Glossary of Terms for Phase Equilibria Diagrams
- Peritectic Point
An invariant point at which the composition of the liquid phase
in equilibrium with the solid phases cannot be expressed in terms of positive
quantities of the solid phases. Whereas the composition of a eutectic point always
lies between or within the composition limits of the solid phases in equilibrium with liquid, the composition
of a peritectic point always lies outside the composition limits.
At a peritectic point the intersecting univariant curves do not produce a minimum point on the
liquidus curve as for a eutectic.
Reference: Levin, E.M., McMurdie, H.F., and Hall, F.P., Phase Diagrams for Ceramists: Volume 1,
The American Ceramic Society, Columbus, Ohio, p. 7, 1956.
Example of Peritectic Point
- Peritectoid
An invariant point composed entirely of crystalline phases, at which the phase
reactions on change of heat content at constant temperature are exactly analogous to those at
a peritectic 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. 7, 1956.
Example of Peritectoid
- Phase
Any portion, including the whole, of a system which is physically homogeneous within
itself and bounded by a surface so that it is mechanically separable from any other portions.
A separable portion need not form a continuous body, as for example, one liquid dispersed in another.
A system composed of one phase is a homogeneous system; a system composed of more than one phase
is heterogeneous; and in order for the phase rule to apply, each phase must be
in homogeneous as well as heterogeneous equilibrium.
Reference: Levin, E.M., McMurdie, H.F., and Hall, F.P., Phase Diagrams
for Ceramists: Volume 1, The American Ceramic Society, Columbus, Ohio, p. 7, 1956.
- Phase Rule
For a system in equilibrium, the sum of the number of phases
plus the number of degrees of freedom must equal the sum of the number of components
plus two, or P + F = C + 2.
Reference: Levin, E.M., McMurdie, H.F., and Hall, F.P., Phase Diagrams
for Ceramists: Volume 1, The American Ceramic Society, Columbus, Ohio, p. 7, 1956.
- Piercing Point
In a quaternary system, the intersection of a univariant curve with a ternary join
at a point other than a ternary invariant point. The univariant curve represents the
compositions of liquids that can exist in equilibrium with three particular solid
phases. The composition of these solid phases usually all lie in the plane of the ternary
join if the intersection is a ternary invariant point, but they cannot all lie in that plane if
the intersection is a piercing point.
Reference: Levin, E.M., McMurdie, H.F., and Hall, F.P., Phase Diagrams
for Ceramists: Volume 1, The American Ceramic Society, Columbus, Ohio, p. 7, 1956.
- Polymorphism
The property possessed by some substances of existing in more than one crystal form, all forms
being of the same chemical composition but differing in crystalline structure and physical
properties, and yielding identical liquid or gaseous phases on melting or evaporating.
Reference: Levin, E.M., McMurdie, H.F., and Hall, F.P., Phase Diagrams
for Ceramists: Volume 1, The American Ceramic Society, Columbus, Ohio, p. 7, 1956.
Example of Polymorphism
- Primary Phase
The only crystalline phase which can exist in equilibrium with liquid of a given
composition. The primary phase is the first crystalline phase to appear on cooling a composition
from the liquid state; or conversely, it is the last crystalline phase to disappear on heating a composition to melting.
Reference: Levin, E.M., McMurdie, H.F., and Hall, F.P., Phase Diagrams
for Ceramists: Volume 1, The American Ceramic Society, Columbus, Ohio, p. 7, 1956.
Example of Primary Phase
- Primary Phase Region
The locus of all compositions in a phase diagram having a common primary phase.
Reference: Levin, E.M., McMurdie, H.F., and Hall, F.P., Phase Diagrams
for Ceramists: Volume 1, The American Ceramic Society, Columbus, Ohio, p. 7, 1956.
Example of Primary Phase Region
- Pseudo System
It is frequently convenient or necessary to refer to portions of a binary or ternary, etc.,
system which are not (true) subsystems. In such instances the term pseudo binary,
or pseudo ternary, etc., is used.
Reference: Levin, E.M., McMurdie, H.F., and Hall, F.P., Phase Diagrams
for Ceramists: Volume 1, The American Ceramic Society, Columbus, Ohio, p. 7, 1956.
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