Engineering Materials

ENGINEERING MATERIALS — QUICK REVISION

BASIC CRYSTALLOGRAPHY

• Crystal: Atoms arranged in a regular, repeating pattern.
• Unit Cell: Smallest repeating unit of a crystal.
• Coordination Number: Number of nearest-neighbour atoms.
• APF: Atomic Packing Factor = Volume occupied by atoms / Volume of unit cell.
• Allotropy: Same element existing in different crystal structures.

• Most closely packed: FCC & HCP.
• BCC: Generally stronger and less ductile than FCC.
• FCC: High ductility and good formability.
• α-Fe: BCC.
• γ-Fe: FCC.
• δ-Fe: BCC.

Crystal Defects

• Point defects: Vacancy, interstitial, substitutional.
• Line defect: Dislocation — edge and screw.
• Surface defects: Grain boundary, twin boundary.
• Volume defects: Voids, cracks, inclusions.

ALLOYS & PHASE DIAGRAMS

• Alloy: Mixture of two or more elements, with at least one being a metal.
• Substitutional alloy: Solute atoms replace solvent atoms. Example: Brass = Cu + Zn.
• Interstitial alloy: Small atoms occupy spaces between larger atoms. Example: Steel = Fe + C.

Important Phase-Diagram Terms

Gibbs Phase Rule

\[ F = C – P + 2 \]

For condensed systems at constant pressure:

\[ F = C – P + 1 \]

• F: Degrees of freedom
• C: Components
• P: Phases

Lever Rule

• Fraction of α = \( \dfrac{C_\beta – C_0}{C_\beta – C_\alpha} \)
• Fraction of β = \( \dfrac{C_0 – C_\alpha}{C_\beta – C_\alpha} \)

IRON–CARBON PHASE DIAGRAM

• Eutectoid composition: ≈ 0.76–0.77% C.
• Eutectoid temperature: ≈ 727°C.
• Eutectoid reaction: Austenite → Ferrite + Cementite.

HEAT TREATMENT

Heat Treatment = Controlled heating + holding + cooling to modify material properties.

• Annealing: Produces soft and ductile material; reduces internal stresses.
• Normalizing: Air cooling; finer grains than full annealing.
• Hardening: Austenitize + quench → forms martensite.
• Tempering: Reheating hardened steel below critical temperature → brittleness ↓ and toughness ↑.
• Quenching severity: Air < Oil < Water < Brine.

TTT & CCT

• TTT: Time–Temperature–Transformation; mainly constant-temperature transformation.
• CCT: Continuous Cooling Transformation; transformation during continuous cooling.
• Martensite: Very hard; forms by rapid quenching; diffusionless transformation.

FERROUS METALS

Ferrous metals: Metals containing iron as the principal constituent.

Cast Iron

Steel

• Mild steel: Low carbon; ductile and weldable.
• Medium-carbon steel: Good strength + toughness.
• High-carbon steel: High hardness and wear resistance.
• Stainless steel: Contains at least about 10.5% Cr; corrosion resistant.
• Tool steel: High hardness and wear resistance.

Important Alloying Elements

NON-FERROUS METALS

NON-METALLIC MATERIALS

Ceramics

• Inorganic, non-metallic materials.
• High hardness and high melting temperature.
• Excellent wear and corrosion resistance.
• Generally brittle.
• Examples: Glass, cement, alumina, porcelain.

Polymers

Composites

• Composite = Matrix + Reinforcement.
• GFRP: Glass Fibre Reinforced Polymer.
• CFRP: Carbon Fibre Reinforced Polymer.
• MMC: Metal Matrix Composite.
• Concrete: Cement matrix + aggregates.

BASICS OF NANO-MATERIALS

• Nanomaterials: Materials having at least one characteristic dimension approximately 1–100 nm.
• At nanoscale, surface area/volume ratio increases significantly.
• Mechanical, electrical, optical and chemical properties can differ from bulk materials.

• CNT: Carbon Nanotube; very high strength and excellent electrical properties.
• Graphene: One-atom-thick carbon layer; very high strength and electrical conductivity.
• Nanoparticles: Used in coatings, catalysts, electronics and biomedical applications.

MECHANICAL PROPERTIES

Important Formulae

• Stress = \( \dfrac{\text{Force}}{\text{Area}} \)
• Strain = \( \dfrac{\text{Change in length}}{\text{Original length}} \)
• Young’s Modulus (E) = \( \dfrac{\text{Stress}}{\text{Strain}} \)
• % Elongation = \( \dfrac{\text{Final length} – \text{Original length}}{\text{Original length}} \times 100 \)
• % Reduction in Area = \( \dfrac{\text{Original area} – \text{Final area}}{\text{Original area}} \times 100 \)

Stress–Strain Curve

• Important points: Proportional limit → Elastic limit → Yield point → Ultimate tensile strength → Fracture.
• Slope of elastic region = Young’s modulus.

MECHANICAL TESTING

Destructive Tests

• Charpy Test: Specimen supported horizontally.
• Izod Test: Specimen held vertically as a cantilever.

Hardness Tests

• Brinell → Ball
• Vickers → Diamond
• Rockwell → Depth

NON-DESTRUCTIVE TESTING (NDT)

NDT: Testing without significantly damaging the component.

Quick Memory: DPT → Surface cracks | MPT → Ferromagnetic | UT → Sound waves | RT → X-ray/Gamma rays

CORROSION

Corrosion: Gradual deterioration of a material due to chemical/electrochemical reaction with its environment.

Types of Corrosion

• Dry Corrosion: Direct chemical reaction, usually without liquid electrolyte.
• Wet/Electrochemical Corrosion: Occurs in presence of an electrolyte.
• Uniform Corrosion
• Galvanic Corrosion
• Pitting Corrosion
• Crevice Corrosion
• Intergranular Corrosion
• Stress Corrosion Cracking
• Erosion Corrosion

Galvanic Corrosion

• Occurs when dissimilar metals are electrically connected in an electrolyte.
• Anode → corrodes.
• Cathode → protected relative to anode.

Rusting of Iron

• \( \text{Fe} \rightarrow \text{Fe}^{2+} + 2e^- \)
• Oxygen and water participate in subsequent reactions.
• Rust mainly consists of hydrated iron oxides.

CORROSION PREVENTION & CONTROL

Cathodic Protection

• Sacrificial Anode: More active metal such as Zn, Mg or Al is connected to the structure; sacrificial metal corrodes.
• Impressed Current: External DC current keeps the protected structure cathodic.

• Galvanizing → Zinc coating
• Tinning → Tin coating
• Cathodic protection → Structure becomes cathode
• Sacrificial anode → Zn/Mg/Al

⚡ ONE-MINUTE REVISION

• BCC: α-Fe, Cr, W → CN 8 → APF 0.68
• FCC: Al, Cu, Ni → CN 12 → APF 0.74
• HCP: Mg, Zn, Ti → CN 12 → APF 0.74
• SC: CN 6 → APF 0.52
• Pearlite = Ferrite + Cementite
• Cementite = Fe₃C
• Eutectoid: ≈0.76–0.77% C at ≈727°C
• Annealing → Softening
• Normalizing → Grain refinement
• Hardening → Hardness ↑
• Tempering → Brittleness ↓, Toughness ↑
• Martensite → Very hard
• Brass = Cu + Zn
• Bronze = Cu + Sn
• Stainless steel → Cr ≥ ~10.5%
• Thermoplastic → Repeatedly softened
• Thermoset → Cannot be remelted easily
• Nanomaterials → ~1–100 nm
• Ductility → Tensile deformation
• Malleability → Compressive deformation
• Toughness → Energy absorption before fracture
• Hardness → Indentation/wear resistance
• Fatigue → Cyclic loading
• Creep → Time-dependent deformation
• DPT → Surface defects
• MPT → Ferromagnetic materials
• UT → Sound waves
• RT → X-ray/Gamma rays
• Galvanizing → Zinc
• Tinning → Tin
• Cathodic protection → Protected structure becomes cathode