ALLOYS. In the commercial sense of the term an alloy is a combination of two or more metals which have been melted together. In a scientific sense, however, the term has a restricted meaning. It signifies a chemical com bination analogous to the common solution of a chemical salt in water; only, in the case of two metals in alloy, each is to be regarded as dissolved in the other. As an illustration of the conditions existing in an alloy let us con sider a solution of common salt in water. In 100 parts of water at 65° F. common salt may be dissolved in any proportions from one part (or less) up to 26.4 parts—at which point the solution is said to be "saturated." In every possible proportion between those limits the solution is entirely homogeneous. If more salt be added, it does not dissolve but remains in the solution as a mere mixture of salt with a solution of salt. In the case of a two-metal or binary alloy the two metals may be desig nated as A and B. The metal having the high est melting point is fused first, and the other is then added. Suppose B to be added to A in this manner. Up to a certain saturation point B dissolves freely in A. Beyond that point, if still more of B is added, a new corn bination will be found present in the alloy, namely, a solution of A in B. The alloy at that point is a mixture of the two solutions in varying proportions, depending upon the amount of B which has been added beyond the saturation point. With a still further ad dition of B a larger proportion of the solution A-in-B prevails in the alloy, and, continuing the process, the solution B-in-A disappears from the alloy, and it becomes a homogeneous solution of A in B.
These considerations relate to alloys in a molten state, at the temperature of the melting point of the most refractory constituent. As the temperature falls toward the point where the alloy becomes solid, several changes may be noted as taking place in its constitution, depend ing upon the proportions and conditions of solution of both A and B. The alloy A-in-B may separate out, generally in a crystalline form, or the alloy B-in-A may separate out. The condition of the mass is then one analo gous to curdled milk. That combination of the
two metals which has the lowest melting point is the last to solidify as the cooling continues, and this is called the eutectic of that alloy.
But, in the process of cooling, other changes may have taken place. In the solution B-in-A B may have crystallized out if it was close to its saturation point, and a similar action may have occurred as to A-in-B. Further complexities arise from the fact that when an alloy has become solid it has not always become stable. With the lapse of time changes are continually taking place either toward greater stability on the one hand, or toward dissolution on the other. Some alloys give evidence of these changes within a brief period. Others show them only after several years. Much has been learned about the constitution of alloys with the microscope, but this form of research has been limited in practice to the temperatures which normally prevail. Attempts to use the microscope on hot alloys have so far proved unsuccessful. For microscopic work the face of a thin slice of the alloy is very carefully and gently polished, and it is then delicately etched with an appropriate acid which serves to reveal many of the details of internal con stitution. (See METALIOGRAPHY). Although it has not been adequately determined whether the fusion and admixture of different metals takes place in some relation to their atomic proportions, or else in some other defi nite ratio, yet the composition of natural alloys such as (lore or electrum, which consists of variable proportions of gold and silver, as well the fact that, as a rule, alloys more readily melt than either of their constituent metals, are cir cumstances that have given rise to a wide spread conviction or belief that nature really does combine them by some law which is as yet undiscovered. Among the many alloys whose density is less than the average density of their constituent metals may be mentioned gold with either silver, copper, iridium, nickel, iron or lead; silver with copper; and iron with antimony, bismuth or lead. Among those whose density is greater than the mean of their constituents is gold with antimony, bismuth, cobalt or zinc.