undefined | unit 3 phase diagram and iron carbon diagram

EMM

UNIT 3

Phase Diagram and Iron Carbon Diagram

Q1) Describe solid solutions and its types?

A1) Solid solutions:-

When homogeneous mixtures of two or more kinds of atoms (of metal) occur in the solid state, they are known as the solid solutions.

the more abundant atomic form is referred as solvent and less abundant atomic form is referred as solute.

For example:- In sterling silver (92.5% of silver of remaining is copper) is a solid solution of silver and copper where silver is solvent and copper is solute.

Types of solid solutions:-

Solid solutions are generally of two types

1) Substitutional solid solutions.

2) Interstitial solid solutions

Substitutional solid solution:- As suggested by the name substitutional means 'replacement’. here in this case when the atom of the parent metal or we can say atom of the solvent replaced by the atom of the solute metal then the solid solution is known as substitutional solid solution.

Interstitial solid solutions:-

In interstitial solid solutions the solute atom does not displace a solvent atom but rather it enters one of the holes or interstices between the solvent atoms.

1) For interstitial solid solutions solute and solvent should have similar electronegativity.

2) Intermetallic compounds are generally formed when one metal is stronger metallic and other is weakly metallic. Example:-

Q2) What is Hume Rothery rule?

A2) Hume Rothery rules defines us the various conditions under which an element court dissolve in a metal forming a solid solution.

These are as follows:

a) Crystal structure factor

b) Relative size factor

c) Chemical affinity sector

d) Relative valence factor

Crystal structure factor:- The crystal structure of solute and solvent must be similar. Example:- either crystal structure should be FCC, or BCC or HCP.

Relative size factor:- The atomic radius of the solute and solvent atoms must differ by not more than 15%. If it’s beyond 15% then it restrict solid solubility.

Chemical affinity factor:- When two metals have lesser chemical affinity then it forms solid solution. If there will be greater chemical affinity then it can cause compound formation. In others we can see that the solute and solvent should have similar electronegativity.

Relative valence factor:- The complete solubility occurs when solute and solvent must have same valency. A metal with lower valency is more likely to dissolve in metal of higher valency.

Q3) Explain nucleation and growth rate?

A3) Nucleation and crystal growth:-

At the solidification temperature, atoms from the liquid such as molten metal begin to band together and starts to form crystal.

the moment a crystal begins to grow is known as nucleus and the point when it occurs is the nucleation point.

When a metal begins to solidify, multiple crystals begin to grow in liquid.

Final size of the individual crystal depends on the number of nucleation points.

The crystals increase in size by the progressive edition of atoms and grow until they impinge upon adjacent growing crystal.

So, by above diagram we understand that gradually after increase of time process of nucleation and growth new tu nuclei are forming and already existing particles are also growing. So overall transformation rate will depend upon nucleation rate and growth rate.

Nucleation rate:- Number of Nutrition events per unit volume per second.

Denoted by 'I'

Unit –

Growth rate:- Rate of increase of the size of the particle.

Denoted by 'G'.

where R is particle size

Unit –

Q4) Differentiate in between solidification of metals and alloys with diagram?

A4) Solidification of metals and alloys:-

Total time taken from point A to point C is total solidification time.

During solidification the liquid changes into solid during cooling.

The energy of liquid is less than that of solid above the melting point. Hence liquid is table above the melting point.

Below the melting point the energy of liquid becomes more than that of solid.

Hence below the melting point liquid gets converted into solid during cooling.

So this transformation of phase from liquid to solid is known as solidification.

This transformation occurs by nucleation and growth.

7. So the basic difference in between solidification of alloys and pure metals is the temperature difference as shown in above graphs, whereas pure metals solidify at constant temperature.

Q5) Explain phase diagram and its types?

A5) Phase diagram and types of phase diagram:-

Phase diagrams are used for quantitative description of the phase transformation and changes

Here, the constitution point are Cu – Ni on the x axis which is expressed in terms of weight percentage of nickel whereas other thermodynamics variable is temperature on y axis.

As we have seen Cu-Ni phase diagram is very simple phase diagram. As you can see in the diagram there are two boundaries, the upper boundary is liquid boundary. At above the liquidus boundary there is liquid phase which is in equilibrium. And the lower boundary is solidus boundary and below this solid solution phase is stable, whitch is represented by . And in between this there is (two phase region).

This phase diagram is in composition temperature space. So we mark any point in diagram, let us suppose 'A'. The X co-ordinate of this point is a composition. In this case if we want to find out the composition, so as shown in diagram it’s 50 weight percent of nickel is X co-ordinate and Y co-ordinate is 1200°C.

So this particular point represents an alloy A office Steve 8% of nickel held at 1200°C in equilibrium.

So such points are called constitution point. That means any point on the phase diagram can be constitution point.

Types of phase diagram

There are four main types of phase diagram

1) Isomorphous system

2) Eutectic system

3) Partial eutectic system

4) Layer system

A) Isomorphous system:-

Two elements have complete solubility in solid as well as liquid.

Example:- Cu-Ni, Au-Ag, Au-Cu

Lever rate ( used to determine mass fraction of each phase diagram)

Percentage amount of liquid

Percentage amount of solid

Cooling behavior of mixture

(B) Eutectic system:-

Two elements added have complete solubility in liquid and in solubility in solid example :- Pb – As, Bi-Cd

M - melting point of A

N - melting point of B

E - eutectic point

MEN – liquidus

MPEQN – solidus

Cooling behavior of eutectic alloys:-

Cooling behavior of hypereutectic alloys:-

Cooling behavior of hypo eutectic alloys:-

Two elements added will have the complete solubility in liquid and the partial solubility in solid state.

Example:- Ag-Cu, Pb-Sn, Sn-Bi, Pb-Sb

M - melting point of A, Solid solutes of B is A

N- melting point of B, Solid solutes of A is B

MNEQN – solidus

MEN – liquidus

(D) Layer system:-

Two elements added are completely in soluble in liquid as well as in solid state.

Example:- Ag-Fe, Cu-Tu, Cu-Me

Q6) Explain Fe-C equlibrium phase diagram?

A6) Fe-C Equilibrium phase diagram

Eutectic point

Eutectoid point

Peritectic point

Peritectoid point

Q7) Define Gibb’s phase rule?

A7) Gibb’s phase rule:-

P+F =C+2

Where, P is number of phases

F is degree of freedom and

C is number of components

Degree of freedom:- It is the total number of independent or intrinsic or extrinsic variables are required to locate or fix the state of any system is known as degree of freedom.

Example:- suppose we have a system maybe water.

Now we have to find out degree of freedom ?

Ans. Case 1:-

Here, P = 1 (only 1 phase that is liquid)

F = ?

C = 1 (only water)

Case 2:- If there will be ice and water, then

Case 3:- When P = 3 (solid liquid gas)

Q8) Define terms degree of freedom and triple point?

A8) Degree of freedom:- It is the total number of independent or intrinsic or extrinsic variables are required to locate or fix the state of any system is known as degree of freedom.

Triple point:- The temperature and pressure at which substance can exist in equilibrium in the liquid, solid and gaseous state.

Q9) Describe cooling curves for alloys and pure metals?

A9) Cooling curve for pure metals:-

Under equilibrium conditions, all metals exhibit a definite melting or freezing point as shown in above diagram.

Cooling curve of alloys:- In this method alloys with different compositions are melted and then the temperature of the mixture is measured at certain time intervals while cooling back to room temperature.

Q10) Describe nucleation graphically with respect to time?

A10) As you can see the friction transforms can take a maximum value of 1, which means 100% transformation has happened and the minimum value will be zero at beginning.

So, initially since nucleation is about to happen and there is no nucleus so overall transformation rate would be 0.

Then as nucleation started there will be few nuclei as shown in figure then after the rate will pick up. but when it reaches at the end of the transformation it will again slowed down as shown in graph. As when it reaches to 1 the overall transformation rate slowdown.

If we will allow for the growth the particle will start impinge among each other.

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