THEORY EXAMINATION (SEM–IV) 2016-17 MATERIAL SCIENCE

MATERIAL SCIENCE (NOE047)

SECTION – A

(Attempt All | 10 × 2 = 20 Marks)

(a) Miller Indices

Miller indices are a set of three integers (h k l) used to describe the orientation of crystal planes in a crystal lattice.

(b) Composite Material

A composite material is made by combining two or more materials with different properties to obtain improved performance.
Examples: Fiberglass, Reinforced concrete.

(c) Eutectic and Eutectoid Reactions

Eutectic reaction:
Liquid → Solid α + Solid β

Eutectoid reaction:
Solid γ → Solid α + Solid β

(d) Packing Efficiency

Packing efficiency is the fraction of volume occupied by atoms in a unit cell.

Packing efficiency=Volume of atoms in unit cellTotal volume of unit cell\text{Packing efficiency}=\frac{\text{Volume of atoms in unit cell}}{\text{Total volume of unit cell}}Packing efficiency=Total volume of unit cellVolume of atoms in unit cell​ 

(e) Gibbs Phase Rule

Gibbs phase rule relates phases, components, and degrees of freedom:

F=C−P+2F = C - P + 2F=C−P+2

where F = degrees of freedom, C = components, P = phases.

(f) Hardness vs Toughness

Hardness: Resistance to indentation or scratching

Toughness: Ability to absorb energy before fracture

(g) Refractory Material

Refractory materials can withstand very high temperatures without melting or deformation.
Example: Fire clay
Property: High melting point
Application: Furnace linings

(h) Ferrous vs Non-Ferrous Materials

Ferrous: Contain iron (e.g., steel)

Non-ferrous: Do not contain iron (e.g., aluminium)

(i) Meissner Effect

The Meissner effect is the expulsion of magnetic field lines from a material when it becomes superconducting below its critical temperature.

(j) Classification of Solids (Energy Gap Basis)

Conductors

Semiconductors

Insulators

Semiconductors: Have small energy gap and moderate conductivity (e.g., silicon).

SECTION – B

(Attempt Any Five | 5 × 10 = 50 Marks)

(a) Stress–Strain Diagram & Hardenability

The stress–strain curve of mild steel shows:

Proportional limit                                           Elastic limit

Yield point                                                      Ultimate tensile strength

Fracture point

Hardenability: Ability of steel to harden in depth during heat treatment.
Factors affecting hardenability: Carbon content, alloying elements, grain size.

(b) Steel Making & Furnaces

Steel making: Conversion of pig iron into steel by removing impurities.
Furnaces:

Wrought iron: Puddling furnace

Cast iron: Blast furnace

(c) Magnetic Hysteresis

Magnetic hysteresis is the lagging of magnetization behind the applied magnetic field.

Retentivity: Ability to retain magnetism

Coercive force: Reverse field required to demagnetize material

Explained using B–H curve.

(d) Corrosion & Injection Moulding

Corrosion: Deterioration of metal due to chemical or electrochemical reaction.
Control: Coating, cathodic protection, alloying.

Injection moulding: Molten plastic is injected into a mould cavity and cooled to form products.

(e) TTT Diagram

TTT diagram is obtained by isothermal heat treatment experiments.
Importance:

Shows transformation kinetics

More practical than Fe–C equilibrium diagram

Helps in selecting heat treatment processes

(f) Short Notes (Any Three)

(i) Semiconductors: Moderate conductivity, temperature-dependent behavior
(ii) Thermistors: Temperature-sensitive resistors
(iii) Meissner Effect: Magnetic field exclusion in superconductors
(iv) High-Temperature Superconductors: Superconduct above 77 K

(g) Thermoplastics vs Thermosetting Plastics

Thermoplastics: Re-mouldable, soft on heating (PVC)

Thermosetting: Hard, cannot be remoulded (Bakelite)

(h) NDT (Non-Destructive Testing)

NDT tests materials without damaging them.

Types:

Ultrasonic testing

Radiography

Magnetic particle test

Ultrasonic testing: Uses high-frequency sound waves to detect internal flaws.

SECTION – C

(Attempt Any Two | 2 × 15 = 30 Marks)

3) Griffith’s Criterion for Crack Propagation

Griffith proposed that crack propagation occurs when strain energy released ≥ surface energy required.

σc=2Eγπa\sigma_c = \sqrt{\frac{2E\gamma}{\pi a}}σc​=πa2Eγ​​

where
σc = critical stress
E = Young’s modulus
γ = surface energy
a = crack length

This is necessary but not sufficient alone, as real materials have plastic deformation.

4) Superconductivity & Diffusion

Superconductivity:
Property of zero electrical resistance below critical temperature.

Diffusion: Movement of atoms due to concentration gradient.

Why Ni diffuses slower than C in Fe:
Nickel atoms are larger and substitute Fe atoms, while carbon occupies interstitial sites.

Self diffusion: Same atoms
Inter diffusion: Different atoms

5) Short Notes (Any Three)

(i) Gun Metal

Copper–tin–zinc alloy used in bearings and valves.

(ii) Duralumin

Aluminium alloy with Cu, Mg, Mn; used in aircraft structures.

(iii) Babbitt Metal

Soft bearing alloy used for journal bearings.

(iv) Heat Resisting Steel

Alloy steels resistant to oxidation at high temperature.

(v) Cyaniding

Surface hardening process using carbon and nitrogen.