MATERIAL SCIENCE AND ENGINEERING METALLURGY

Model question with key

PART- A (15 X 2 marks = 30 marks)

Answer ALL Questions. Each question carries 2 marks

1. Define (a) Crystalline solid and (b) Amorphous solid.

2. Give an example of an amorphous material that is not derived by melting.

3. Draw the [1 1 2] direction vector in cubic unit cell.

4. Define the term: Stress relaxation.

5. State Griffith’s theory.

6. Distinguish between elastic and plastic deformation.

7. What is an isothermal transformation in the solid state?

8. Define hardenability of a steel.

9. Name the three basic heat – treatment steps to strengthen a precipitation –

hardenable alloy.

10.What is necking?

11.Define ‘True Stress’ and ‘True Strain’.

12.What do you mean by ‘Flexural Strength’?

13.Define the term: spherodizing.

14.What is the normalizing heat treatment for steels?

15.What are the advantages of martempering?

PART – B (5 X 14 marks = 70 marks)

(Answer ALL questions. Each question carries 14 marks)

16.a) Calculate the atomic packing factor for the FCC structure. (6)

b) How are miller indices for a crystallographic plane in a cubic unit cell

determined? What generalized notation is used to indicate them? (8)

[OR]

c) Describe and illustrate the following imperfections that can exist in crystal

lattice:

(i) Frenkel imperfection and

(ii) Schottky imperfection

17.a) Discuss the difference between the slip and twinning mechanism of plastic

deformation of metals. (7)

b) Describe two principal mechanisms whereby primary recrystallization can

occur. (7)

[OR]

c) Describe four major factors that affect the fatigue strength of metal. (8)

d) Draw a typical creep curve for a metal under constant load and at a relatively

high temperature and indicate on it all three stages of creep. (6)

18.a) Draw time-temperature cooling paths for 1080 steel on an isothermal

transformation diagram that will produce the following microstructures. Start with

the steels in the austenitic condition at time = 0 and 850C.

(i) 100% martensite

(ii) 50% martensite and 50% coarse pearlite,

(iii) 100% fine pearlite,

(iv) 50% martensite and 50% upper bianite,

(v) 100% upper bianite and

(vi) 100% lower bianite.

[OR]

b) What are some of the limitations of plain – carbon steels for engineering

designs? (5)

c) Explain how the dielectric and magnetic properties of materials such as

BaTiO3 and ferrites can be controlled using solid solution formation. (9)

19.a) Compare the engineering stress and strain with the true stress and strain for

the tensile test of a low-carbon steel that has the following test values:

Load applied to specimen = 75 kN,

Initial specimen diameter = 12.5 mm

Diameter of the specimen under 75 kN = 12 mm. (8)

b) Discuss the major factors that affect the fatigue strength of a metal. (6)

[OR]

b) With the help of suitable sketches, discuss the following:

(i) R.R. Moore reversed – bending fatigue machine (7)

(ii) Cyclic stresses (7)

20.a) Describe the tempering process for plain carbon steel. (7)

b) What are the advantages and limitations of Austempering process? (7)

[OR]

c) Describe the martempering process for a plain – carbon steel. Draw a cooling

curve for a martempered austenitized eutectoid plain –carbon steel by using an

IT diagram. What type of microstructure is produced after martempering this

steel?

1. Crystalline Solid: If the atoms (or) ions of a solid that has are arranged in a pattern that repeats itself in three dimension they form a solid that has long range order and is referred to as a Crystalline SolidAmorphous Solid: Materials with only shot-range ordered in their atomic structure are classified as amorphous Solid

2.

3.

4. Stress relaxation:

5. Decrease in the stress for a material held under constant strain as a function of time , observed in visco elastic material

6. Elastic deformation of the material that is recovered instantaneously when the applied load is removed.Plastic deformation: Permanent deformation of a material when a load is applied, then removed.

7. When the amount of a transformation of a particular temperature depends on the time permitted for the transformation

8. Ease with which steel can form martensite

9.

10. Local deformation causing reduction in the cross-sectional area of a tensile specimen

11. True Strain: The strain calculate using actual and not original dimensionTrue Stress: The load divided by the actual cross sectional area of the specimen at the load.

12. The stress required to fracture a specimen in a bend test.

13.

14. Is a heat treatment which the steel is heated in the austentic region and the cooled in still air

15. Modified quenching process used for steel to minimize distortion and cracking that may develop during uneven cooling of the heat treated material.

16

1. Packing Factor =

for FCC

Packing Factor =

b.1. Choose a plane that does not pass through the origin at (0,0,0).2. Determine the intercepts of the plane in terms of the crystallographic x, y, and z

axes for a unit cube.3. From the reciprocals of these intercepts.4. Clear fraction and determine the smallest set of whole number that are in the

same ratio as the intercepts.

c.i)If a positive cation moves into a interstitial site in an ionic crystal, a cation vacancy is created in the normal vacancy ion site. This vacancy is called frenkel imperfection.ii)When two oppositely charged ions are missing from an ionic crystk , a cation –anion divacancy is created that is known as schottky imperfection.

17 a.SLIP TWINNING

In this process atoms moves over each other

In this process a part of the atomic lattice is deformed so that it forms a mirror image of the undeformed lattice next to it

The crystallographic plane of symmetry between the undeformed and deformed part of the metal lattice is called twinning plane.

Occurs in a specific direction called slip direction

Occurs in a specific direction called twinning direction

Atoms move equal distance on one side of the slip

Atoms move distance proportional to their distance from twinning plane

Leaves serious of steps Leaves small but well defined region of crystal deformed

b.c.d.

1. Stress concentration2. Surface roughness3. Surface condition4. Environment.

The microstructure of the martempered steel is martensite.

19a. Area at start A0 =

Area under load Ai

Assuming no volume change during extension A0l0= AIlI (or) li/l0=A0/Ai

Engineering stress =

Engineering stress =

True stress =

True strain = ln

b.c. page 283 and 285 fig GREEN BOOK

20 a) The steel is first austenitized and the quenched at a rapid rate to produce martensite and to avoid the transformation of austentite to ferrite and cementite. The steel is then subsequently reheated at a temperature below the eutectoid temperature to soften the martensite by transforming it to a structure of iron carbide particle in a matrix of ferrite.b) Advantage:

1. Improved ductility and impact resistance of certain steel over those values obtained by conventional quenching and tempering

2. Decreased distortion of the quenched materialDisadvantage:

21. The need for a special molten salt bath22. The fact that the process can be used for only a limited number of steel.

c) (vii) Austenitizing the steel(viii) Quenching it in hot oil or molten salt at a temperature just slightly

above the Ms temperature.(ix) Holding the steel in the quenching medium until the temperature

is uniform throughout and stopping this isothermal treatment before the austenite-to-bainite transformation begins

(x) Cooling at a rate to room temperature to prevent large temperature difference.