In this article we will discuss about:- 1. Meaning of Phase Diagram 2. Type of Phase Diagram 3. Various Type of Phase Diagram Reaction 4. Congruent Phase Transformations 5. Influence of Alloying Elements.
Meaning of Phase Diagram:
A phase diagram is also called an equilibrium or constitutional diagram. It shows the relationship between temperature, the compositions and the quantities of phases present in an alloy system under equilibrium conditions.
When temperature is altered many microstructure develop due to phase transformation. It may involve transition from one phase to another phase. Thus, these diagram are helpful in predicting phase transformation and the resulting microstructures.
Types of Phase Diagram:
1. Unary Phase Diagram (Single Component):
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It is used mainly for carbon and pure metal.
There is very limited practical utilities of such diagram plotted between temperature and pressure axis.
Example:
Water, graphite, metallic carbon, diamond.
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2. Binary Phase Diagram (Two Components):
These are of following type:
Type 1 (Binary Isomorphous Systems):
The material which are completely soluble in liquid as well as solid state:
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Since, the two metals are completely soluble in solid state, the only type of solid phase formed will be a substitution solid state. The two metals will generally have the same type of crystal structure and differ in atomic radii by less than 8%.
The corresponding such diagram shown below:
This phase diagram consists of two points, two lines and three areas. The two points of the two pure metals A & B. The upper line, obtained by connecting the points showing the beginning of solidification is called liquidius line, and the lower line, determined by connecting the points showing the end of solidification is called the solidus line.
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Area above the liquidus line is single phase region and any alloy in that region will consist of homogeneous liquid solution. Similarly, lower region is homogeneous in nature. In between the solidus and liquidus line there exist a two phase region. It is the mixture of both solid and liquid, it is mushy in type.
There are two rules to find the actual composition and relative amounts of the two phases that are present:
Rule 1 – Chemical Composition of Phases:
To determine the actual chemical composition of the phase of an alloy, in equilibrium at any specified temperature in a two-phase region, draw a horizontal temperature line (called a tie line) to the boundaries of the field.
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These points of intersection are dropped to the base line, and the composition is read directly, e.g., in the below figure ‘mo’ tie line to the boundaries of the field.
Point ‘m’, the intersection of the tie line with the solidus line, when dropped to the base line, gives the composition of the phase that exists at that boundary.
In this case, the phase is a solid solution a of composition 90A-10B. Similarly, point 0, when dropped to the base line, will give the composition of the other phase constituting the mixture, in this case liquid solution of composition 74A-26B.
Rule 2 – Relative Amount of Each Phase:
To determine the relative amount of the two phase in equilibrium at any specified temperature in a two phase region, draw a vertical line representing the alloy and a horizontal temperature line to the boundaries of the field. The vertical line will divide the horizontal line into two parts whose lengths are inversely proportional to the amount of phase present. This is known as lever rule.
The point where the vertical line intersects the horizontal line may be considered as the fulcrum of a lever system. The relative length of the lever arms multiplied by the amounts of the phase present must balance. In the previous figure, the vertical line representing the alloy 20B, divides the horizontal tie line into two parts ‘mn’ and ‘no’.
If the entire length of the tie line mo is taken to represent 100% or the total weight of the two phases present at temperature T, the lever rule may be expressed mathematically as:
For such phase diagram, there is one degree of freedom.
So, in this case Gibb’s rule converts into
Basically, when the material transform from one phase to another phase two type of process occur:
Note:
When process is carried out below the recrystallization temperature (0.3Tm to 0.5Tm) these are known as cold working and above of it is termed as hot working. Here Tm = Melting temperature.
The above figure shows the following:
i. This phase is a substitutional solid solution of Cu and Ni atoms and having FCC structures.
ii. At temperature below 1085°C Cu and Ni are mutually soluble in each other in solid state for all composition as both have same FCC crystal structure and nearly identical atomic radii and electronegativity.
iii. Liquid (L)- It is a homogenous liquid solution composed of both Cu and Ni.
iv. (α + L)- Phase between liquidus line and solidus line. This is a mushy zone where both Cu and Ni are present in varying compositions.
v. Solidus and liquidus lines intersect at two extremities (composition) point of intersection represents the melting point of pure metal (Cu and Ni).
vi. Melting or freezing occurs over a range of temperatures between solids and liquidus lines.
Phase diagrams give three kinds of informations:
(i) Phases present
(ii) Composition of phases present
(iii) Percentages of the phase
Binary Phase Diagram of Type-II:
i. The material which are completely soluble in liquid state but partially soluble in solid state (eutectic phase diagram).
Binary Eutectic system consists of three phase:
(i) α
(ii) β {Solid solutions}
(iii) Liquid phase (L)
ii. Here a eutectic point is defined, where degree of freedoms are zero.
Example 1:
Cu-Ag eutectic Phase Diagram:
i. Eutectic means in latin word Eu means: nice & tectic means: melting.
Here number of phase equal to 3.
ii. As per Gibb’s law F + P = C + 1
iii. At eutectic point
F = 0, So by Gibb’s law O + P = 2 + 1; P = 3.
So three phase α, β, L will be there at eutectic point.
α Phase:
It is rich in copper and Ag is present as solute and has FCC structure.
β Phase:
It is rich in Ag and Cu is present as solute and again has FCC structure.
(α + β) phase constitutes of pure copper and pure silver.
iv. Below line CEG (779°C), there is only partial solubility of Ag in Cu (α phase) and Cu in Ag (β-phase). Maximum solubility of Ag in Cu occurs at 779°C and is 8%. Likewise, maximum solubility of Cu is 8.8% in Ag again at 779°C.
v. Solves line demarcates between α and (α + β) phases and β and (α + β) phases.
Salient Features:
i. In liquid regions as silver is added to copper, we see that on the liquids line AE, the melting point of copper decreases with silver addition. Same is true for copper addition to silver.
ii. Point E on the phase diagram it is called invariant point at this point an important reaction takes place at constant temperature Called Eutectic reaction where liquid (L) is directly converted to (α) and β phase.
iii. Line CEG is called eutectic isotherm. It is also seen that single phase region (α or β) are separated from each other by a two phase region. Invariant point is also termed as eutectic point.
iv. Invariant reaction: The eutectic reaction, in which a liquid transforms into two solid phases, is just one of the possible three-phase invariant reaction that can occur in binary system those are not isomorphus.
Example 2:
Sn-Pb Eutectic System:
i. For a 40% Sn – 60% Cu alloy at 150°C. Calculate composition of phase present.
ii. Amount of each phase present in terms of main fractions and volume fractions. Given density of Pb and Sn are 11.23 kg/cm2 and 7.24 g/cm3.
Binary Phase Diagram of Type-III:
The material which are completely soluble in liquid state and completely insoluble in solid state.
Example:
Alloy of Bi.
Various Type of Phase Diagram Reaction:
i. Eutectoid Reaction:
a. Here a solid phase (α) directly transforms to two other solid phase (β) and (γ). Eutectoid means eutectic like.
b. It is also an invariant reaction. It is denoted by point E.
ii. Peritectic Reaction:
a. It involved three phases at equilibrium. In this reaction upon cooling a solid and a liquid, phase transform isothermally to a solid phase (β).
b. The new solid formed is usually an intermediate phase but in some case it is terminal solid solution.
c. Peritectic reaction occurs in alloys having large difference in melting point.
iii. Peritectoid Reaction:
a. It involves reaction between two solid and final product will be solid.
b. The peritectoid reaction has the same relationship to the peritectic reaction as the eutectoid has to the eutectic. The new product is intermediate alloy.
Note:
Eutectic temperature is lower to melting point of all other composition. Eutectic means nice melting.
Summary of invariant reaction in binary system:
iv. Monotectic Reaction:
v. Syntetic Reaction:
In this two liquid phases react to form a solid phase.
Congruent Phase Transformations:
These are phase transformation for which there are no alteration in composition of phases. e.g. Allotropic transformations and melting of pure metals.
Incongruent transformations, at least one of the phase will experience change in composition. e.g. Eutectic and Peritectic reactions.
These are carried out isothermally.
Relationship between the Phase Diagram and Properties of Material:
i. Fluidity is maximum for pure material at eutectied. It is directly proportional to the area of mushy zone. Larger the area of mushy zone. Lower will be the fluidity.
ii. Corrosion resistance is poor for eutectic composition. It increase both side of eutectic point.
iii. Generally density is higher for low melting point materials.
iv. Hardness of pure material is very poor and maximum strength will be at the point of maximum solid solubility.
Influence of Alloying Elements on Phase Diagram:
Alloys elements addition always reduces eutectoid composition i.e.% of C while they may increase or decrease the eutectoid temperature. (Ni) specially is an austenite stabilizer.
Effect of Alloying Element on Fe:
1. Carbon:
By increase in percentage of carbon in iron, ductility decreases and brittleness increases and when carbon % increase from more than 2.1 % in iron, iron convert into cast iron.
2. Sulfur:
Sulfur in commercial steel is generally kept below 0.05%. Sulfur formed iron sulfide with iron, Iron sulfide forms a low melting point eutectic alloy with iron which tends to concentrate at the grain boundaries.
When the steel is forged or rolled at elevated temperatures, the steel becomes brittle or hot-short due to the melting of the iron sulfide eutectic which destroys the Cohesion between the grains allowing cracks to develop, and material got failed due to brittle fracture.
To overcome from this problem Manganese is added with which Sulfur formed manganese sulfide rather than FeS. Manganese sulfide improve the machinability as due to presence of more number of sulfide inclusions which breakup the chips, thus reducing tool wear. It is noted that Mn has low shear strength.
3. Manganese:
It promotes the soundness of steel casting through its deoxidizing action on liquid steel.
Large quantity of manganese in steel formed MnS as well as Mn3C which is formed associated with cementite. Hadfield steel have 12% Mn and to make Bull-dozer rolls.
4. Phosphorus:
Phosphorus generally kept below 0.4%. This small quantity tends to dissolve in ferrite, increasing the strength and hardness slightly. Large quantity of phosphorus tend to crack the steel when cold worked making steel cold-short.
5. Silicon:
Its amount generally varies in between 0.05% to 0.3%. It dissolves in ferrite, increasing the strength of steel without gradually decreasing the ductility.
By addition of Silicon, eutectic point shift towards left and graphite formation takes place at much lower percentage of Carbon. If act as deoxidizer.