The equilibrium diagram is a chart, plotted upon the composi tion of the alloys in percentages as base line and with temperature as ordinate. A series of lines indicate the temperatures at which the alloys undergo changes, which are generally accompanied by evolutions of heat on cooling and absorptions of heat on heating. Starting with a molten alloy, the first change which occurs on cooling is the commencement of solidification or freezing. This is readily determined by means of cooling curves which show thermal irregularities as the alloy cools. The freezing points or, rather, the points where freezing begins, are connected by a curve in the equilibrium diagram which is known as the "liquidus." Similarly, the temperatures at which the alloys become com pletely solid on cooling are connected by a line or lines known as the "solidus." Beyond these two curves or groups of curves, the diagram is divided into areas or "fields" which delimit the con ditions of composition or temperature under which the various possible constituents of the alloys can exist in equilibrium. This state of complete equilibrium, however, is not always, or indeed, often, reached by alloys, although with very slow cooling or after prolonged heating at suitable temperatures, the constitution of the alloys tends to approach closely to the equilibrium condi tion. Alloys as ordinarily cooled, however, are usually in an intermediate or meta-stable state, the exact condition depending on the rate of cooling. The diagrams as determined for the various alloy systems follow certain well-defined types; only two examples of these types can be given here. Fig. 3 is the diagram of the alloys of a pair of metals which, while completely soluble in one another in the liquid state, separate into distinct crystals on cooling. Alloys of this type are known as "eutectiferous" since a low-melting "eutectic" is formed along the horizontal line PQ of the figure.
The structure typical of such an alloy, cor responding to the point C of the diagram, is illustrated in fig. 2, Plate 1. Fig. 4 repre sents an alloy system of the extreme op posite type, in which the two metals re tain in the solid state the mutual miscibility which exists when they are molten, the alloys forming a continuous series of what are known as "solid solutions." These al
loys, when very slowly cooled, exhibit the same structure throughout as a pure metal, although the alloys may differ widely from one another in physical properties, colour, etc. There are inter mediate types, and others in which the formation of definite inter metallic compounds is indicated by a break or by a maximum in the liquidus curves.
The equilibrium diagram gives a clue not only to the micro structure and thermal behaviour of the alloys, but also to their physical properties. Conversely, all these properties have been used for the determination of the equilibrium diagram. Reference has already been made to thermal methods whereby the tem peratures at which a specimen crosses certain lines of the equilib rium diagram can be determined by irregularities—halts or retardations—in the heating and cooling process. The micro structure, both of slowly cooled specimens and of specimens rapidly quenched from a definite temperature, also affords an in sight into the location of the lines of the diagram. Thus, if a specimen of an alloy is quenched from a temperature above the "solidus," i.e., in a condition where traces of liquid are present, these can be recognized as so-called "fusion spots" in the struc ture. Such spots are illustrated in fig. 8, Plate 1.
Dilatometry consists in the measurement of the changes in length of small rods of an alloy when it is slowly heated or cooled. Apart from the normal thermal expansion or contraction, it is found that alloys exhibit comparatively abrupt changes of length when they undergo internal changes such as allotropic transforma tions or the appearance of a new constituent or the disappearance of one previously present. The points at which such changes begin or end are fairly clearly marked on the dilatometer curves, usually recorded photographically by means of dilatometers of the Chevenard type. This method of studying alloys has the advantage that very slow rates of heating and cooling can be employed so that some approach to equilibrium conditions is obtained. This advantage also applies to the study of alloys by measurement of the changes of electrical conductivity with tem perature. If this method is carefully used, with adequate pre cautions against accidental disturbing factors, it is capable of giving valuable results. It is not, however, certain that changes of electrical conductivity always occur at the precise points which are sought for the purpose of determining equilibrium diagrams.