EMM I UNIT Q&A

7/31/2019 EMM I UNIT Q&A

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UNIT- 1

PART-A

1. Define Peritectic and Eutectoid reactions.

Peritectic reactions:

i. Peritectic reactions, upon cooling, a solid and liquid phase transformsisothermally and reversibly to a solid phase having a different composition.The peritectic reaction can be written as

Eutectoid reactions:

i. Eutectic reaction is the reversible isothermal reaction of thea liquid whichtransforms two different solid phases upon cooling. The Eutectic reaction canbe written as

ii. Eutectic systems are found in many metallic and ceramic.

2. State the conditions under which two metallic elements will exhibit unlimited solid solubility.

i. A phase diagram in which constituent exhibit complete solid solubility is

known as an isomorphous phase.ii. In binary equilibrium condition two components are completely soluble ineach other

iii. In the solid state, and also there is only one solid phase or getting exhibitunlimited solid solubility.

3. Define the terms "ferrite" and "austenite" in iron-carbon alloy system.

i. Ferrite is a primary solid solution based on iron having BCC structure.ii. Its nothing but interstitial solid solution of carbon in iron.iii. Ferrite is soft, ductile and highly magnetic.iv. It can undergo extensive cold work.

4. Define solute and solvent.

i. Solute is the minor part of the solution or materials which is dissolved.ii. Solvent, the material which contributes the major portion of the solution.

5. What are the different types of solid solutions?

i. subtitutional solid solution:


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a. Randomb. Ordered

ii. Intersitial solid solution.

6. What is phase diagram? And its importance.

i. Phase diagrams are the graphical representation of what phase are present ina materials system at various temperature, pressure and composition.

ii. Phase diagrams are also known as Equilibrium diagrams or constitutionaldiagrams.

iii. Phase diagrams show in graphical form, the constitution of alloy as a functionof the temperature under equilibrium condition.

Importance:

i. Its used to identify What condition is the material in? Is composition uniform

throughout?ii. If not, how much of each component is present?iii. If something present in that may give undesired properties?iv. What will happen if temperature is increased or decreased; pressure is

changed; or composition is varied?v. Phase diagram are used by engineering and scientist to understand and to

predict many aspect of the behavior of materials.

7. Define cementite, pearlite.

Cementite:i. cementite is the name given to the carbide of iron ( Fe3C )

ii. Its the hard, brittle, intermetalic compound of iron with 6.69% of carbon.iii. The hardness and brittleness of cast iron is believed to be due to the presence

of the cementite.iv. It is magnetic below 250oC.

Pearlite:i. Pearlite is the eutectoid mixture of ferrite (87.5%) and cementite (12.5%)ii. Its formed when when austentite decompose during cooling. It contains 0.8%

of carbon.iii. It consists of alternates thin layer (or lamellae) of ferrite and cementiteiv. The name derives from its lustrous appearance when viewed in white light

under a microscope.

8. What is meant by hypo eutectoid, hypereutectoid steel?

i. Steels having less than 0.8% C are known as Hypoeutectic steels.ii. Steels having more than 0.8% C are known as Hypereutectoid steels.

9. What is cooling curve?

i. Cooling curves are obtained by plotting the measurement temperature at equalintervals during the cooling period of melt to a solid.


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PART-B1. Discuss the similarities and differences between substitutional and interstitial solid solution?

Two kind of solid solutions formed arei. Substitutional solid solution

a ) Random

b ) Orderedii. interstitial solid solution

1) Substitutional solid solutions:i. When the solute atoms (impurites) substitute for parent solvent atoms

in a crystal lattice, they are called substitutional atoms, and themixture of the two elements is called a substitutional solid solution.

ii. In substitutional solid solution, the atom of the solvent substituteforatoms of the solute in lattice structure of the solvent.

iii. This type of solid solution is quite common among various metalsystem.

iv. A Cu Ni system Shown in figure is an example for a substitutionalsolid solution. These two elements are completely soluble in oneanother at all proportions. This system also satisfies all the HumeRotherys rulen that govern the degree of solubility, as below

a) The atomic radii for copper and nickel are 1.28 & 1.25 Respectively.

b) Both the Cu and Ni Have the FCC crystal structure.c) The most common valencies are +1 for Cu and +2 For Ni;d) The electro negativities of Cu and Ni are 1.9 and 1.8

respectivelye) Thus Cu-Ni system forms an extensive substitutional solid

solution.a) Random or Disorder substitutional solid solution :

In random substitutional solid solution, there is no order in the


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substitution of the two elements; the solute and solvent atomsare ramndomly.

In the formation of a substitutional solid solution, the soluteatoms do not occupy any specific position but are distributed atrandom in lattice structure of the solvent. This alloy is said tobe in a random or disordered condition.

A random substitutional solid solution of copper-zinc system.Here the crystal pattern is not altered.

b) Ordered substitutional solid solution: If the solute and solvent atoms take up some preferred

position, then the solution is called ordered substitutionalsolid solution or super lattice.

Figure shows substitutional solid solutions. Diffusion (that takes place during cooling) tends to produce

uniform distributed of solute and solvent atoms. Thus thesolute atom move into definite orderly posinions in thelattice.

Au-Cu and Cu2MnAl have ordered crystal structure.2) Intersitial solid solution:

i) In interstitial solid solution, the solute atoms fit into the space betweenthe solvent or parent atoms. These spaces or voids are calledinterstices.

ii) Interstitial solid solutions can from only when one atoms is muchlarger than another.

iii) The atoms which have atomic radii less than 1 are likely to form aninterstitial solid solution.Such atoms are hydrogen (0.46 ), carbon

(0.71 ), and oxygen (0.6 ).


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iv) An interstitial solid solution that is formed by carbon in FCC ironjust above 9120 C. in this case, the atomic radius of carbon is 0.75 and that of iron is 1.29 , and so there is an atomic radius difference of42%.

v) Like substitutional solid solutions, Intersitial solid solution also dependon size, valency, and electronegativity factors, But they do not dependon the type of crystal structure.

2. What is cooling curve? How does the time temperature cooling curve of an alloy of eutecticcomposition different from that of a pure metal? That of non-eutectic composition?

Fig: Cooling curveCooling curve:

i. Cooling curves are obtained by plotting the measured temperature at equalintervals during the cooling period of a melt to a solid.

ii. The data obtained from these cooling curves are useful in constructing theequilibrium.


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Cooling curves for pure metal:

i. Figure (a) shows the cooling curve for a pure metal or a compound,

ii. From P to Q, the curve proceeds at a uniform rate and at point Q, the firstcrystals being to form.

iii. As solidification proceeds, the latent heat of fusion is liberated in such amountthat the temperature remains constant from Q to R until whole mass hasentirely solidified. The period QR knows as horizontal thermal arrest.

iv. Further cooling form point R will cause the temperature to drop along thecurve RS. The slopes of PQ and RS curves depend upon the specific heats ofthe liquid and solid metals respectively.

v. In the region PQ, only liquid phase is present while in the region RS on;y solidphase.

vi. Applying Gibbs condensed phase rule, we get

F= C P + 1 = 1 1 + 1 = 1 ( one degree of freedom )

Thus the temperature can be varied independently without altering equilibrium.

3. Explain:


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Iron-carbon phase diagram, showing the eutectoid transformation between austenite () andpearlite.

i). Eutectic reaction:

i. This type of reaction is an invariant reaction, because it is in thermalequilibrium;

ii. Another way to define this is the Gibbs free energy equals zero. Tangibly,this means the liquid and twosolid solutions all coexist at the same time andare in chemical equilibrium.

iii. There is also a thermal arrestfor the duration of the reaction.

iv. The resulting solid macrostructurefrom a eutectic reaction depends on afew factors.

v. The most important factor is how the two solid solutions nucleate and grow.The most common structure is a lamellar structure, but other possiblestructures include rodlike, globular, and acicular.
http://en.wikipedia.org/w/index.php?title=Invariant_reaction&action=edit&redlink=1http://en.wikipedia.org/wiki/Thermal_equilibriumhttp://en.wikipedia.org/wiki/Thermal_equilibriumhttp://en.wikipedia.org/wiki/Gibbs_free_energyhttp://en.wikipedia.org/wiki/Solid_solutionhttp://en.wikipedia.org/wiki/Solid_solutionhttp://en.wikipedia.org/wiki/Chemical_equilibriumhttp://en.wikipedia.org/wiki/Thermal_arresthttp://en.wikipedia.org/wiki/Thermal_arresthttp://en.wikipedia.org/wiki/Macrostructurehttp://en.wikipedia.org/wiki/Macrostructurehttp://en.wikipedia.org/wiki/Lamellar_structurehttp://en.wikipedia.org/wiki/Lamellar_structurehttp://en.wikipedia.org/wiki/Thermal_equilibriumhttp://en.wikipedia.org/wiki/Thermal_equilibriumhttp://en.wikipedia.org/wiki/Gibbs_free_energyhttp://en.wikipedia.org/wiki/Solid_solutionhttp://en.wikipedia.org/wiki/Chemical_equilibriumhttp://en.wikipedia.org/wiki/Thermal_arresthttp://en.wikipedia.org/wiki/Macrostructurehttp://en.wikipedia.org/wiki/Lamellar_structurehttp://en.wikipedia.org/w/index.php?title=Invariant_reaction&action=edit&redlink=1


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Four eutectic structures: A) lamellar B) rod-like C) globular D) acicular.

ii. Eutectoid reactions:

i. The eutectoid reaction describes the phase transformation of one solid into twodifferent solids.

ii. In the Fe-C system, there is a eutectoid point at approximately 0.8wt% C,723C.

iii. The phase just above the eutectoid temperature for plain carbon steels isknown as austenite or gamma.

iv. We now consider what happens as this phase is cooled through the eutectoid

temperature (723C).
http://www-g.eng.cam.ac.uk/mmg/teaching/typd/addenda/definition.htmlhttp://upload.wikimedia.org/wikipedia/commons/a/a3/Various_eutectic_structures.pnghttp://www-g.eng.cam.ac.uk/mmg/teaching/typd/addenda/definition.html


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Eutectoid reactionsiii). Peritectic reaction

i. Peritectic transformations are also similar to eutectic reactions. Here, a liquidand solid phase of fixed proportions react at a fixed temperature to yield asingle solid phase.

ii. Since the solid product forms at the interface between the two reactants, it canform a diffusion barrier and generally causes such reactions to proceed muchmore slowly than eutectic or eutectoid transformations.

iii. Because of this, when a peritectic composition solidifies it does not show the

lamellar structure that is found with eutectic solidification.iv. Such a transformation exists in the iron-carbon system, as seen near the upper-left corner of the figure.

v. It resembles an inverted eutectic, with the phase combining with the liquid toproduce pure austenite at 1,495 C (2,723 F) and 0.17% carbon.

iv). Peritectoid reaction (4)i. An isothermal reversible reaction in which a solid phase on cooling reacts with

another solid phase to form a third solid phase.ii. A peritectoid transformation is a type ofisothermal reversible reaction that

have two solid phases reacting with each other upon cooling of a binary,ternary, ... , alloy to create a completely different and single solid phase.[11]The reaction plays a key role in the orderand decomposition ofquasicrystalline phases in several alloy types
http://en.wikipedia.org/wiki/Isothermalhttp://en.wikipedia.org/wiki/Reversible_reactionhttp://en.wikipedia.org/wiki/Phase_(matter)http://en.wikipedia.org/wiki/Alloyhttp://en.wikipedia.org/wiki/Eutectic_system#cite_note-Gold_Book_PAC.2C1994.2C66.2C588-10http://en.wikipedia.org/wiki/Decompositionhttp://en.wikipedia.org/wiki/Quasicrystallinehttp://en.wikipedia.org/wiki/Isothermalhttp://en.wikipedia.org/wiki/Reversible_reactionhttp://en.wikipedia.org/wiki/Phase_(matter)http://en.wikipedia.org/wiki/Alloyhttp://en.wikipedia.org/wiki/Eutectic_system#cite_note-Gold_Book_PAC.2C1994.2C66.2C588-10http://en.wikipedia.org/wiki/Decompositionhttp://en.wikipedia.org/wiki/Quasicrystalline


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4. Draw Iron -Carbide equilibrium diagram and mark on it all salient temperature andcomposition fields

Iron carbon equilibrium is very much useful in understanding the micro structureand properties of cast iron and carbon steel.

It is also to understand the basic difference among iron alloy and the control of theirproperties.

This phase diagram is constructed by plotting the carbon composition (Weight percent) along the X-axis and temperature along Y- axis.

The iron carbon (Fe3C)Phase diagram is shown in figure.

This phase diagram presents the phase present at various temperatures for very slowlycooled Iron-carbon alloys with upto 6.67% carbon.

The pure iron exists in three allotropy forms i.e. iron iron, and iron before itmelts. At room temperature the stable form, called ferrite (or iron). Exists with aBCC crystal structure. Upon heating, ferrite transforms to FCC austenite (or iron).At this temperature the FCC austenite (or iron) at 912C (1674 F). This austenitecontinues till 1394C (2541F); at this temperature the FCC austenite transforms backto a BCC phase knows as ferrite. Then finally the iron melts at 1538 C (2800F). All


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these changes are seen in figure along the vertical axis of the phase diagram. Carbon is an interstitial impurity in iron and forms a solid solutions with each of

ferrite and also with austenite, as indicated by the , and single phase field asshown in figure.

The important information that can be obtained from the Fe3C can be studied underthe following topics:

i. Solid phase in the diagram.ii. Invariant reaction in the phase diagramiii. Eutectoid, hypoeutectoid and hyperectectoid steel.iv. Eutectic, hypoeutectic and hypereutectic cast iron.

i. Solid phase in the diagram:

The Fe3C diagrams contains solid phases.i. Ferriteii. Ferriteiii. Auteniteiv. Cementide (Fe3C)

i) Ferrite: This phase is an interstitial solid solution of carbon in the BCC

iron crystal lattice. The solid solubility of carbon in ferrite is a maximum of

0.02% at 723 and decrease to 0.05% at 0 C.ii) Austenite:

The interstitial solid solution of carbon in iron is calledaustenite.

Austenite has FCC crystal structure and a much higher solidsolubility for carbon then ferrite.

As indicated by figure the solid solubility of carbon in austenite

is a maximum of 2.08% at 1148 C and decreases to 0.8% at723C.iii) Cementide:

cementite is the name given to the carbide of iron ( Fe3C ) Its the hard, brittle, intermetalic compound of iron with

6.69% of carbon. The hardness and brittleness of cast iron is believed to be due

to the presence of the cementite. It is magnetic below 250oC.

iv) Ferrite: Intersitialsolid solution of carbon in iron is called Ferrite. Ferritehas a BCC crystal structure.

As shown in figure the maximum solid solubility of carbon in Ferrite is 0.09% at 1465o C.

ii. In variant reactions in the phase diagram:

The three important invariant reaction associated with the Fe3C diagram are peritectic, eutectic and eutectoid reactions.

1. Peritectic reaction. At this peritectic reaction point shown in figure, liquid


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of 0.53%C combines with ferrite of 0.09%C to form austenite of 0.17%.

This pertectic reaction, which occurs at 14950 C, can bewritten as

The perictic reaction affects only solidification of steelswith less than 0.55%C.

2. Eutectic reaction. At the eutectic reaction point (shown in figure) liquid

of 4.3% forms austenite of 2.08% C and theintermetallic compound Fe3 C (cementide), whichcontains 6.67%C.

This eutectic reaction, which occurs at 11480C, can be

written as

This reaction is pf great importance in cast iron.3. Eutectic reaction.

At the eutectic reaction point shown in figure, solidaustenite of 0.8%C produces ferrite with 0.02%Cand Fe3 C that contains 6.677%C.

This eutectic reaction, which occurs at 7230C, can bewritten as

This reaction gains much importance for the heattreatments of steels.

iii. Solidifications of iron-Carbon alloys: As we know, there are three types of ferrous alloy based on carbon content. They are:

1. Iron2. Steel3. Cast iron

Composition upto 0.008% carbon are regarded as commercially pure iron,those from 0.008% to 2% carbon present steel, and those above 2%carbone represent cast iron.

Steels are further subdivided into.Eutectoid steels:

Steels that contain 0.8% C are called eutectoid steels.


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Hypoeutectoid steel:Steels having less than 0.8% C are known as Hypo eutectoid steels.

Hyper eutectoid steels:Steels having more than 0.8%C are known as Hyper eutectoid steels.

5. (a) Elements A & B melt at 7000 C and 10000 C respectively. Draw a typical isomorphousphase diagram between the elements A & B.

(Two metals completely soluble in the liquid and solid states; Phase diagrams forbinary isomorphous alloy systems)

A mixture of teo metals is called a binary alloy. In some binary alloy system, the two elements are completely soluble in each other in

both the liquid and solid states. In these system only a single type of crystal structureexists for all composition of the components, and therefore they are called isomorphoussystems.

The equilibrium phase diagram for the isomorphous system is shown in figure.

The common examples of isomorphous systems are: Copper Nickel (Cu-Ni) System Antimony Bismuth (Sb-Bi) System Gold Silver (Au-Ag) System Cromium Molybdenum (Cr-Mo) System Tungsten Molybdenum (W Mo) System Copper Gold (Cu-Au) System

The phase diagram is divided into three separate areas by two phase


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boundaries namely the liquid and solidus. Above the liquid there is a uniform liquid solution, while below the solid, there

is a single solid solution. Between the liquidus and solidus, there is a single solidsolution. Between the liquidus and solidus, both liquid and solid solutioncoexists.

Unlike pure metals, alloy freez over a range of temperature and that the regionbetween the liquidus and solidus curves represents the temperature intervalduring which the alloys are in a past condition.

(b) Elements A & B melt at 7000 C & 10000 C respectively. They form a eutectic at 35%A atTemperature 5000 C. Draw a typical phase diagram between A & B.

( Phase Diagram for binary eutectic alloy system) As we know, many materials exhibits partial solid solubility. Each element is soluble in the other element up to a certain limit is a function of

temperature. If the two metals A and B are not completely soluble through all range in

composition, then second phase will form at grain boundaries. The equilibrium phase diagram for an alloy of the two elements A and B that

exhibits partial solubility is shown in figure. The common examples of this system include:

Copper Silver system Lead Tin system. Aluminium- copper system.

First of all, six phase region/field are found on the diagram:


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Liquid phase Solid solution phase solid solution phase solid + liquid phase solid + liquid solution

+ solid solution phase. The phase is a solid solution in which element of metal A is more than that ofmetal B. The phase is a solid solution in which element of metal B is more thanthat of metal . technically, pure metal A and B are considered to be and phaserespectively.

The figure AEF is known as liquidus line. Liquidus line is the line or boundary thatseparates liquid and liquid + solid phase region.

In figure, the line ABCGF is known as solidus line . solidus line is the line orboundary that line or boundary that separate solid an solid + liquid phasediagram.

In figure the line BC and GH are known as solves lines separate single- phase solidregion from two phase solid region from two-phase maximum solubility limits of

metal A in B and of metal B in A respectively. The introduction of metal B decreases the melting temperature of metal A along

the liquidus line AE. Similarly the Addition of metal. A decreases the meltingtemperature of metal B along the liquidus line FE. These two liquidus lines melt atthe point E on the phase diagram. This point E is known as eutectic point.

The corresponding temperature (TE) and composition (CE) in the phase diagramare known as the eutectic temperature ant eutectic composition respectively.

In other words, eutectic temperature is the minimum temperature at which abinary system is fully melted.

It can also be noted that the horizontal isothermal line BEG also passes throughthe point E. This horizontal solidus line at TE is called the eutectic isotherm.

When liquidus of eutectic composition is slowly cooled to the eutectic temperature,

the single liquid phase transforms simultaneous into two solid forms. Thistransformation is known as the eutectic reaction.

The eutectic reaction can be written as.

The eutectic reaction is also called an invariant reaction since it occurs underequilibrium conditions at a specific temperature and alloy composition that cannotbe varied.

During the progress of the eutectic reaction. The liquid phase is in equilibriumwith the two solid solution and . thus during a eutectic reaction, three phasecoexist and are in equilibrium.

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EMM I UNIT Q&A