eutectic point gives the second halt in cooling, due to the simultaneous
formation of lead crystals and tin crystals. In the case
of this pair of metals, or indeed of any metallic alloy, we cannot
see the crystals forming, nor can we easily filter them off and
examine them apart from the liquid, although this has been done
in a few cases. But if we polish the solid alloys, etch them if
Fig. 6
necessary, and examine them microscopically, we shall find that
alloys on the lead side of the diagram consist of comparatively
large crystals of lead embedded in a minute complex, which is
due to the simultaneous crystallization of the two metals during
the solidification at the eutectic temperature. If we examine
alloys on the tin side we shall find large crystals of tin embedded
in the same complex. The eutectic alloy itself, fig. 2 (Plate),
shows the minute complex of the tin-lead eutectic, photographed
by J. A. Ewing and W. Rosenhain,
and fig. 3 (Plate),
photographed by F. Osmond, shows
the structure of a silver-copper
alloy containing considerably
more silver than the eutectic.
Here, the large dark masses are
the silver or silver-rich substance
that crystallized above the eutectic
temperature, and the more minute
black and white complex represents the eutectic. It is not
safe to assume that the two ingredients we see are pure silver
and pure copper; on the contrary, there is reason to think that
the crystals of silver contain some copper uniformly diffused
through them, and vice versa. It is, however, not possible to
detect the copper in the silver by means of the microscope.
This uniform distribution of a solid substance throughout the
mass of another, so as to form a homogeneous material, is called
“solid solution,” and we may say that solid silver can dissolve
copper. Solid solutions are probably very common in alloys,
so that when an alloy of two metals shows two constituents under
the microscope it is never safe to infer, without further evidence,
that these are the two pure metals. Sometimes the whole alloy
is a uniform solid solution. This is the case with the copper-tin
alloys containing less than 9% by weight of tin; a microscopic
examination reveals only one material, a copper-like substance,
the tin having disappeared, being in solution in the copper.
Much information as to the nature of an alloy can be obtained by placing several small ingots of the same alloy in a furnace which is above the melting-point of the alloy, and allowing the temperature to fall slowly and uniformly. We then extract one ingot after another at successively lower temperatures and chill each ingot by dropping it into water or by some other method of very rapid cooling. The chilling stereotypes the structure existing in the ingot at the moment it was withdrawn from the furnace, and we can afterwards study this structure by means of the microscope. We thus learn that the bronzes referred to above, although chemically uniform when solid, are not so when they begin to solidify, but that the liquid deposits crystals richer in copper than itself, and therefore that the residual liquid becomes richer in tin. Consequently, as the final solid is uniform, the crystals formed at first must change in composition at a later stage. We learn also that solid solutions which exist at high temperatures often break up into two materials as they cool; for example, the bronze of fig. 1, which in that figure shows two materials so plainly, if chilled at a somewhat higher temperature but when it was already solid, is found to consist of only one material; it is then a uniform solid solution. The difference between softness and hardness in ordinary steel is due to the permanence of a solid solution of carbon in iron if the steel has been chilled or very rapidly cooled, while if the steel is slowly cooled this solid solution breaks up into a minute complex of two substances which is called pearlite. The pearlite when highly magnified somewhat resembles the lead-tin eutectic of fig. 2 (Plate). In the case of steel (see Iron and Steel) the solid solution is very hard, while the pearlite complex is much softer. In the case of some bronzes, for example that with about 25% of tin, the solid solution is soft, and the complex into which it breaks up by slow cooling is much harder, so that the same process of heating and chilling which hardens steel will soften this bronze.
If we melt an alloy and chill it before it has wholly solidified, we often get evidence of the crystalline character of the solid matter which first forms. Fig. 4 (Plate) is the pattern found in a bronze containing 27·7% of tin when so treated. The dark, regularly oriented crystal skeletons were already solid at the moment of chilling; they are rich in copper. The lighter part surrounding them was liquid before the chill; it is rich in tin. This alloy, if allowed to solidify completely before chilling, turns into a uniform solid solution, and at still lower temperatures the solid solution breaks up into a pearlite complex. The analogy between the breaking up of a solid solution on cooling and the formation of a eutectic is obvious. Iron and phosphorus unite to form a solid solution which breaks up on cooling into a pearlite. Other cases could be quoted, but enough has been said to show the importance of solid solutions and their influence on the mechanical properties of alloys. These uniform solid solutions must not be mistaken for chemical compounds; they can, within limits, vary in composition like an ordinary liquid solution. But the occasional or indeed frequent existence of chemical compounds in alloys has now been placed beyond doubt.
We can sometimes obtain definite compounds in a pure state by the action of appropriate solvents which dissolve the rest of the alloy and do not attack the crystals of the compound. Thus, a number of copper-tin alloys when digested with hydrochloric acid leave the same crystalline residue, which on analysis proves to be the compound Cu3Sn. The bodies SbNa3, BiNa3,
SnNa4, compounds
Fig. 7 of iron and molybdenum and many other substances, have also been isolated in this way. The freezing-point curve sometimes indicates the existence of chemical compounds. The simple type of curve, such as that of lead and tin, fig. 6, consisting of two downward sloping branches meeting in the eutectic point, and that of thallium and tin, the upper curve of fig. 7, certainly give no indication of chemical combination. But the curves are not always so simple as the above. The lower curve of fig. 7 gives the freezing-point curve of mercury and thallium; here A and E are the melting-points of pure mercury and pure thallium, and the branches AB and ED do not cut each other, but cut an intermediate rounded branch BCD. There are thus two eutectic alloys B and D, and the alloys with compositions between B and D have higher melting-points. The summit C of the branch BCD occurs at a percentage exactly corresponding to the formula Hg2Tl. It is probable that all the alloys of compositions between B and D, when they begin to solidify, deposit crystals of the compound; the lower eutectic B probably corresponds to a solid complex of mercury and the compound. The point B is at −60° C., the lowest temperature at which any metallic substance is known to exist in the liquid state. The higher eutectic D may correspond to a complex of solid thallium and the compound; but the possible existence of solid solutions makes further investigation necessary here. The curves of fig. 7 were determined by N. S. Kurnakow and N. A. Puschin. Sometimes a freezing-point curve contains more than one intermediate summit, so that more than one compound is indicated. For example, in the curve for gold-aluminium, ignoring minor singularities, we find two intermediate summits, one at the percentage Au2Al, and another at the percentage AuAl2. Microscopic examination fully confirms the existence of these compounds. The substance AuAl2 is the most remarkable compound of two metals that has so far been discovered; although it contains so much aluminium its melting-point is as high as that of gold. It also possesses a splendid purple
