Not only may there be these three different types of curves, but there may also be combinations of these. Thus the two metals may not only form compounds, but one of the metals may not separate out in the pure state at all, but form mixed crystals. In this case the freezing point may rise (as in the case of silver and zinc), and one of the eutectic points will be absent.
Iron-Carbon Alloys.—Of all the different binary alloys, probably the most important are those formed by iron and carbon: alloys consisting not of two metals, but of a metal and a non-metal. On account of the importance of these alloys, an attempt will be made to describe in brief some of the most important relationships met with.
Before proceeding to discuss the applications of the Phase Rule to the study of the iron-carbon alloys, however, the main
facts with which we have to deal may be stated very briefly. With regard to the metal itself, it is known to exist in three different allotropic modifications, called α-, β-, and γ-ferrite respectively. Like the two modifications of sulphur and of tin, these different forms exhibit transition points at which the relative stability of the forms changes. Thus the transition point for α- and β-ferrite is about 780°; and below this temperature the α- form, above it the β- form is stable. For β- and γ-ferrite, the transition point is about 870°, the γ- form being the stable modification above this temperature.
The different modifications of iron also possess different properties. Thus, α-ferrite is magnetic, but does not possess the power of dissolving carbon; β-ferrite is non-magnetic, and likewise does not dissolve carbon; γ-ferrite is also non-magnetic, but possesses the power of dissolving carbon, and of thus giving rise to solid solutions of carbon in iron.
Various alloys of iron and carbon, also, have to be distinguished. First of all, there is hard steel, which contains varying amounts of carbon up to 2 per cent. Microscopic examination shows that these mixtures are all homogeneous; and they are therefore to be regarded as solid solutions of carbon in iron (γ-ferrite). To these solutions the name martensite has been given. Pearlite contains about 0.8 per cent. of carbon, and, on microscopic examination, is found to be a heterogeneous mixture. If heated above 670°, pearlite becomes homogeneous, and forms martensite. Lastly, there is a definite compound of iron and carbon, iron carbide or cementite, having the formula Fe3C.
A short description may now be given of the application of the Phase Rule to the two-component system iron—carbon; and of the diagram showing how the different systems are related, and with the help of which the behaviour of the different mixtures under given conditions can be predicted. Although, with regard to the main features of this diagram, the different areas to be mapped and the position of the frontier lines, there is general agreement; a final decision has not yet been reached with regard to the interpretation to be put on all the curves.
The chief relationships met with in the case of the
iron-carbon alloys are represented graphically in Fig. 75.[[309]] The curve AC is the freezing-point curve for iron,[[310]] BC the unknown freezing-point curve for graphite. C is an eutectic point. Suppose, now, that we start with a mixture of iron and carbon, represented by the point x. On lowering the temperature, a point, y, will be reached at which solid begins to separate out. This solid phase, however, is not pure iron, but a solid solution of carbon in iron, having the composition represented by y′ (cf. p. [185]). As the temperature continues to fall, the