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 

Alloy

          A substance which is composed of two or more than two chemical elements such that metallic atoms predominates in composition and the metallic bond predominates is called an "Alloy". The element which is present in largest proportion is called "base metal" and all other elements present are known as "alloying elements".

         Alloys are classified as binary alloys, composed of two components; as ternary alloys, composed of three components; or as multicomponent alloys. Most commercial alloys are multicomponent. The composition of an alloy is described by giving the percentage (either by weight or by atoms) of each element in it.

          Metal alloys by virtue of composition, are often grouped into two classes : Ferrous and Non-Ferrous. Ferrous alloys are those in which iron is the principal constituent, include steels and cast irons. The non-ferrous alloys are all alloys that are not iron based.

          Alloys are widely used in industry because their physical and chemical properties can be easily varied to suit the exact individual requirement. One can achieve this by preparing alloys of different metals.

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.,