Glossary of Terms for Phase Equilibria Diagrams

A B C D E G H I J L M P S T V

Peritectic Point
Peritectoid
Phase
Phase Rule
Piercing Point
Polymorphism
Primary Phase
Primary Phase Region
Pseudo System

  • 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

    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

    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

    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

    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

    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.






Privacy Statement/Security Notice
Disclaimer | FOIA

NIST is an agency of the
U.S. Commerce Department

Date created: 08 February 2001
Last updated: 01 July 2008


Technical inquiries:
Ceramics Division
NIST, 100 Bureau Drive, Stop 8520, Gaithersburg, MD 20899-8520.
Phone (301) 975-6119
Fax (301) 975-5334

Ceramics website comments:
Ceramics Webmaster

General NIST inquiries:
Public Inquiries Unit:
Phone (301) 975-NIST (6478)
TTY (301) 975-8295