Two metals mutually soluble in all proportion in the liquid state but only partially soluble in the solid state

Two metals mutually soluble in all proportion in the liquid state but only partially soluble in the solid state
This case is in effect intermediate between the two dealt with in
Cases II and III and the thermal equilibrium diagram is, in consequence,
a sort of hybrid of these two
in this case it is a eutectic of two solid solutions
instead of two pure metals. It is in fact the general case and purists may
argue that, as such, it should have been dealt with before either of the
preceding particular cases. The systems have, however, been dealt with in
order of complexity rather than logical classification in order to assist the student
who may be encountering the subject for the first time.

The only new feature of this system, as compared with those foregoing,
is that we have phase boundaries occurring below the solidus, indicating
that phase changes can take place in the solid. On the tin-lead thermal equilibrium
diagram (Fig. 9.12), which we shall use as an example of this
type of system, such phase boundaries as these are indicated by the lines
BC and FG. A line such as BC or FG is termed a solvuse and indicates a change in
Solubility of one metal in another in the solid state