colour, more remarkable than that of any other metal or alloy. Many other inter-metallic compounds have been indicated by summits in freezing-point curves. For example, the system sodium-mercury has a remarkable summit at the composition NaHg2. This compound melts at 350° C., a temperature far above the melting-point of either sodium or mercury. In the system potassium-mercury, the compound KHg2 is similarly indicated. In the curve for sodium-cadmium, the compound NaCd2 is plainly shown. These three examples are taken from the work of N. S. Kurnakow. Various compounds of the alkali metals with bismuth, antimony, tin and lead have been prepared in a pure state. Such are the compounds SbNa3, BiNa3, PbNa2, SnNa4. Of these, the first three are well indicated on the freezing-point curves. The intermediate summits occurring in the freezing-point curves of alloys are usually rounded; this feature is believed to be due to the partial decomposition of the compound which takes place when it melts. The formulae of the group of substances last mentioned are in harmony with the ordinary views of chemists as to valency, but the formulae NaHg2, NaCd2, NaTl2, AuAl2 are more surprising. They indicate the great gaps in our present knowledge of the subject of valency. We must not take it for granted, when the freezing-point curve gives no indication of the compound, that the compound does not exist in the solid alloy. For example, the compound Cu3Sn is not indicated in the freezing-point curve, and indeed a liquid alloy of this percentage does not begin to solidify by the formation of crystals of Cu3Sn; the liquid solidifies completely to a uniform solid solution, and only at a lower temperature does this change into crystals of the compound, the transformation being accompanied by a considerable evolution of heat. Until recently the vast subject of inter-metallic compounds has been an unopened book to chemists. But the subject is now being vigorously studied, and, apart from its importance as a branch of descriptive chemistry, it is throwing light, and promises to throw more, on obscure parts of chemical theory.
The graphical representation of the properties of alloys can be extended so as to record all the changes, thermal and chemical, which the alloy undergoes after, as well as before, solidification, including the formation and breaking up of solid solutions and compounds. For an example of such a diagram, see the Bakerian Lecture, 1903, Phil. Trans., A. 346. The Phase Rule of Willard Gibbs, especially as developed by Bakhuis Roozeboom, is a most useful guide in such investigations.
So far we have been considering alloys containing two metals;
the phenomena they present are by no means simple. But when
three or more metals are present, as is often the case in useful
alloys, the phenomena are much more complicated. With three
component metals the complete diagram giving the variations in
any property must be in three dimensions, although by the use of
Fig. 8
contour lines the essential facts can be represented in a plane
diagram. The following method, depending
on the constancy of the sum of the
perpendiculars from any point on to the
sides of an equilateral triangle, can be
adopted:—Let ABC (fig. 8) be an
equilateral triangle, the angular points
corresponding to the three pure metals A, B, C.
Then the composition of any alloy can be
represented by a point P, so chosen that
the perpendicular Pa on to the side BC
gives the percentage of A in the alloy, and the perpendiculars
Pb and Pc give the percentages of B and C respectively.
Points on the side AB will correspond to binary alloys
containing only A and B, and so on. If now we wish to
represent the variations in some property, such as fusibility,
we determine the freezing-points of a number of alloys
distributed fairly uniformly over the area of the triangle, and,
at each point corresponding to an alloy, we erect an ordinate
at right angles to the plane of the paper and proportional
in length to the freezing temperature of that alloy. We can
then draw a continuous surface through the summits of all
these ordinates, and so obtain a freezing-point surface, or
liquidus; points above this surface will correspond to wholly
liquid alloys. The ternary alloys containing bismuth, tin and
lead have been studied in this way by F. Charpy and by E. S.
Shepherd. We have here a comparatively simple case, as the
metals do not form compounds. The solid alloy consists of
crystals of pure tin in juxtaposition with crystals of almost pure
lead and bismuth, these two metals dissolving each other in solid
solution to the extent of a few per cent only. If now we cut the
freezing-point surface by planes parallel to the base ABC we get
curves giving us all the alloys whose freezing-point is the same;
these isothermals can be projected on to the plane of the triangle
and are seen as dotted lines in fig. 9. The freezing surface, in
this case, consists of three sheets each starting from an angular
point of the surface, that is, from the freezing-point of a pure
metal. The sheets meet in pairs along three lines which
themselves meet in a point. In fig. 9, due to F. Charpy, these lines are
projected on to the plane of the triangle as Ee, E′e and E″e.
The area of the triangle is thus divided into three regions. The
region PbEeE′ contains all the alloys that commence their
solidification by the crystallization of lead; similarly, the other
two regions correspond to the initial crystallization of bismuth
and tin respectively; these areas are the projections of the three
sheets of the freezing-point surface. The points E, E′, E″ are
the eutectics of binary alloys. Alloys represented by points on
Ee, when they begin to solidify, deposit crystals of lead and
bismuth simultaneously; Ee is a eutectic line, as also are E′e and
E″e. The alloy of the point e is the ternary eutectic; it deposits
the three metals simultaneously during the whole period of its
solidification and solidifies at a constant temperature. As the
lines of the surface which correspond to Ee, &c., slope downwards
to their common intersection it follows that the alloy e has the
lowest freezing-point of any mixture of the three metals; this
freezing-point is 96° C., and the alloy e contains about 32% of
lead, 15·5% of tin and 52·5% of bismuth.
Fig. 9.
It is evident that any other property can be represented by similar diagrams. For example, we can construct the curve of conductivity of alloys of two metals or the surface of conductivity of ternary alloys, and so on for any measurable property.
The electrical conductivity of a metal is often very much decreased by alloying with it even small quantities of another metal. This is so when gold and silver are alloyed with each other, and is true in the case of alloys of copper. When a pure metal is cooled to a very low temperature its electrical conductivity is greatly increased, but this is not the case with an alloy. Lord Rayleigh has pointed out that the difference may arise from the heterogeneity of alloys. When a current is passed through a solid alloy, a series of Peltier effects, proportional to the current, are set up between the particles of the different metals, and these create an opposing electromotive force which is indistinguishable experimentally from a resistance. If the alloy were a true chemical compound the counteracting electromotive force should not occur; experiments in this direction are much needed.
