PHASE DIAGRAMS

Prof. H. K. Khaira

Professor

Deptt. of MSME

M.A.N.I.T., Bhopal

Phase

• Phase is a homogenous, physically distinct and mechanically separable part of the material with a given chemical composition and structure.

Properties of a Materials

Properties of a materials depend on 1. Number of phases present2. Types of phases present3. Amount of phases present and 4. Distribution of the phases present

The Properties can be changed by altering these quantities. In order to make these changes, it is essential to know the conditions under which these quantities exist and the conditions under which a change in phase will occur.

Phase diagrams

Phase Diagram

• A phase diagram is a type of chart used to show conditions at which thermodynamically distinct phases can occur at equilibrium at different temperature, pressure and composition.

• The best method to record the data related to phase changes in many alloy systems is in the form of phase diagrams, also known as equilibrium diagrams or constitutional diagrams.

Phase diagrams

• In order to specify completely the state of a system in equilibrium, it is necessary to specify three independent variables.

• These variables, which are externally controllable, are temperature, pressure and composition.

• Phase diagram is the graphical presentation of the phases present in a system under different conditions of pressure, temperature and composition.

Components of a Phase Diagram• Common components of a phase diagram are lines of equilibrium or phase

boundaries, which refer to lines that mark conditions under which multiple phases can coexist at equilibrium. Phase transitions occur along lines of equilibrium.

• Triple points are points on phase diagrams where lines of equilibrium intersect. Triple points mark conditions at which three different phases can coexist. For example, the water phase diagram has a triple point corresponding to the single temperature and pressure at which solid, liquid, and gaseous water can coexist in a stable equilibrium.

• The solidus is the temperature below which the substance is stable in the solid state.

• The liquidus is the temperature above which the substance is stable in a liquid state. There may be a gap between the solidus and liquidus; within the gap, the substance consists as a mixture of solid and liquid (like a "slurry"). [1]

A typical phase diagram for Water

• The simplest phase diagrams are pressure-temperature diagrams of a single simple substance, such as water.

• The axes correspond to the pressure and temperature.

• The phase diagram shows, in pressure-temperature space, the lines of equilibrium or phase boundaries between the three phases of solid, liquid, and gas.

Phase diagrams in Metallurgical Systems

• In metallurgical systems, the pressure is usually taken as atmospheric pressure. Thus the phase diagram shows the phases present at different compositions and temperatures.

• With pressure assumed to be constant at atmospheric pressure, the equilibrium diagram indicates the structural changes due to variation of temperature and composition.

• Phase diagrams show the phases present under equilibrium conditions, that is, under conditions in which there will be no change with time.

• Equilibrium conditions may be approached by extremely slow heating and cooling, so that if a phase change is to occur, sufficient time is allowed.

Types of Phase Diagrams

• The phase diagrams may be divided into the following three catagories– 1. Unary Phase Diagram– 2. Binary Phase Diagram– 3. Ternary Phase Diagram

Unary Phase Diagrams

Unary Phase Diagrams

• Unary Phase Diagrams are phase diagrams for single component systems.

• Phase diagram for water, discussed earlier, is an example of Unary Phase Diagram.

• Another example is the diagram showing allotropy of iron or any other element.

Binary phase diagrams

Binary phase diagrams• The phase diagrams which

contains two components are known as binary phase diagrams.

• In that case concentration becomes an important variable.

• Phase diagrams are usually plotted with composition on X axis and the temperature on Y axis.

• The iron–iron carbide (Fe–Fe3C) phase diagram is shown here. The percentage of carbon present and the temperature define the phases of the iron carbon alloy.

Types of Binary Phase Diagrams

• Binary phase digrams can be of three types1. Complete solid solution type2. Eutectic type3. Peritectic type

Complete solid solution type

Complete solid solution type

• The phase diagram in which both the constituents are soluble in each other in solid state at all proportions, is known as complete solid solution type phase diagram.

• The phase diagram in Figure displays an alloy of two metals which forms solid solution at all concentrations of the two species

Complete solid solution typeNickel-Copper Phase Diagram

Complete solid solution typeGermanium-Silicon Phase Diagram

Eutectic Type

Eutectic Phase Diagram

• A phase diagram having a eutectic reaction is known as eutectic phase diagram.

• In eutectic reaction, a liquid phase (L) decomposes into a mixture of two solid phases (α and β) at a constant temperature on cooling.

• Eutectic reaction is : L = α (s) + β (s)

• It is a reversible reaction.

Fe-C Diagram

• A phase diagram of great technological importance is that of the iron-carbon system for less than 6.67% carbon.

• The x-axis of such a diagram represents the concentration variable of the mixture.

Eutectic Diagram• Figure shows the eutectic phase diagrams. • The components are A and B, and the possible phases are pure crystals of A, pure

crystals of B, and liquid with compositions ranging between pure A and pure B.• Compositions are plotted across the bottom of the diagram. • Temperature is plotted on the vertical axis.

Eutectic Diagram• The curves separating the fields of A + Liquid from Liquid and B + Liquid from

Liquid are termed liquidus curves. • The horizontal line separating the fields of A + Liquid and B + Liquid from A + B all

solid, is termed the solidus.

Eutectic Diagram• At the eutectic point in this two component system, all three phases, that is Liquid,

crystals of A and crystals of B, all exist in equilibrium.• Note that the eutectic is the only point on the diagram where this is true.

Eutectic Diagram• The eutectic point is an invariant point. If we change the composition of the liquid

or the temperature, the number of phases will be reduced to 2. • If the system contains only pure A, then the system is a one component system

and phase A melts at only one temperature, the melting temperature of pure A, TmA. If the system contains only pure B, then it is a one component system and B melts only at the melting temperature of pure B, TmB.

Eutectic Diagram• For all compositions between pure A and pure B, the melting temperature is

reduced, and melting begins at the eutectic temperature TE.

• The eutectic composition melts at only one temperature, TE. • For all compositions between A and B, other than eutectic composition, the

melting occurs over a range of temperatures between the solidus and the liquidus. This is true for all compositions except one, that of the eutectic.

Eutectic Phase DiagramTin-Lead Phase Diagram

Eutectic Phase DiagramGold-Germanium Phase Diagram

Eutectic Phase DiagramCopper-Silver Phase Diagram

Peritectic Phase Diagram• It is a phase diagram

containing peritectic reaction.• Peritectic reaction is :

L + α (s) = β (s)• In peritectic reaction, a liquid

(L) and a solid (α) transform in to another solid (β) on cooling.

• It is a reversible reaction.• Figure shows phase diagram

for Fe–C system (dotted lines represent iron-graphite equilibrium).

Application of Phase Diagrams

• Phase diagram gives us – Overall Composition– Solidus line– Liquidus line– Limits of Solid Solubility– Chemical Composition of Phases at any temperature– Amount of Phases at any temperature– Invariant Reactions– Development of Microstructure

Overall Composition

• Concentration: Relative amounts of each constituent

• It is the horizontal axis in all binary phase diagrams

• The scale can be in weight %, atomic % or mole %

• Normally concentration is denoted by weight % on X axis.

Solidus and Liquidus Points

• Solidus point– Temperature up to which alloy is completely solid– Temperature at which melting begins

• Liquidus point– Temperature up to which alloy is completely liquid– Temperature at which solidification begins

Chemical Composition of Phases

• It is the chemical composition of each phase in the system

• In a system having more than one phase, each phase will have a unique chemical composition which will be different from each other, and will also be different from the overall composition

• Not to be confused with overall composition

Invariant Reactions in Phase Diagrams

• Eutectic: L = α (s) + β (s); e.g., Pb-Sn• Peritectic: α (s) + L = β (s); e.g., Pb-In• Monotectic: L1 = α (s) + L2; e.g., Cu-Pb• Syntectic: L1 + L2 = α (s); e.g., Na-Zn• Metatectic: β (s) + α (s) = L1 e.g., U-Mn

THANKS