Phase Diagrams
The phase diagram is a convenient graphical method of displaying the state of a
given system that is stable under a defined set of conditions. Phase diagrams are
known as thermal equilibrium diagrams or constitutional diagrams .
The equilibrium state is the state of minimum free energy of the binary
system at a given temperature and composition at constant pressure. Thus the
analysis of a phase diagram is the subject of thermodynamics. The equilibrium
conditions in phase diagrams can be greatly satisfied by using very slow heating
and cooling which give sufficient time for phase changing.
Definitions and Terminology of Phase Diagrams:Phase diagrams are used to represent some of the changes which metals
undergo when they are slowly heated or cooled. This may involve melting,
freezing, and some type of heat-treating. As a preliminary to the equilibrium
diagrams, some definitions are given below:
Equilibrium:-The equilibrium is the basic idea in physics and chemistry where
free energy of a system at a minimum value for a given temperature ,composition
,and pressure the system becomes in equilibrium. The practical meaning is at
equilibrium there will be no microstructure changes with respect to time
.Mechanical systems are in equilibrium when all the bodies taking part are at rest
and occupy positions where their potential energy is at a minimum. Thermal
equilibrium is reached between two bodies enclose thermal contact when no heat
passes between them. Chemical equilibrium occurs when the concentrations of all
substances taking part in a chemical reaction no longer change .
Components: -The chemical elements which make up the alloy .
Alloys:- A metallic alloy is a mixture of a metal with other metals or nonmetals. For example, Copper (Cu) and Zinc (Zn), when mixed, form the
alloy brass. Iron (Fe) and carbon (C) mix to give carbon steel .
System: - A system is considered as a body of matter completely isolated from its
surrounding. This isolation is not a practical possibility but from a theoretical
viewpoint is a valuable concept. The word system is used to denote a series of
alloys containing the same components. For instance, the copper-zinc system refers
to all possible alloys of Cu-Zn .
Phase: - Phase is the part of an alloy with the same physical and chemical
properties and the same composition . It may contain one or more components. A
single phase system is called homogeneous system and systems with two or more
phases are mixtures or heterogeneous systems.
Solution:-It is consisting of two parts, one being the solute while the other is
the solvent. The main portion of the solution consists of the solvent with the
solute being dissolved in the solvent. Solutions are therefore homogeneous
mixtures in which the atoms and/or the molecules of the solute are
uniformly dispersed in the solvent.
Solid Solution: -is a solution in the solid state consisting of two types of
atoms combined in a single space lattice. There is a significant difference
between solid solutions and the usual liquid solutions. In the liquid solution
the relative size of the atoms or molecules is usually unimportant, while in
solid solution the relative size of the solute atom or molecule with respect to
the solvent atom or molecule is very important since it affects the solubility
of the solute in the solvent. Solid solution may be substitutional or
interstitial.
Substitutional Solid Solution:-It results when the solute atoms take up the
positions of the solvent metal in the crystal lattice. The solvent and solute atoms
must have the same number of valencey electrons , the same crystal structure,
the atomic radii of them differ by less than 15%, and small difference in
electronegativity.
Interstitial Solid Solution:- It results when the solute atoms are small enough to
fit into the interstices of the metal lattice. The elements that can form interstitial
solid solutions are hydrogen, carbon, nitrogen and boron.
Liquidus Line: - The locus of temperatures above which all compositions
are liquid. During cooling, the first solid nuclei to form upon passing the
liquidus point. During heating of the alloy, the structure becomes
completely liquid upon passing through the liquidus point but just below this
point some solid will still exist.
Solidus Line: -The locus of temperatures below which all compositions are
solid . During heating this is the temperature where melting of the metal
begins.
Every phase diagram for two or more component must show a
liquidus and solidus, and intervening freezing range, whether the
components are metals or nonmetals. There are certain locations on the
phase diagram where the liquidus and solidus meet. For the pure
component , this point lies at the edge of the diagram. When the pure
metal is heated, it will remain solid until its melting point is reached and will
then change entirely to liquid .The solidus and liquidus must also meet at
the eutectic.
Chemical Composition of Phases: - A phase diagram shows the chemical
compositions of the phases that are present under conditions of equilibrium after
all reactions have been completed.
Regions of Phase Diagrams:-Depending on the type of phase diagram, there
are three types of regions .Limited point to the type of region is number of
phases. Regions are :-One phase area: - the determination of the chemical composition of a single
phase is automatic. It has the same composition as the alloy. This also
holds when the location in the phase diagram involves a single phase solid
solution.
-Two phase area: - the determination of the chemical compositions of two
phases can be handled on a rote basis. The chemical composition of the
two phases are located at the two ends of the isotherm across two phases.
-Three Phase Area: -A liquid that has the analysis of the eutectic
composition when
the system separates into two solid phases at the
eutectic temperature. Thus, at this temperature only, three phases of this
system melt into a one phase liquid .
Solvus line: -It is a line consists of series of reaction points. Each point
denotes the amount of solubility that exists between the two metals of the
alloy. This degree of solubility varies with change in temperatures, as the
temperature falls the amount of one element absorbed in the lattice
structure of the second element decreases. When the temperature rises
the degree of absorption increases .
Phase diagrams are usually plotted with temperature, in degrees
centigrade or Fahrenheit or Kelvin, as the ordinate and the alloy
composition in weight percentage as the abscissa .For composition the
weight percentage of only one component is plotted , the balance of the
composition being the other component .Sometimes more convenient for
certain types of scientific work to express the alloy composition in atomic
percent .for two metals A and B, the weight % of component A is defined as
follows:
WA=( weight of component A/∑weights of all components)*100
The atom (or mole) % of component A can also be introduced :
MA= (number of atoms (or moles)of component A /∑number of atoms (or moles)of all
components)*100
Note that from MA and MB ,one can calculate WA and WB and reverse by
this way :
WA= MA* aA / (MA* aA+ MB* aB) :WB= MB* aB /(MA* aA + MB* aB)