Phase: A phase may be defined as a homogeneous portion of a system that has uniform physical and chemical characteristics. Every pure material is considered to be a phase;
If more than one phase is present in a given system, each will have its own distinct properties, and a boundary separating the phases will exist across which there will be a discontinuous and abrupt change in physical and/or chemical characteristics. When two phases are present in a system, it is not necessary that there be a difference in both physical and chemical properties; a disparity in one or the other set of properties is sufficient. Also, when a substance can exist in two or more polymorphic forms (e.g., having both FCC and BCC structures), each of these structures is a separate phase because their respective physical characteristics differ.
Equilibrium: A system is at equilibrium if its free energy is at a minimum under some specified combination of temperature, pressure, and composition. Phase equilibrium is reflected by a constancy with time in the phase characteristics of a system.
There are three externally controllable parameters that will affect phase structure—viz. temperature, pressure, and composition—
and phase diagrams are constructed when various combinations of these parameters are plotted against one another.
Lever rule for phase analysis, which follows the rule of mass conservation, states that the quantities of the individual components present in an alloy must be equal to the sum of their quantities in the liquid and the solid phases. Lever rule also indicates that quantities of phases being in equilibrium are inversely proportional to respective phases, in case of, two components.
