UNIT 5: MATERIALS TECHNOLOGY - COMPREHENSIVE NOTES

Based on analysis of RGPV past papers (Jun 2022 - Jun 2025). Focus on definitions, diagrams, processes, and applications.

1.0 FUNDAMENTALS OF ENGINEERING MATERIALS & THEIR PROPERTIES

1.1 Definition, Classification & Selection

Definition: Materials from which things are made. Selection is based on required properties (mechanical, physical, chemical) and application/structure.
Classification by Structure:
- Metals: Ferrous (Fe-based) & Non-ferrous.
- Ceramics: Inorganic, non-metallic (e.g., oxides, carbides).
- Polymers: Organic, long-chain molecules (thermoplastics, thermosets).
- Composites: Two or more distinct materials combined (e.g., fiberglass).

1.2 Key Mechanical Properties

Property	Definition & Measurement	Significance
Ductility	Ability to undergo plastic deformation before fracture. Measured by % elongation and % reduction in area in tensile test.	Important for forming processes (rolling, drawing, forging).
Malleability	Ability to be hammered/rolled into thin sheets without cracking.	Specific to sheet forming operations.
Hardness	Resistance to localized plastic deformation (indentation, scratching). Measured by Brinell (HB), Rockwell (HR), Vickers (HV).	Indicates wear resistance and often correlates with strength.
Toughness	Ability to absorb energy up to fracture (area under stress-strain curve). Measured by Charpy/Izod impact test (energy absorbed in J).	Critical for design against shock/impact loads.
Brittleness	Tendency to fracture with little plastic deformation. Low toughness.	Undesirable in most structural applications.
Creep	Time-dependent deformation under constant load at elevated temperatures. Stages: Primary (decelerating), Secondary (steady-state), Tertiary (accelerating to rupture).	Governs design life of high-temperature components (boilers, turbines, engines).
Fatigue	Failure under repeated/reversed cyclic stresses below UTS. Represented by S-N curve (Stress vs. Number of cycles). Fatigue limit (endurance limit) is stress below which failure doesn't occur.	Major cause of service failures in rotating/cyclic components (axles, bridges).

1.3 Stress-Strain Analysis (Mild Steel)

Key Phases on Curve: Proportional Limit → Elastic Limit → Yield Point (Upper & Lower) → Strain Hardening → Ultimate Tensile Strength (UTS) → Necking → Fracture.

Brittle vs. Ductile Fracture: Ductile shows significant necking; Brittle shows little/no plastic deformation, often transgranular. Griffith's Theory: Brittle fracture occurs when stress concentration at micro-cracks exceeds theoretical cohesive strength.

DiagramCANVAS: Draw a typical stress-strain curve for mild steel. Label: Proportional Limit (PL), Elastic Limit (EL), Upper Yield Point (UYP), Lower Yield Point (LYP), Yield Strength (σ_y), Ultimate Tensile Strength (UTS/σ_u), Fracture Point. Show distinct necking region after UTS. Inset: Brittle fracture curve (no necking, linear to fracture) vs Ductile curve.

2.0 ATOMIC & CRYSTAL STRUCTURE

2.1 Crystal Lattice & Unit Cells

Crystal Lattice: 3D geometric arrangement of atoms/molecules.
Common Bravais Lattices in Metals: Simple Cubic (SC), Body-Centered Cubic (BCC), Face-Centered Cubic (FCC), Hexagonal Close-Packed (HCP).

2.2 Atomic Packing Factor (APF)

Fraction of volume in a unit cell occupied by atoms (assuming hard spheres).

FCC: APF = 0.74 (Highest packing efficiency).
BCC: APF = 0.68.
SC: APF = 0.52.

2.3 Crystal Planes & Directions: Miller Indices

Planes (hkl): 1. Take intercepts on axes (in terms of lattice parameters a,b,c). 2. Take reciprocals. 3. Clear fractions to smallest integers. 4. Enclose in parentheses ().
Directions [uvw]: 1. Vector from origin to point (u,v,w in terms of a,b,c). 2. Reduce to smallest integers. 3. Enclose in brackets [].

2.4 Crystal Imperfections/Defects

Defect Type	Examples	Influence on Properties
Point Defects	Vacancy, Interstitial, Substitutional atom.	Increase electrical resistivity; affect diffusion.
Line Defects	Dislocations (Edge, Screw).	Primary cause of plastic deformation. Strain hardening occurs due to dislocation interaction.
Surface Defects	Grain boundaries, twin boundaries, phase boundaries.	Hinder dislocation motion → increase yield strength (Hall-Petch relationship).

2.5 Atomic Bonding

Metallic Bonding: Non-directional "electron sea" model. Explains malleability, ductility, high electrical/thermal conductivity.
Other Bonds:
- Ionic: Electrostatic attraction (e.g., NaCl). Brittle, poor conductivity in solid.
- Covalent: Shared electrons (e.g., diamond, Si). Hard, high melting point, brittle.
- van der Waals: Weak dipole/dispersion forces (e.g., graphite, polymers). Low strength, low melting point.

3.0 PHASE DIAGRAMS & SOLID SOLUTIONS

3.1 Solid Solutions

Substitutional: Solute atoms replace solvent atoms (e.g., Cu-Ni). Requires similar atomic radii (<15% difference), same crystal structure, similar electronegativity, similar valency (Hume-Rothery Rules).
Interstitial: Small solute atoms (C, N, H) occupy voids in solvent lattice (e.g., C in γ-Fe (austenite)).

3.2 Binary Phase Diagrams

Isomorphous (Complete Solid Solubility): Simple eutectic-free system (e.g., Cu-Ni). Alloy properties vary smoothly with composition.
Eutectic System: Liquid ↔ α + β at eutectic composition/temperature (e.g., Pb-Sn). Cooling curve shows thermal arrest at eutectic temperature.

3.3 Iron-Iron Carbide (Fe-Fe₃C) Phase Diagram

CRITICAL DIAGRAM. Must be drawn and labeled.

Key Phases: δ-ferrite (BCC), γ-austenite (FCC), α-ferrite (BCC), Fe₃C (cementite).

Key Invariant Reactions:

Eutectic (4.3% C, 1147°C): L → γ (2.14% C) + Fe₃C (6.67% C) → Ledeburite.

Eutectoid (0.76% C, 727°C): γ (0.76% C) ↔ α (0.022% C) + Fe₃C (6.67% C) → Pearlite.

Peritectic (0.16% C, 1495°C): δ + L ↔ γ.

Microstructures: Austenite (γ), Ferrite (α), Cementite (Fe₃C), Pearlite (lamellar α+Fe₃C), Bainite, Martensite (non-equilibrium).

DiagramCANVAS: Draw complete Fe-Fe3C phase diagram. Clearly label all phase fields (δ, γ, α, L, α+Fe3C, γ+Fe3C, etc.), all invariant reaction points (Eutectic, Eutectoid, Peritectic) and their temperatures/compositions. Label key lines (GS, ES, PQ, etc.).

3.4 Time-Temperature-Transformation (TTT) Diagram for Eutectoid Steel

Construction: Isothermal transformation of austenite. Start (C-curve) and Finish curves.

Products:

Above 550°C: Pearlite (coarse to fine).

250-550°C: Bainite (Upper: feathery; Lower: plate-like).

Below 250°C (Ms): Martensite (diffusionless, shear transformation). Mf is finish temperature.

Significance over Equilibrium Diagram: Shows kinetics of non-equilibrium phase formation (martensite, bainite). Essential for designing isothermal heat treatments (austempering, martempering).

DiagramCANVAS: Draw typical TTT diagram for eutectoid (0.8% C) steel. Label C-curve (nose at ~550°C), Ms, Mf lines. Show transformation products in different regions (Pearlite, Bainite, Martensite). Indicate cooling paths for austempering (hold in bainite region) and martempering (hold above Ms).

3.5 Age-Hardening (Precipitation Hardening)

Example: Al-Cu system (up to 10% Cu).

Stages:

Solution Treatment: Heat to single-phase region (above solvus) → homogeneous solid solution.

Quenching: Rapid cool → supersaturated solid solution (soft).

Aging: Reheat to intermediate temperature (below solvus) → precipitation of fine, dispersed second phase (e.g., θ'' Al₃Cu) → hardening.

Over-aging: Coarse precipitates form → softening.

3.6 Relationship Between Phase Diagrams & Alloy Properties

Phase diagram indicates phase composition and microstructure at equilibrium.
Properties depend on phase fractions (lever rule), phase distribution, and grain size.
Alloy design uses diagrams to achieve desired combinations (e.g., two-phase alloys for strength).

4.0 HEAT TREATMENT OF STEELS

4.1 Objectives

Relieve stresses, refine grain, modify microstructure, alter mechanical properties (hardness, strength, toughness, ductility).

4.2 Annealing

Full Annealing: Heat to above A₃/A₁, hold, slow cool in furnace. → Coarse pearlite/ferrite. Soft, machinable, relieve stresses.
Process Annealing: Below A₁, slow cool. → Stress relief without phase change.
Stress Relief Annealing: Below A₁, slow cool. → Remove residual stresses.

4.3 Normalizing

Heat to above A₃/A₁, hold, air cool (moderate cooling rate).
Microstructure: Finer pearlite/ferrite than annealed. Higher strength & hardness than annealed. Refines grain.

Annealing vs. Normalizing:

| Feature | Annealing | Normalizing |

| :--- | :--- | :--- |

| Cooling | Furnace (slow) | Air (moderate) |

| Microstructure | Coarse | Finer |

| Properties | Softest, most ductile | Stronger, harder, tougher |

4.4 Hardening

Process: Austenitize (heat above A₃/A₁) → Quench rapidly (water, oil, polymer).
Product: Martensite (supersaturated BCT structure). Very hard & brittle.
Quench Cracks: Due to high thermal stresses and martensite transformation volume expansion.

4.5 Tempering

Theory: Reheat hardened steel below A₁. Decomposes martensite → Tempered Martensite (ferrite + dispersed carbides). Reduces internal stresses, decreases hardness, increases toughness.
Types:

Low Temp (150-250°C): Minimal hardness loss, high toughness. Tempered Martensite.

Medium Temp (300-450°C): Troxite (or tempered troostite). Good toughness.

High Temp (500-650°C): Sorbitic (or tempered sorbite). High toughness, moderate hardness.

4.6 Special Heat Treatment Processes

Austempering: Austenitize → Quench to bainite region (250-400°C) → Hold until transformation complete → Cool in air. Product: Bainite ( Ausferrite). Advantages: No martensite, low distortion, good toughness.
Martempering (Marquenching): Austenitize → Quench to just above Ms (150-300°C) → Hold until temperature uniform → Cool in air through Ms. Purpose: Reduce thermal stresses before martensite forms. Minimizes cracking.

4.7 Hardenability

Definition: Ability of a steel to be hardened (form martensite) to a specified depth under given conditions. NOT the same as hardness.
Factors: Carbon content (↑), alloying elements (Cr, Mo, Ni, Mn ↑), austenite grain size (↑), quenching severity (↑).
Jominy End Quench Test: Standardized test. Specimen austenitized, one end quenched with water. Hardness measured along length → hardenability curve.
Quench Severity: Cooling media: Brine > Water > Oil > Air. Severity = H-value (Grossmann's).

4.8 Surface Hardening / Case Hardening

Distinction:

Case Hardening: Diffusion of C/N into surface → hardened case, tough core.

Surface Hardening: No diffusion. Rapid surface heating & quenching → hardened surface.

Process	Principle	Typical Applications
Carburizing	Add C to surface (pack, liquid, gas) at 900-950°C.	Low-C steels (e.g., gears, cams).
Nitriding	Add N to surface (gas, salt bath) at 500-570°C. Forms hard nitrides.	Alloy steels (Cr, Mo, Al). High wear, no quench needed.
Cyaniding	Short-time (20-30 min) C+N diffusion in molten cyanide salt (~850°C).	Small parts (tools, gears). Fast, but toxic.
Carbo-nitriding	Simultaneous C+N diffusion (gas) at 700-900°C.	Similar to cyaniding, less toxic.
Flame Hardening	Localized oxy-acetylene flame heating → water quench.	Large gears, rails, shafts.
Induction Hardening	High-frequency AC induces surface heating → quench.	Shafts, gears, precise case depth.
Ion Implantation	Bombard surface with high-energy ions (N, B, C) in vacuum.	Precision components, tool steels.

5.0 STEELS & CAST IRONS

5.1 Classification of Steels

Basis	Types	Properties & Applications
Carbon Content	Low-C (<0.25% C): Ductile, weldable.	Sheets, structural sections, pipelines.

| Medium-C (0.25-0.6% C): Balanced strength/ductility. | Axles, gears, rails. |

| High-C (>0.6% C): Hard, brittle. | Springs, cutting tools, wires. | | Alloying | Low-Alloy (≤5% total): Improved hardenability, strength. | Structural, automotive. |

| High-Alloy (>5% total): Special properties. | Stainless, tool, heat-resistant steels. |

| Tool Steels: High hardness, wear resistance (T, H, O, A, D, S series). |

| Stainless Steels: >12% Cr for corrosion resistance. |

| Maraging Steels: Ni-based, age-hardenable, ultra-high strength. | | Deoxidation | Killed: Fully deoxidized, uniform composition. | High-quality steels. |

| Semi-killed: Partial deoxidation, some surface blowholes. | General structural. |

| Rimmed: Most Mn oxidized, rim pure Fe. | Sheet, strip (good surface). |

| Boiling (沸腾钢): Strong deoxidation, CO gas evolution. | Cheapest, poor surface/quality. |

5.2 Role of Alloying Elements

Element	Primary Effect(s)
C	Increases hardness/strength (martensite), decreases weldability.
Mn	Deoxidizer, combines with S (MnS), increases hardenability.
Si	Deoxidizer, increases strength (solid solution), magnetic.
Cr	Increases hardenability, wear/corrosion resistance, forms carbides.
Ni	Increases toughness, strength, corrosion resistance (austenite stabilizer).
Mo	Increases hardenability, high-temperature strength, reduces temper brittleness.
V	Forms stable VC → refines grain, increases hardness, resists temper softening.
W	Similar to Mo, for hot-work tools.
Cu	Increases corrosion resistance (weathering steels), precipitation hardening.

5.3 Special Steels

Hadfield (Manganese) Steel: ~12% Mn, 1% C. High impact strength, work-hardens severely. Applications: Railway switches, crusher jaws.
Stainless Steels:
- Martensitic (Cr 12-18%, C 0.1-1%): Hardenable, magnetic. Cutlery, valves.
- Ferritic (Cr 12-30%, C <0.1%): Non-hardenable, magnetic, corrosion resistant. Automotive trim.
- Austenitic (Cr 16-26%, Ni 6-22%, C <0.08%): Non-magnetic, excellent corrosion/weldability. Chemical plants, food.
- Duplex (Cr 22-25%, Ni 5-7%, Mo 2-3%): Mixed austenite/ferrite. High strength, corrosion resistance. Offshore.

5.4 Cast Irons

All contain >2% C.

Type	Graphite Form	Microstructure	Properties	Applications
Grey Cast Iron	Flakes	Pearlite/Ferrite matrix + graphite flakes.	Good castability, machinability, damping. Weak in tension, strong in compression.	Engine blocks, pipes, machine bases.
White Cast Iron	Cementite (Fe₃C)	Hard, brittle cementite matrix.	Very hard, wear resistant, brittle.	Grinding balls, crushing rolls.
Malleable Cast Iron	Clusters (from white iron annealed)	Ferrite/pearlite matrix + tempered carbon clusters.	More ductile/tough than grey.	Fittings, brackets, agricultural parts.
Ductile (S.G.) Iron	Spheroids (nodular)	Pearlite/ferrite matrix + spherical graphite.	High strength, ductility, toughness (like steel).	Pipes, gears, automotive components.

6.0 NON-FERROUS METALS & ALLOYS

6.1 Copper & Its Alloys

Pure Cu: Excellent conductivity, corrosion resistant, ductile. Electrical wires, heat exchangers.
Brasses (Cu-Zn):
- Cartridge Brass (70% Cu, 30% Zn): Good strength/ductility. Ammunition, fixtures.
- Muntz Metal (60% Cu, 40% Zn): α+β structure. Used for hot working (ship sheathing).
Bronzes:
- Cu-Sn Bronze (88% Cu, 12% Sn): α phase. High corrosion resistance, low friction. Bearings, gears, bells.
- Al Bronze (Cu-Al): High strength, corrosion resistance. Marine hardware.
- Be Bronze (Cu-Be): High strength, non-sparking. Springs, tools.
Cupronickel (Cu-Ni): Corrosion resistant (seawater). Condenser tubes, coins.
Nickel Silver (Cu-Zn-Ni): Silvery appearance. Cutlery, musical instruments.

6.2 Aluminium & Its Alloys (AA Designation)

Series	Main Alloying Element	Key Properties	Typical Applications
1xxx	None (99%+ Al)	High conductivity, corrosion resistant.	Electrical conductors, chemical equip.
2xxx	Cu	High strength (heat-treatable).	Aircraft structures (2024).
3xxx	Mn	Moderate strength, good workability.	Beverage cans, roofing (3003).
4xxx	Si	Low melting point, good castability.	Welding rods, brazing alloys.
5xxx	Mg	Excellent corrosion resistance, weldable.	Marine, pressure vessels (5052, 5083).
6xxx	Mg-Si	Heat-treatable, good extrudability.	Architectural, automotive (6061, 6082).
7xxx	Zn	Very high strength (heat-treatable).	Aerospace (7075, 7050).
8xxx	Others (Li, Fe)	Specialized.	Aerospace (Al-Li alloys).

6.3 Nickel & Its Alloys

Monel Metal (Ni-Cu, ~67% Ni, 30% Cu): High strength, corrosion resistant (seawater, acids). Chemical plants, marine hardware.
Inconel (Ni-Cr-Fe): Excellent high-temperature strength, oxidation/corrosion resistance. Jet engines, nuclear reactors.
Nimonic (Ni-Cr-Co): Similar to Inconel, for high-temperature turbine blades.

6.4 Titanium Alloys

Alpha Alloys (e.g., CP Ti, Ti-6Al-4V): HCP structure. Good creep resistance, weldable. Ti-6Al-4V: Workhorse alloy. Properties: High strength/weight ratio, excellent corrosion resistance.
Applications: Aerospace (airframes, engines), chemical processing, biomedical implants.

6.5 Bearing Alloys (Babbitt Metals)

Tin-Based (White Metal): Sn-Sb-Cu. Excellent embeddability, conformability. Uses: Heavy-duty bearings.
Lead-Based: Pb-Sn-Sb. Cheaper, lower melting point. Uses: Light/medium duty bearings.

7.0 METALLURGICAL ANALYSIS & TESTING METHODS

7.1 Chemical Analysis

Method	Principle	Use in Metallurgy
Volumetric (Titration)	Reactant of known concentration reacts with analyte. Volume measured.	Determine %C, S, Mn, etc.
Gravimetric	Analyte converted to insoluble compound → weighed.	Accurate % determination (e.g., SiO₂).
Spectrophotometric	Beer-Lambert Law: Absorbance ∝ Concentration.	Trace element analysis (colorimetry).
Optical Emission Spectrometry	Spark excites atoms → emit characteristic wavelengths.	Rapid multi-element analysis (production control).

7.2 Spot Test Method

Principle: Apply chemical reagent to a small spot on metal surface. Observe color change/precipitate for identification.
Application: Rapid, qualitative identification of metals in scrap, workshops.

7.3 Destructive Mechanical Testing

Test	What it Measures	Key Feature
Tensile	UTS, YS, %EL, %RA, modulus.	Full stress-strain curve.
Hardness (B, R, V)	Resistance to indentation.	Non-destructive, quick, correlated to strength.
Impact (Charpy/Izod)	Toughness (energy absorbed).	Notched specimen, fracture energy. Logic: Notch concentrates stress, simulates flaw.
Creep	Strain vs. time at constant stress/temp.	Determines steady-state creep rate → design life.
Fatigue	S-N curve (stress vs. cycles to failure).	Rotating beam or axial loading. Endurance limit for steels.

7.4 Non-Destructive Testing (NDT) for Cracks

Method	Principle	Suitable For
VT (Visual)	Direct/remote visual inspection.	Surface defects.
PT (Liquid Penetrant)	Penetrant seeps into surface cracks → developer shows.	Non-porous materials, surface cracks.
MT (Magnetic Particle)	Magnetic field + particles collect at flux leakage (cracks).	Ferromagnetic materials, surface/subsurface.
ET (Eddy Current)	Coil induces eddy currents → changes indicate flaws.	Conducting materials, surface/subsurface.
UT (Ultrasonic)	Pulse-echo; sound wave reflects from flaws.	Thickness, internal flaws (deep).
RT (Radiographic)	X-rays/Gamma rays; film shows internal density variations.	Internal voids, inclusions, welds.

8.0 METALLURGICAL RAW MATERIALS & REAGENTS

8.1 Main Ores

Manganese Ores:
- Pyrolusite (MnO₂): Main ore. Used in dry cells, steel deoxidizer.
- Rhodochrosite (MnCO₃): Carbonate ore.
- Hausmannite (Mn₃O₄): Important source.
- Industrial Use: Steelmaking (deoxidizer, sulfur fixer, alloying for Hadfield steel).
Chromium Ores:
- Chromite (FeCr₂O₄): Only important ore. Spinel structure.
- Industrial Use: Stainless/heat-resistant steels (Cr for passivation), refractories (chromite-magnesite).

8.2 Chemical Reagents in Metallurgy

Acids (HCl, H₂SO₄): Pickling (scale removal), leaching, analysis.
Fluxes (Limestone, Fluorspar): Smelting (remove impurities as slag).
Inhibitors (e.g., NaNO₃): Quenching media (reduce severity).
Reducing Agents (C, CO, Al): Smelting, thermite welding.
Analysis Reagents: Titrants (KMnO₄, EDTA), precipitants (NH₄OH for Al), indicators.

> [!TIP] EXAM FOCUS:

Draw & Label: Stress-strain curve, Fe-Fe₃C diagram, TTT diagram.
Distinguish Clearly: Annealing vs. Normalizing; Hardness vs. Hardenability; Case vs. Surface hardening; Ductile vs. Brittle fracture.
Process Steps: Know stages of age-hardening, steps of austempering/martempering, Jominy test procedure.
Applications: Link alloy composition to use (e.g., Al-6xxx series = extrusions; Hadfield steel = impact wear).
NDT Methods: Match principle to application (UT for thick welds, MT for surface cracks in steel).