TERMS USED IN EQUILIBRIUM DIAGRAMS

The following terms are frequently used in the study of solid phases and phase diagrams:

 i. Alloy

 ii. System

 iii. Components

 iv. Phase

 v. Phase diagram

 vi. Phase Equilibrium

 vii. Phase Transformation

 viii. Structural constituents

 ix. GIBB’S PHASE RULE OR THE PHASE RULE OR CONDENSED PHASE RULE 

System

          A system may be composed of solids, liquids, gases or their combinations and may have metals and non-metals separately or in any combination. It is explained as the whole complex of phases of one or several components at different pressures and compositions. A system is so isolated from its surroundings that it is unaffected by solids, liquids, gasses or their combinations and is subjected to the change in the overall composition, temperature, pressure or total volume, only to the extent permitted by the investigator.

          A system is classified according to the number of components that constitute the system.

Components

          These are the substances, either chemical elements or chemical compounds, whose presence is essential and sufficient to make a system. A system consist of one or more components, A pure metal is called a one-component system, an alloy of two metals is called a binary or two component system, etc.,

Phase

          It is a homogeneous portion of a system that has uniform physical and chemical characteristics. The number of phases in a system is the number of different substances that exist in it in a homogeneous system. A homogeneous liquid solution is a single-phase system; a mixture of crystals of two types, differing in composition and structure separated by an interface, or the coexistence of the liquid alloy and its crystals comprise two-phase system.