NATURE OF MATERIALS - Materials for Civil and Construction Engineers - page 73

eutectic mixture is composed of an intimate mixture of fine crystals of A and
B, usually in a plate-like structure.

Partially Soluble Materials In between purely soluble and insoluble materi-
als are the materials that are partially soluble. In other words, there is a
solubility limit between the components of A and B. If the percent of com-
ponent B is less than or equal to the solubility limit, on cooling all of the B
atoms will be in solution with the A component. If the percent of the B com-
ponent is above the solubility limit, the atoms in excess of the amount that
will go into solution will form separate grains of the component B. The
result is shown on the phase diagram in Figure 2.16. Note that the only dif-
ference between this phase diagram and the one shown in Figure 2.15 is the
presence of the solid solution regions on each side of the graph. The com-
position analysis of the two-phase region is the same as that described for
Figure 2.15.

Eutectoid Reaction Up to now, the phase diagram has been used to describe
the transition between liquid and solid phases of materials. However, the
lattice structure of some elements (e.g., iron) is a function of the temperature
of the solid. As a result of the lattice transformation, the microstructure of
the solid material changes as a function of temperature, as shown by the
phase diagram in Figure 2.17. When this occurs, a eutectoid reaction occurs
on the phase diagram; it has characteristics similar to the eutectic reaction,
but is for a lattice structure transformation of the material rather than for a
liquid-solid transformation. The rules for the analysis of the components of
the material are the same as those discussed for the eutectic material. As
with the phase transformation at the eutectic temperature, the transforma-
tion of the lattice structure at the eutectoid temperature will result in fine-
grained materials.

Liquid

Liquid b

a

Liquid

a b

a b

FIGURE 2.16 Binary phase
diagram, partially soluble material.

A B

Composition