htm question bank final 2013

T.Manikandan, Asst. Professor, Aeronautical Engg, Jeppiaar Engg College, Chennai-119

JEPPIAAR ENGINEERING COLLEGE

DEPARTMENT OF AERONAUTICAL ENGINEERING

AE 2354- HIGH TEMPERATURE MATERIALS

UNIVERSITY QUESTION BANK


JEPPIAAR ENGINEERING COLLEGE

DEPARTMENT OF AERONAUTICAL ENGINEERING

AE 2354- HIGH TEMPERATURE MATERIALS

UNIVERSITY QUESTION BANK

UNIT-1: CREEP

2 Marks

1. Define creep

Creep is large plastic deformation of material on application of stress or elevated

temperature or a combination of both applied for a prolonged time. A material

subjected to a constant stress may fail well below its yield stress if load is applied for

prolonged time.

2. Define creep formation

It involves time-dependent deformation and high temperature creep cracking

generally develops in an intercrystalline manner in components of engineering

importance that fail over an extended time.

3. What is meant by diffusion process?

At low stress and high temperature atoms diffuse from sides to the top and bottom.

The grain becomes longer as the applied stress does work. Atomic diffusion in one direction

is same as vacancy diffusion in opposite direction.

4. Why the rate of creep is more at elevated temperature?

Creep is one of the most serious high temperature damage mechanisms. As temperature

increases mobility of atom increases, frequently increasing the diffusion mechanism and

has significantly influence on high temperature mechanical properties like creep and

fracture. So the rate of creep is more at elevated temperature.

5. Define slip and climb.

6. What is edge dislocation?


7. What are the problems associated with materials used at elevated temperature?

Generally a material subjected to high temperature reduces the strength of the material

and failure mechanisms affecting the functional life of components are totally

different. At high temperature the strength of material has strong dependence on time.

8. What are the factors influencing functional life of components at elevated temp?

1) Creep

2) Corrosion

3) High temperature fracture

4) Thermo mechanical Fatigue

5) Interaction of all above with each other

6) Metallurgical ageing and metallurgical stability

7) Micro structural changes

9. Which types of materials are preferred for creep application?

A material which posses the following features to avoid creep formation and get creep

resistant at high temperatures.

1. Higher creep resistance is observed with metals having high melting point. Creep

becomes significant above 0.4 Tm .

2. A coarse grain metal has high creep resistance than fine grained metals.

3. Single crystal have excellent creep resistance because they have no grain boundary.

10. What are the deformation modes?

1. Dislocation climb and glide

2. Vancancy diffusion

3. Dislocation Creep

4. Diffusion Creep

5. Grain Boundary Sliding


11. Draw a typical creep curve and mark the various stages.

12. Define super plasticity.

Super plasticity is the ability to withstand very large deformation in without necking.

13. What is sub grain formation?

14. Derive a relation between ‘strength’ and ‘dislocation density’.

15. Write a relation between ‘creep strain’ and ‘time’ for various stages of creep.

(Refer notes )

16. Draw typical creep curve, mark the various stages and mention the factors

influencing each stage. Refer Q.no .10

1. Primary Or Transient Creep – Strain or work hardening

2. Secondary Or Steady State Or Recovery Creep- The rmal Softening or Annealing

3. Tertiary Creep – Void Propagation and Grain boundary sliding


17.What are effects of stress and temperature on the creep curve?

18. Write any two high creep resistance materials.

1. Tungsten

2. Nickel based super alloys

3. Iron based super alloys

4. Cobalt based super alloys, 5. Molybdenum

19. Name two metallurgical factors that affect creep rate.

1. Work hardening

2. Thermal Softening

3. Grain boundary sliding

4. Diffusion and cavitation

16 Marks

1. (i) What are the structural changes involved during creep? Explain in detail.

(NOV/DEC 2008)

(ii)Briefly explain the three steps of creep with a creep curve.

2. Discuss the mechanism of creep deformation briefly with necessary sketches.

(NOV/DEC 2008)


3. It is well known that at high temperature materials are subjected to creep corrosion

and micro structural changes. Analyse each of the things in limiting functional life on

components.

4. Write technical notes on various creep mechanisms.

5. It is well known that the functional life time of material put into service at high

temperature are subjected to three dimensional damages by way of creep, corrosion and

micro structural changes. For each type write on the various damage mechanisms and

the method to combat them. (MAY/JUNE 2007)

6. It is well known that to predict long term properties accelerated short term tests are

to be carried out and extrapolating procedure by parametric approach is to be followed.

Analyse how various parameters are evolved based on the material behaviour and how

to choose the best parameters for Master curve. (MAY/JUNE 2007)

7. Write the constitutive equations for plastic and creep deformations from high stress

and low temperature to low stress and high temperature and comment on the

mechanism of deformation for each equation. (APR/MAY 2008)

8. Explain in detail about the factors influencing functional life of components at

elevated temperatures. (MAY/JUNE 2012)

9. List out the metallurgical factors influencing various stages of creep using a suitable

diagram, explain these stages. (MAY/JUNE 2012)


UNIT-2: DESIGN FOR CREEP RESISTANCE

2 Marks

1. What are Material aspects for creep resistance?

v Creep resistance improved if diffusion rates are decrease.

v An increased modulus improves resistance to dislocation creep

v When creep is controlled by dislocation climb and glide process, similarly material

having high shear modulus display a better creep resistance.

v Stress raises such as sharp corners are avoided so that the stress concentration

diminish or vanish considerably and materials with finer grain size are utilized.

v Surface irregularities and cracks are removed by polishing surface.

v Compressive stresses are introduced at the surface by process such as short sand

blasting.

v Nitriding and carburising operations are performed to create strong surface layers.

2. What is the role of Larsen- Miller parameter in predicting time to rupture?

(Refer notes )

3. Why is transient creep time crucial in designing materials for high temperature

applications?

4. Define rupture life of creep?

Rupture life of creep is defined as the maximum stress that will allow the member to

withstand creep without failure at the specified period of time. In creep testing the

main goal is to determine the minimum creep rate in stage II. Once a designer knows

the materials will creep and has accounted for this deformation a primary goal is to

avoid failure of the component.


5. Distinguish between Ductile material and brittle material.

Ductile material brittle material

Ductile materials undergo extend plastic

deformation and absorb significant energy

before fracture.

Brittle material undergoes very low plastic

deformation and low energy absorption prior to

breaking.

A crack, formed as a result of the ductile

fracture, propagates slowly and when the stress

is increased.

A crack, formed as a result of the brittle

fracture, propagates fast and without increase

of the stress applied to the material.

6. Define Monkmann-Grant formula.

Monkman grant relationship predicts time of failure due to creep mechanisms.

Monk man grant relationship relates minimum strain rate and time to failure. Monk man

grant relation is given as

Where C is a constant

Minimum strain rate for a given stress and temperature is given by t he relation

7. Define Transient creep time.

8. What is meant by strain hardening?

Work hardening or strain hardening phenomenon can also be explained from the

intersection of various dislocations moving through the active slip planes. The

resistance of the metal to deformation is increased to work or strain the metal in both

the cases. This implies an increase in strength and hardness.


9. What are the types of Hardening (Strengthening) mechanisms?

1. Strain Hardening

2. Hardening by grain refinement

3. Solid solution strengthening

4. Precipitation hardening

10. Define Time hardening and Strain Hardening for step loading for the design or

primary creep.

(Refer notes )

11. Explain the term Creep Ductile Fracture and Brittle Fracture.

Ductile fracture:

Ductile fracture is characterized by extensive plastic deformation and absorbs

significant energy before fracture. A crack, formed as a result of the ductile fracture,

propagates slowly and when the stress is increased.

Brittle fracture:

Brittle fracture is characterized by very low plastic deformation and low energy

absorption prior to breaking. A crack, formed as a result of the brittle fracture, propagates fast

and without increase of the stress applied to the material.

12. Define time hardening and strain hardening in transient creep.

(Refer notes )

14. Define Monkman-Grant relationship and its significance as a Master curve.

Minimum strain rate is defined as the strain rate that is prescribed by the designer for

creep resistance. It is specified by the designer for safe operation of the component without

going to rupture stage. Once the minimum strain rate for creep design is specified, Monk man

grant relationship relates minimum strain rate and time to failure.

Monk man grant relation is given as


16 Marks:

1. Analyse time hardening and strain hardening mathematically and pictorially and

prove that for gradual loading strain hardening reduces strain hardening.

(MAY/JUNE 2007)

2. It is Obvious that as the material creeps it proceeds from primary stage into a steady

state and accelerates into the tertiary stage with a end result of fracture.

(i) For creep ductile failure or rupture the area of cross section that supports load

reduces to zero. Analyse how to compute time to rupture based on initial conditions.

(MAY/JUNE 2007)

(ii) For creep brittle fracture void nucleate and grow form the onset of steady state and

in tertiary state voids coalesce into cracks which propagates to produce fracture. This

can be modelled by defining a damage parameter w which changes from zero at virgin

state to unity at rupture .Based on this model compute time for brittle fracture.

3. Analyse hidden variables or internal variables on the primary and steady state creep

deformation. (APR/MAY 2008)

4. Derive expression for Creep ductile and creep brittle fractures .Also comment on the

instantaneous fractures in both cases. (APR/MAY 2008)

5. Analyse time hardening strain hardening pictorially and mathematically and prove

that at constant stress the strain hardening reduces to time hardening.

6. Based on steady state creep rate derive expressions for creep ductile and brittle

fracture.

7. (I) How will you obtain creep resistance through strain hardening? (NOV/DEC 2008)

(ii)Explain stress rupture test at elevated temperature.

9. What are the approaches there for design under conditions of creep-fatigue

interaction? Explain aii with neat sketches.

11. Derive the various methods adopted in representing rupture life of creep.

(MAY/JUNE 2007)


12. (i)Briefly explain the influence of brittle and ductile materials on creep.

(ii) Discuss the conditions favourable for creep cavitation. (MAY/JUNE 2012)

UNIT-3 - FRACTURE

2 Marks.

1. Define Fracture?

Fracture is a process of breaking a solid into number of pieces as a result of stress.

Fracture denotes the complete destruction of the material, resulting separation of a portion

of the material body.

2. What are types of fracture?

Generally fracture can be divided into 2 types such

1. Brittle fracture (eg. Cast iron)

2. Ductile fracture (eg. Mild steel)

Further it can e classified,

Depends on the appearance as 1.shearing fracture and 2.cleavage fracture and

crystallographic nature as1. fibrous and 2.granular fracture

3. What is ductile fracture, Brittle fracture?

Brittle fracture : The word ‘brittle’ is associated with a minimum of plastic

deformation, i.e. with a brittle fracture the material fractures with very rapid propagation of

crack with very little or no plastic deformations like a china cup.

http://www.substech.com/dokuwiki/doku.php?id=tensile_test_and_stress-strain_diagram


Ductile fracture: This signifies large plastic deformation, and occurs after extensive

plastic deformation prior to and during the propagation of the crack. This requires

considerable energy which is absorbed in forming dislocations and other imperfections

(defects) in metals.

3. Explain the terms Chisel Edge fracture and Point Edge fracture.

Chisel Edge fracture : An incomplete fracture of the head of the radius, in which the

fracture line extends distally from the centre of the articular surface

4. What is meant by Equicohesive Temperature (ECT)?

It is the temperature at which grain boundary strength and grain strength are equal

Points to be noted about ECT

1) It is not a fixed value

2) It depends on stress, temperature and strain rate.

Decreasing the strain rate lowers the ECT and encourages inter granular fracture.

5. Distinguish between ductile fracture and brittle fracture?

Ductile fracture Brittle fracture

Materials fractures after plastic deformation and

slow propagation of crack.

Materials fractures with very little or no plastic

deformation, e.g. in a china clay, glass etc.

Fractured surfaces are dull or fibrous in

appearance.

Fractured surfaces are crystalline in appearance

Percentage elongation is about 30% prior to

fracture occurs.

Percentage elongation is about 0.5% or almost nil

prior to fracture occurs.

There is reduction in cross-sectional area of the

specimen.

There is virtually no change in the cross-sectional

area.

Fracture takes place after necking with little

sound.

Fracture occurs rapidly often accompanied by a

Loud noise.


6. Define cleavage fracture.

In brittle crystalline materials, fracture can occur by cleavage as the result of tensile stress

acting normal to crystallographic planes with low bonding (cleavage planes). After the

formation of micro crack described above, if the crack propagates along a weak

crystallographic plane it is known as cleavage fracture.

7. What are high temperature fracture modes?

Refer Intergranular fracture

8. Define Rupture?

Rupture is defined as the total destruction of material at loading conditions. When the

applying load is relatively high enough to destructive the material at very short time.

9. Define Transgranular fracture.

After the formation of micro crack and if temperature is high the grain boundaries are

stronger than the grains and hence the crack propagates through the grains piercing the

grain boundary and through the grains as shown below.

10. What is Inte rgranular fracture?

After the formation of micro crack and if the temperature is low the grain stronger than grain

boundaries and hence the crack propagates along grain boundaries as shown below.

(Refer fig in Notes)

http://en.wikipedia.org/wiki/Cleavage_%28crystal%29

http://en.wikipedia.org/wiki/Tensile_stress


10. What is fracture toughness?

Fracture toughness is a property which describes the ability of a material containing

a crack to resist fracture, and is one of the most important properties of any material

for many design applications. The linear-elastic fracture toughness of a material is

determined from the stress intensity factor ( ) at which a thin crack in the materia l

begins to grow. It is denoted KIc and has the units of or .

11. How micro void coalescence occurs?

MVC proceeds in three stages: nucleation, growth, and coalescence of microvoids.

The nucleation of microvoids can be caused by particle cracking or interfacial failure

between precipitate particles and the matrix. Microvoids grow during plastic flow of

the matrix, and microvoids coalesce when adjacent microvoids link together or the

material between microvoids experiences necking. Microvoid coalescence leads to

fracture.

12. Explain the mechanisms of micro void coalescence.

MVC proceeds in three stages: nucleation, growth, and coalescence of microvoids.

The nucleation of microvoids can be caused by particle cracking or interfacial failure

between precipitate particles and the matrix. Microvoids grow during plastic flow of

the matrix, and microvoids coalesce when adjacent microvoids link together or the

material between microvoids experiences necking. Microvoid coalescence leads to

fracture.

13. Analyse how micro void coalescence occurs.

Microvoids grow during plastic flow of the matrix, and microvoids coalesce when

adjacent microvoids link together or the material between microvoids experiences

necking. Microvoid coalescence leads to fracture.

16 Marks

1. Derive an expression for cleavage I failure based on Orowan and Griffith Theory and

comment how the various terms in these equations are responsible for strengthening

and toughening the materials. (APR/MAY 2008)

http://en.wikipedia.org/wiki/Fracture

http://en.wikipedia.org/wiki/Stress_intensity_factor

http://en.wikipedia.org/w/index.php?title=Precipitate_particles&action=edit&redlink=1

http://en.wikipedia.org/wiki/Necking_%28engineering%29

http://en.wikipedia.org/w/index.php?title=Precipitate_particles&action=edit&redlink=1




2. Derive an expression for cleavage II failure and ductile- brittle transition based on

Micro void formation by plastic deformation and comment on the various metallurgical

parameters affecting ductile-brittle transition. (APR/MAY 2008)

3. Analyse technically the following fractures (4X4=16)

(i) Trans Granular Ductile fracture.

(ii) Inter granular creep fracture. (MAY/JUN 2007)

(iii) Pure diffusional Fracture.

(iv) Rupture.

4. Draw fracture maps for a bcc material or an oxide and explain various regimes.

5. What are the approaches there for design under conditions of creep-fatigue

interaction? Explain all with neat sketches.

6. Discuss fractures at elevated temperature with fracture mechanism map. (NOV/DEC

2008)

7. Draw the fracture maps.

(i) For pure aluminium and commercially pure aluminium and explain the absence of

intergranular creep failure field and large transgranular regime for pure aluminium.

(MAY/JUN 2007)

(ii)For nickel, a nickel base super alloy and thoria dispersed nickel and explain the

shrink and disappearance of some regimes.

(iii)For a bcc or a ceramic and mark all the regimes of fracture.

8. Briefly explain the theory of Ductile to Brittle transition with neat diagram.

(NOV/DEC 2008)

9. Derive Orowon and Griffith theory of cleavage I failure. (MAY/JUN 2012)

10. What are the various types of fracture? Classify the fracture on the basis of type of

loading extent of deformation and appearance.


UNIT-4:

OXIDATION AND HOT CORROSION

2-Marks

1. Define oxidation process.

Oxidation means the loss of electrons. The oxidation of a metal occurs when the metal

loses one or more electrons, so that the atoms of the metal go from the neutral state

and become a positively charge ion. This commonly results in the formation of a

metal oxide (in the case of iron, that is known as rust).

2. Define Pilling Bed worth theory.

The Pilling-Bedworth ratio, (P-B ratio) R, of a metal oxide is defined as the ratio of the

volume of the metal oxide, which is produced by the reaction of metal and oxygen, to the

consumed metal volume:

M and D are the molecular weight and density of the metal oxide whose composition

is (Metal)a(oxygen)b; m, and d are the atomic weight and density of the metal.

3. What are the different stages of hot corrosion?

1. Type -1 hot CORROSION (Temp Range from 850-950’c)

2. Type -1I hot CORROSION (Temp Range from 650-800’c)

4. What are the Kinetic Laws of Oxidation?

1. Parabolic rate law

2. logarithmic rate law

3. Linear rate law

5. What is meant by Fluxing mechanism?


The process of non protective reaction product is formed to dissolute the oxide layer

on metal surface is called fluxing mechanism.

6. Define hot corrosion. What are the methods used for combat hot corrosion?

Hot corrosion is defined as an accelerated, often catastrophic surface attack super

alloy by hot-gas path component. This is severe in the temperature range 750-1000’c

and has affected A/C engines and industrial gas turbines.

There are three methods to combat hot corrosion such as

1. Aluminide (diffusion) coatings

2. Thermal barrier coatings

3. Overlay coatings

7. Name some oxidation resistance materials?

Conventionally Aluminium, chromium and copper materials are majorly used towards

corrosion resistance. Some metals have naturally slow reaction kinetics, these include such

metals as zinc, magnesium, and cadmium. While corrosion of these metals is continuous

and ongoing, it happens at an acceptably slow rate. An extreme example is graphite, which

releases large amounts of energy upon oxidation, but has such slow kinetics that it is

effectively immune to electrochemical corrosion under normal conditions.

7. Define basic fluxing and acidic fluxing in hot corrosion.

Basic fluxing: The term basic fluxing describes oxide dissolution, when O-2 contains

with the oxide to form a soluble MO-2 radical. It is generally associated with

temperatures > 900’C and termed as high temperature hot corrosion.

ü These results support a basic fluxing reaction, i.e. corrosive attack by forming

a basic solute of the protective scale.

4Ni + Na2SO4 = Na2O + 3 NiO + NiS

Acidic Fluxing: It defines dissolution , when a soluble metal M2+ and O-2 are

produced from oxide . The acidic fluxing is generally associated with temperature

range of 650’C to 800’C and it is termed as low temperature hot corrosion.

Acidic fluxing takes place when the O-2 activiyy in the molten salt is markedly

lowered , it leads to much more severe oxidation compared to basic fluxing.

7. What is the difference between n-type and p-type oxides?

http://en.wikipedia.org/wiki/Chemical_kinetics

http://en.wikipedia.org/wiki/Zinc

http://en.wikipedia.org/wiki/Magnesium

http://en.wikipedia.org/wiki/Cadmium

http://en.wikipedia.org/wiki/Graphite

http://en.wikipedia.org/wiki/Oxidation


N-type : Zirconium oxide ZrO2 contains an excess of four negatively charged

electrons in place of two vacant anion sites. Zirconium oxide is termed as n-type

semiconductor since it contains an excess of negatively charged electronic current

carriers/ electrons.

P-type : nickel oxide NiO is a metal deficient oxide. For each nickel ion vacancy ,

there are two trivalent Ni ions in normal lattice positions. The trivalent ions can be

considered as divalent ion and an associated “electron hole” (Absence of electron).

This is called p-type oxides.

8. Write any two methods to prevent corrosion.

1. Control or removal of aggressive impurities.

2. Development of new alloys with higher resistance to hot gas corrosion and greater

mechanical properties.

3. Development of protective surface coatings.

9. Define Pilling-Bedworth ratio on oxidation and its significance.

10. The Pilling-Bedworth ratio, (P-B ratio) R, of a metal oxide is defined as the ratio of

the volume of the metal oxide, which is produced by the reaction of metal and

oxygen, to the consumed metal volume:

If P-B ratio is more than unity, the oxide layer will be compression and will

uniformly cover the metal surface and be protective. for excessively large R, large

compressive stresses are likely to exist in metal oxide, leading to buckling and

spalling.

If P-B ratio is less than 1, a metal oxide tends to be porous and non-protective

because it cannot cover the whole metal surface. ,

11. Define hot corrosion.

Hot corrosion is an high- temperature analog of aqueous atmospheric corrosion. A thin

film deposit of fused salt on an alloy surface in a hot oxidizing gas causes accelerated

corrosion kinetics. (OR) Refer Q.no:6


12. Comment on the various kinetic laws of oxidation.

The parabolic rate law assumes that the diffusion of metal cations or oxygen anions is

the rate controlling step. The diffusivity of the oxide layer is also assumed to be

invariant. This assumption implies that the oxide layer has to be uniform, continuous

and of the single phase type.

13. Give two methods for prevention of hot corrosion

14. What is PB ratio?

Refer Q.no: 2

15. Distinguish b/w type-I and type-II hot corrosion.

Refer Notes

16 Marks:

1. Compare the parameters in controlling aqueous and dry corrosion. Discuss the

kinetic laws of high temperature corrosion with special effect on the doping on the

corrosion kinetics. (APR/MAY 2008)

2. Analyse the hot corrosion mechanisms involved in basic and acidic fluxing.

(APR/MAY 2008)

3. (i) Analyse how doping of a n-type and p-type semiconductor oxides influence

corrosion kinetics.

(ii)Analyse various kinetic laws of oxidation.

4. What are the kinematics principles in the Oxidation? Discuss in detail. (NOV/DEC

2008)

5. Describe the different metal-gas reactions and their remedies. (NOV/DEC 2008)

6.(i)It is well known that for corrosion resistance at high temperature there is a need for

a formation of a adherent oxide scale to prevent the diffusion of deleterious species such

as oxygen and sulphur through the scale and into the alloy. This is nearly impossible

since the oxide scales are non-stoichiometric and defective such as n-type or a p-type


oxide. Analyse how alloying or doping can influence the defect population and corrosion

in defect structures. (8) (MAY/JUNE 2007)

(ii)Analyse the various stages of hot corrosion with respect to exposure time various

chemical equations and degradation mechanisms. (8) (MAY/JUNE 2007)

7. Fluxing in hot corrosion in loss of protective oxide due to dissolution. Analyse the

various models for basic and acidic fluxing and effect of electroche mical polarisation on

fluxing of various oxides with the help of phase stability diagrams. (MAY/JUNE 2007)

8. Explain the various stages of hot corrosion with mechanisms and super alloys.

(MAY/JUNE 2012)

9. Discuss the kinetic laws of oxidation and the fundamentals behind various method of

combating corrosion. (MAY/JUNE 2012)

UNIT-5:

SUPER ALLOYS AND OTHER MATERIALS

2-Marks

1. Define Super alloys. What are the properties of super alloys?

Super alloys, or high performance alloys, are alloys that exhibit excellent mechanical

strength and creep resistance at high temperatures, good surface stability, and corrosion

and oxidation resistance. They typically have an austenitic face-centered cubic crystal

structure with a base alloying element of nickel, cobalt, or nickel-iron.

2. Why are super alloys preferred for high temperature application?

Super alloys, or high performance alloys, are alloys that exhibit excellent mechanical

strength and creep resistance at high temperatures, good surface stability, and

corrosion and oxidation resistance. The development of super alloys has primarily

been driven by the aerospace and power industries.


3. What are intermetallics? What are cera?

Intermetallics are made up of two or more elements, producing a new phase with its

own composition, crystal structure and properties. Intermetallics compounds are

almost always very hard and brittle.

Ceramic materials are complex chemical compounds containing both metallic and

non metallic elements. Alumina (Al2O3) is a ceramic composed of both metallic

aluminum and non metallic oxygen atoms. Ceramics are formed because of ionic or

covalent bonding, hence are usually hard, brittle and have high melting point.

4. What properties of intermetallics compounds offers creep resistance?

Refer notes with Fig

5. What are the strengthening mechanisms used for alloys?

1. Solid solution strengthening

2. Precipitation hardening by gamma prime

3. Grain boundary strengthening

4. Strain hardening.

6. Name a few examples for super alloys applicable for high temperature

applications.

Examples of superalloys are Hastelloy, Inconel (e.g. IN100, IN600, IN713),

Waspaloy, Rene alloys (e.g. Rene 41, Rene 80, Rene 95, Rene N5), Haynes alloys,

Incoloy, MP98T, TMS alloys, and CMSX (e.g. CMSX-4) single crystal alloys.

7. Define precipitation strengthening.

Precipitation hardening, also called age hardening, is a heat treatment technique used

to increase the yield strength of malleable materials, including most structural alloys of

aluminium, magnesium, nickel, titanium, and some stainless steels. In super alloys, it is

known to cause yield strength anomaly providing excellent high temperature strength.

8. Define Embrittlement.

n Embrittlement is a loss of ductility of a material, making it brittle. Various materials

have different mechanisms of embrittlement. Hydrogen embrittlement is the effect of

hydrogen absorption on some metals and alloys.

http://en.wikipedia.org/wiki/Hastelloy

http://en.wikipedia.org/wiki/Inconel

http://en.wikipedia.org/wiki/Waspaloy

http://en.wikipedia.org/wiki/Rene_41

http://en.wikipedia.org/wiki/Heat_treatment

http://en.wikipedia.org/wiki/Malleable

http://en.wikipedia.org/wiki/Aluminium

http://en.wikipedia.org/wiki/Magnesium

http://en.wikipedia.org/wiki/Nickel

http://en.wikipedia.org/wiki/Titanium

http://en.wikipedia.org/wiki/Stainless_steel

http://en.wikipedia.org/wiki/Superalloys

http://en.wikipedia.org/wiki/Yield_strength_anomaly


9. What properties of ceramic compound offer high temperature applications?

General characteristics of ceramics:

1) High melting point

2) Low electrical and thermal conductivity

3) Good chemical and thermal stability.

4) High compressive strengths.

10. Define corrosion fatigue.

Corrosion fatigue is fatigue in a corrosive environment. It is the mechanical

degradation of a material under the joint action of corrosion and cyclic loading.

Nearly all engineering structures experience some form of alternating stress, and are

exposed to harmful environments during their service life. The environment plays a

significant role in the fatigue of high-strength structural materials like steel, aluminum

alloys and titanium alloys.

11. What is grain boundary cracking? Give remedy for it

Intergranular fractures are cracks that take place along the grain boundary of a material.

Straight edges of the grain and shiny surface may be seen. There are several processes that

can lead to intergranular fracture. Grain-boundary strengthening is a method of

strengthening materials by changing their average crystallite (grain) size. So, by changing

grain size one can influence dislocation movement and yield strength.

12. What are the types of Ni-base alloys?

Ni-base alloys are Hastelloy, Inconel (e.g. IN100, IN600, IN713), Waspaloy, Rene

alloys (e.g. Rene 41, Rene 80, Rene 95, Rene N5), Haynes alloys, Incoloy,

13. Define TCP phases and its significance in super alloys.

In some alloys, if composition has not been carefully controlled, undesirable phases

can form either during heat treatment or, more commonly, during service. These

phases affect the creep resistance; fracture strength, premature cracking, and yield

strength. These precipitates are known as TCP phasesTopologically close-packed

(TCP) type phases, which are plate-like or needle- like phases such as σ, and μ that

http://en.wikipedia.org/wiki/Fatigue_%28material%29

http://en.wikipedia.org/wiki/Corrosion

http://en.wikipedia.org/wiki/Hastelloy

http://en.wikipedia.org/wiki/Inconel

http://en.wikipedia.org/wiki/Waspaloy




may form for some compositions and under certain conditions. These cause lowered

rupture strength and ductility.

14. What are Super alloys and their applications?

∑ Super alloys are generally in three ways such as Nickel base , cobalt base and iron

base super alloys. These superalloys are majorly available in many industries such

Aerospace, Turbine blades and jet/rocket engines, Marine industry, Submarines,

Chemical processing industry, Nuclear reactors, Heat exchanger tubing, Industrial gas

turbines .

15. Define directional solidifications and its benefits.

Directional solidification and progressive solidification describe types of

solidification within castings. Directional solidification describes solidification that

occurs from farthest end of the casting and works its way towards the sprue.

Progressive solidification, also known as parallel solidification. Directional

solidification can be used as a purification process. Since most impurities will be

more soluble in the liquid than in the solid phase during solidification, impurities will

be "pushed" by the solidification front, causing much of the finished casting to have a

lower concentration of impurities than the feedstock material, while the last solidified

metal will be enriched with impurities. This last part of the metal can be scrapped or

recycled.

16. Explain why the single crystal turbine blades perform better than directionally

solidified and coarse grained cast products.

Single crystal has the mechanical advantage of being able to operate at a much higher

temperature than crystalline materials. Creep is a common cause of failure in turbine

blades and is in fact the life limiting factor. When temperatures of a material under high

stress are raised to a critical point, the creep rate quickly increases .The single crystal

structure has the ability to withstand creep at higher temperatures than crystalline turbine

blades due to the lack of grain boundaries present.

17. Name two chemical composition of super alloys.

Many wrought nickel-base superalloys contain 10 to 20% Cr, up to

about 8% Al and Ti combined, 5 to 15% Co, and small amounts of boron,

zirconium, magnesium, and carbon. (Refer notes)

http://en.wikipedia.org/wiki/Solidification

http://en.wikipedia.org/wiki/Casting

http://en.wikipedia.org/wiki/Sprue_%28manufacturing%29


18. Mention the name of two high temperature ceramics

EXAMPLES OF HIGH TEMPERATURE CERAMICS:

Aluminum silicate, Alumina, Quartz, Porcelain

19. Define and list various TCP phases and explain whether they are beneficial or

detrimental for high temperature properties.

The composition has to be carefully controlled in order to avoid topologically close-

packed (TCP) phases, for example σ phase, μ phase or Laves. These phases can be

formed under certain conditions, usually during service. They are distinguished by

their plate like or needle like shapes. Alloys containing transition metals, such as

tantalum, niobium, chromium, tungsten or molybdenum, are the alloys the most

vulnerable to the formation of TCP phases.

16-Marks

1. In developing a super alloy for high temperature application how alloy chemistry can

be judiciously employed to produce resistance for creep, corrosion and micro structural

instability.

2. It is also possible to produce resistance to the above three dimensional damage by

different processing routes. Analyse please.

3. Write a technical note on the evolution of super alloys.

4. Analyse how the processing parameters can influence the mechanical properties of

super alloys at high temperatures.

5. Briefly explain the major phases of nickel-base alloys

6. Explain strengthening of cobalt-base super alloys.

7. Draw fracture maps for pure Ni chrome a solid solution and a precipitation hardened

nickel base super alloy and explain how the boundaries shifts due to alloying and

precipitation.

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