Glossary of Terms for Phase Equilibria Diagrams
- Enantiotropic Forms
Polymorphic forms which possess an inversion point
at which they are in reversible equilibrium, that is, they are interconvertible; for example,
 - and  -2CaO·SiO and - and
-quartz. In such cases the vapor-pressure curves intersect
below the melting point of the highest temperature polymorphic form.
Reference: Levin, E.M., McMurdie, H.F., and Hall, F.P., Phase Diagrams for
Ceramists: Volume 1, The American Ceramic Society, Columbus, Ohio, p. 6, 1956.
- Equilibrium
From the theoretical, thermodynamic standpoint, the conditions for equilibrium can be exactly and
precisely defined; because for any reversible process, no useful energy passes from or into the system.
From the practical, experimental standpoint, however, the actual attainment of an equilibrium state
within a system may be very difficult to assess. Three criteria have been used variously either
singly or together: (1) The time criterion, based on the constancy of phase properties with the
passage of time; (2) the approach from two directions criterion, yielding under the same conditions
phases of identical properties, e.g., from undersaturation and supersaturation, or from raising and
lowering the temperature to the same value; and (3) the attainment by different procedures criterion,
producing phases having the same properties when the same conditions, with respect to the variants, are reached.
None of these criteria are entirely adequate for excluding metastable relationships.
In silicate systems, in particular, metastable equilibrium is common and may persist for long periods of time and at high
temperatures. In the final analysis, interpolation and judgment by the investigator are of prime importance.
Reference: Levin, E.M., McMurdie, H.F., and Hall, F.P., Phase Diagrams
for Ceramists: Volume 1, The American Ceramic Society, Columbus, Ohio, p. 6, 1956.
- Eutectic
A eutectic represents an invariant point (unique temperature, pressure, composition)
for a system at which the phase reaction on the addition or
removal of heat results in an increase or decrease, respectively, of the proportion of liquid to solid phases,
without change of temperature. At a eutectic point the composition of the liquid phase in equilibrium
with the solid phases can always be expressed in terms of positive quantities of the solid phases. The eutectic composition
is that combination of components in a simple system having the lowest melting point
of any ratio of the components and is located at the intersection of the two solubility curves
in a binary system and of the three solubility surfaces in a ternary system.
Reference: Levin, E.M., McMurdie, H.F., and Hall, F.P., Phase Diagrams
for Ceramists: Volume 1, The American Ceramic Society, Columbus, Ohio, p. 5, 1956.
Example of Eutectic
- Eutectoid
An invariant point composed solely of crystalline phases, at
which the phase reaction on change of heat content at constant temperature results in a change in
proportions of the solid phases exactly analogous to that at a eutectic point, in which
one of the phases is liquid.
Reference: Levin, E.M., McMurdie, H.F., and Hall, F.P., Phase Diagrams
for Ceramists: Volume 1, The American Ceramic Society, Columbus, Ohio, p. 5, 1956.
Example of Eutectoid
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