SOLIDIFICATION OF ALLOYS AND MAGMAS wie 
to its completion. With points for commencement and completion 
of solidification obtained from similar cooling-curves representing a 
complete series of mixtures, we may plot a new diagram with the 
composition on the X axis, and the corresponding freezing tempera- 
tures on the Y axis, and obtain the solubility curve as shown on the 
right of Fig. 2. In metallography this is usually designated the 
“ freezing-point curve.” CF,D and CF,D are the loci of the upper 
and lower transition points as obtained from the cooling-curves, and 
represent, therefore, the commencement and completion of solidifica- 
tion for the entire range of mixtures. 
With this type of freezing, a mixture of composition F, for exam- 
ple, will commence to solidify at /; with a separating-out of the first 
frozen particle of composition G, a consequent relative enrichment 
of the molten metal in element A, and a lowering of the temperature 
of solidification. During freezing, therefore, the temperature- 
composition locus of the molten material will shift along the upper 
curve from /’, toward C, and that of the solid material along the lower 
curve from G,; toward C. If cooling is sufficiently slow because of 
the complete solubility in the solid, the equilibrium is completed by 
the diffusion of the different solid particles until finally the resultant 
alloy has a uniform composition Ff, the same as the original molten 
mixture. 
With complete solubility, therefore, the freezing is selective but 
not rigid, and the initial heterogeneousness is effaced by diffusion. 
Under the microscope, the structure should be uniform throughout 
if diffusion is complete. 
Of this type are the Sb-Bi, Ag-Au, Fe-Mn alloys. 
The type of complete insolubility is shown in Fig. 3. On the left 
is the cooling-curve, with a break at K,, indicating commencement 
of solidification, and a jog K,K, denoting completion of freezing at 
a constant temperature. At the right is the freezing-point diagram, 
with FE,G and CE,D as the loci of commencement and completion 
of solidification. A mixture of composition K will commence to 
freeze at K,. The solid separating out will be element A only, 
because of the complete insolubility; there will be a relative enrich- 
ment of the molten solution in B, and a gradual shifting of the tem- 
perature composition range from K, to F;. At E, the whole of the 
