals. In the appearances which accompany the union of the metals, and in the properties of the resulting products, we observe that which characterises the manifestation of affinity, that is, an evolution of heat and light, resulting in the formation of substances having a definite composition, distinct crystalline form, and a variety of properties different from those of the constituents. If a piece of clean sodium is rubbed in a mortar with a quantity of dry mercury, the metal dissolves, producing a harsh sound resembling the immersion of red hot iron in water. This phenomenon is due to the large evolution of heat which accompanies the combination, as the mercury rises rapidly in temperature on the addition of each successive piece of sodium. If the mass is allowed to cool after the action, long needles of a white brilliant alloy of definite composition crystallise from the middle of the liquid, from which the excess of mercury may be separated by decantation. Platinum, iridium, gold, rhodium, ruthenium, and silver unite with tin, producing an evolution of heat; if the tin is in excess after cooling, a metallic ingot is obtained resembling closely the original substance; but if the mass is treated with strong hydrochloric acid, the excess of tin is dissolved and crystals remain of a definite alloy of tin and the precious metal. These alloys are insoluble in strong hydrochloric acid, which dissolves tin so easily; but they are soluble in aqua regia, even when the precious metal contained therein (rhodium, ruthenium, iridium) is in the free state absolutely insoluble. This is no proof that the industrial alloys are always the result of one definite combination dissolved in excess of one of the metals, as many combinations are able to co-exist in the same alloy. This may be proved by taking an alloy of tin, lead, and bismuth, which melts below the boiling point of water, heating to a temperature of 25° C., and observing the rate of cooling by means of a thermometer. The thermometer falls at first regularly as far as a certain degree, where it remains stationary for some time, after which it descends to a lower temperature, where it is again similarly arrested. These two stoppages in the rate of cooling can only be explained by admitting the production of a less fusible alloy in the fluid mass, which solidifies with an evolution of heat, rendering the thermometer stationary for a time. Each successive arrest will therefore correspond to the formation of more fusible combinations. Thus the metals form amongst themselves true chemical combinations; and alloys are often formed by the mixture of one or more of these compounds with excess of one of the constituents.
Recently hydrogen, which, although a gaseous substance, has chemical properties resembling those of the true metals, has been combined with palladium, sodium, and potassium, producing compounds similar in properties to the recognised alloys.
Properties of Alloys. Density.— If the density of any alloy is calculated from that of the components— assuming that there is no condensation of volume—the resulting number is sometimes greater than, equal to, or less than, the experimental result. Thus the alloys of gold and silver are less dense than the theoretical mean density; whereas brass and the alloys of lead and antimony vary in the opposite direction. The former are therefore produced through an expansion, the latter through a condensation of their constituents. In the formation of many alloys there is no alteration of volume, and then the calculated density is correct. Colour.— This is generally grey, unless when we have a coloured metal like copper or gold present in sufficient quantity. Hardness, Ductility, and Tenacity.— Alloys are for the most part harder and more brittle, and are generally less ductile and possess less tenacity than the constituent metal that has these properties in excess. Aluminium bronze is an exception, as its tenacity is greater than that of either of the components. Fusibility. —This is always greater than that of the least fusible metal entering into the composition of the alloy, and is sometimes greater than in any of the components. Thus an alloy, composed of 5 parts of bismuth, 3 of lead, and 2 of tin, melts at 91° C. Alloys of lead and silver, containing a small quantity of the latter, are more fusible than lead, and potassium and sodium form an alloy fluid at the ordinary temperature of the air. Liquation.— The constituents of an alloy heated gradually to near its point of liquefication frequently unite anew in such proportions as to form a mass that is fusible at the given temperature. If the fluid portion is poured off, there remains a solid alloy less fusible than the original. Copper is separated from silver by this process. Decomposition. —When the alloy contains a volatile metal like zinc or mercury, heat decomposes it, but the temperature required to expel the last trace of the volatile metal must be considerably higher than that metal’s normal temperature of ebullition. Temper.— The alloy of 94 parts of copper and 6 parts of tin forms a bronze so brittle that it may be pulverised with a hammer when it has been slowly cooled; but if, on the contrary, it is cooled rapidly by tempering it in cold water, it becomes malleable. Influence of the Constituent Metals.— Mercury, bismuth, tin, and cadmium give fusibility to alloys into which they enter; tin also gives hardness and tenacity if present in considerable quantity; lead and iron give hardness; arsenic and antimony render alloys brittle.
Composition of Alloys. —A statement of the average proportions in which the metals enter into the best known alloys, the composition of which is generally very variable; is given in the following table :—
[table]
Preparation of Alloys.- —The metals are generally fused together under a layer of charcoal to prevent oxidation, thoroughly mixed by agitating, and the mass left to cool slowly. This process can only be employed when the constituent metals are all non-volatile at the temperature required for combination. If the mixture contains volatile metals, like sodium, potassium, magnesium, or zinc, they are added after the more refractory metal is fused.
