Materials Technology (ME-303) - Unit 1 Short Notes

1.0 Introduction to Engineering Materials

• Definition: Materials engineered for specific applications based on desired properties.

• Classification:

  | Class | Bonding Type | Properties | Examples | Applications | |-----------------|-----------------------|-----------------------------------------|-----------------------------|--------------------------------------| | Metals | Metallic | High strength, ductility, conductivity | Fe, Cu, Al, Ti | Structural, electrical, aerospace | | Ceramics | Ionic/Covalent | Brittle, high T, corrosion-resistant | Al₂O₃, SiC, ZrO₂ | Refractories, cutting tools, bio-implants | | Polymers | Covalent (Van der Waals) | Low density, flexible, insulating | PE, PVC, Nylon | Packaging, textiles, automotive | | Composites | Mixed | Tailorable properties, high specific strength | CFRP, GFRP | Aerospace, sports equipment |

[!TIP]

Exam Focus: Be ready to classify any given material (e.g., tungsten carbide → ceramic matrix composite). Link properties to bonding type.

2.0 Atomic Structure, Bonding, and Crystal Imperfections

2.1 Crystal Lattice & Unit Cell

• Crystal Lattice: 3D periodic arrangement of atoms.

• Unit Cell: Smallest repeating unit defining lattice geometry.

• Common Metal Crystal Structures:

  | Structure | Atoms/Unit Cell | Coordination Number | APF | Example Metals | |---------------|---------------------|------------------------|-----------|--------------------| | SC | 1 | 6 | 0.52 | Polonium (rare) | | BCC | 2 | 8 | 0.68 | Cr, α-Fe, W, Mo | | FCC | 4 | 12 | 0.74 | γ-Fe, Al, Cu, Ni | | HCP | 2 | 12 | 0.74 | Mg, Zn, Ti, Cd |

APF Calculation:

• FCC: $$\displaystyle \text{APF} = \frac{4 \times \frac{4}{3}\pi r^3}{a^3} $$, where $$\displaystyle a = 2\sqrt{2} r $$

• BCC: $$\displaystyle \text{APF} = \frac{2 \times \frac{4}{3}\pi r^3}{a^3} $$, where $$\displaystyle a = \frac{4r}{\sqrt{3}} $$

\boxed{\text{APF}{\text{FCC}} = 0.74,\ \text{APF}{\text{BCC}} = 0.68}

2.2 Miller Indices

• Notation system for crystallographic planes/directions.

• Steps:

  1. Find intercepts of plane with axes in terms of lattice parameters $a, b, c$.

  2. Take reciprocals.

  3. Clear fractions to smallest integers.

  4. Enclose in parentheses (hkl) for planes, [uvw] for directions.

• Example: Plane intercepts at $a, 2b, \infty c$ → reciprocals $$\displaystyle 1, \frac{1}{2}, 0 $$ → Miller indices (1 1 0).

2.3 Atomic Bonding

[!TIP]

Common Pitfall: Don't confuse covalent network (diamond) with molecular covalent (I₂). Only network solids are hard/brittle.

2.4 Crystal Imperfections

• Point Defects:

  • Vacancy: Missing atom.

  • Interstitial: Extra atom in space.

  • Substitution: Foreign atom replaces host.

• Line Defects: Dislocations (edge/screw) → enable plastic deformation.

• Surface Defects: Grain boundaries → impede dislocation motion → increase strength (Hall-Petch).

2.5 Solid Solutions

• Intermediate Phases: New crystal structure, fixed/compositional range (e.g., σ-phase, intermetallics like Ni₃Al). Contrast with solid solutions (single phase, variable composition).

3.0 Mechanical Properties and Material Behavior

3.1 Stress-Strain Curve (Polycrystalline)

graph LR

    A[Proportional Limit] --> B[Elastic Region]

    B --> C[Yield Point]

    C --> D[Plastic Deformation]

    D --> E[Ultimate Tensile Strength UTS]

    E --> F[Necking]

    F --> G[Fracture]

Key Points:

• Proportional Limit (σₚ): Stress ∝ Strain (Hooke's Law: σ = Eε).

• Yield Point (σᵧ): Onset of plastic flow (upper/lower for some steels).

• UTS (σₘ): Maximum stress.

• Fracture: Final failure.

• Elastic Recovery: Reversible deformation.

• Plastic Deformation: Permanent (dislocation motion).

3.2 Mechanical Properties

Griffith's Theory of Brittle Fracture: Crack propagates when stress intensity factor $K$ reaches critical $$\displaystyle K_{IC} $$ (fracture toughness). Flaws/defects concentrate stress.

3.3 Time-Dependent Deformation

• Creep: Time-dependent strain under constant stress at elevated T.

  • Stages:

    1. Primary (Transient): Strain rate decreases (work hardening).

    2. Secondary (Steady-state): Constant strain rate (balance of hardening/recovery) → design parameter.

    3. Tertiary: Strain rate accelerates → rupture (void growth).

  • Effect of Temperature: Creep significant when T > 0.3Tₘ (absolute).

• Fatigue: Failure under cyclic stress < UTS.

  • S-N Diagram: Stress amplitude (S) vs. number of cycles to failure (N).

  • Fatigue Limit (Endurance Limit): Stress below which failure doesn't occur (for steels, Al has no true limit).

  • Factors: Surface finish, size, temperature, residual stresses, corrosion.

3.4 Effect of Grain Size

• Hall-Petch Relationship: $$\displaystyle \sigma_y = \sigma_0 + k_y d^{-1/2} $$

  • $$\displaystyle \sigma_y $$: Yield strength

  • $$\displaystyle \sigma_0 $$: Friction stress

  • $$\displaystyle k_y $$: Hall-Petch slope

  • $ d $: Average grain diameter

• Finer grains → more grain boundaries → impede dislocations → higher strength, toughness, creep resistance (but may reduce ductility at very fine sizes).

3.5 Elastic, Anelastic, Viscoelastic Behavior

4.0 Phase Diagrams and Solid-State Transformations

4.1 Binary Phase Diagrams

• Types:

  • Isomorphous: Complete solubility (e.g., Cu-Ni).

  • Eutectic: Liquid ↔ α + β (e.g., Pb-Sn).

  • Eutectoid: Solid ↔ α + β (e.g., γ ↔ α + Fe₃C in steel).

  • Peritectic: Liquid + α ↔ β (e.g., Fe-γδ at 1495°C).

• Cooling Curves: Show thermal arrests at phase transformations (eutectic: one arrest; eutectoid: one arrest in solid state).

• Lever Rule (for two-phase region):

$$ \text{Weight fraction of phase} = \frac{\text{Alloy composition - other phase composition}}{\text{Phase composition - other phase composition}} $$

\boxed{W_\alpha = \frac{C_0 - C_\beta}{C_\alpha - C_\beta},\quad W_\beta = \frac{C_\alpha - C_0}{C_\alpha - C_\beta}}

4.2 Iron-Carbon (Fe-Fe₃C) Phase Diagram

• Key Phases:

  • δ-ferrite (BCC): Solubility max 0.02% C at 1495°C.

  • γ-austenite (FCC): Solubility max 2.14% C at 1147°C.

  • α-ferrite (BCC): Solubility max 0.022% C at 727°C.

  • Fe₃C (Cementite): Orthorhombic, 6.67% C, hard/brittle.

• Invariant Reactions:

  | Reaction | Temp (°C) | Composition | Products | Type | |-----------------|---------------|-----------------|---------------------|----------------| | Eutectic | 1147 | 4.3% C (L) | γ (2.14% C) + Fe₃C | L → γ + Fe₃C | | Eutectoid | 727 | 0.76% C (γ) | α (0.022% C) + Fe₃C | γ → α + Fe₃C | | Peritectic | 1495 | 0.16% C (L + δ) | γ (0.09% C) | L + δ → γ |

• Microstructural Constituents:

  • Pearlite: Lamellar α + Fe₃C (eutectoid product, 0.76% C).

  • Ledeburite: Eutectic mixture (γ + Fe₃C, 4.3% C).

  • Martensite: Supersaturated solid solution (diffusionless γ → α').

  • Bainite: Fine plate/needle mixture (α + Fe₃C, forms below 550°C).

4.3 TTT Diagram for Steel

• Construction: Isothermal transformation curves (start/finish) from austenite.

• Key Features:

  • "Nose": Minimum incubation time → critical cooling rate.

  • CCT vs. TTT: Continuous Cooling Transformation (CCT) diagrams account for cooling rate; TTT assumes instantaneous quench to constant T.

• Importance over Fe-C Diagram: Fe-C shows equilibrium phases; TTT shows non-equilibrium products (martensite, bainite) and kinetics (time required).

• Information Supplied: Transformation start/finish times, product microstructure at given T, effect of alloying (shifts nose right → improves hardenability).

4.4 Partial Al-Cu Phase Diagram (≤10% Cu)

• Eutectic: L (33% Cu) ↔ α (5.65% Cu at 548°C) + θ (Al₂Cu) at 548°C.

• Engineering Significance (4-5% Cu):

  • Lies in α + θ two-phase region at room T.

  • Age-hardening: Supersaturated solid solution (α) from quenching → precipitation of θ (Al₂Cu) → hardening.

  • Solvus Curve: Boundary of α phase solubility (max Cu dissolves decreases with T). Solution treat above solvus → quench → age.

4.5 Eutectoid Solid Solution & Solvus Curve

• Eutectoid Solid Solution: Single phase (γ in Fe-C) that transforms into two phases (α + Fe₃C) at eutectoid T. Significance in Steelmaking: Austenite (γ) is the parent phase for all heat treatments (quenching, annealing).

• Solvus Curve: Phase boundary showing maximum solubility of solute in solvent. Role in Age-hardening: Defines solution treatment temperature (above solvus to dissolve solute) and aging temperature (below solvus for precipitation).

5.0 Heat Treatment of Metals

5.1 Objectives of Heat Treatment

5.2 Fundamental Processes

5.3 Austempering & Martempering

5.4 Surface Hardening Methods

Case Hardening vs. Surface Hardening:

• Case Hardening: Diffusive process (C/N added) → hardened case + tough core.

• Surface Hardening: General term including case hardening and thermal methods (flame/induction) that harden only surface without changing composition.

5.5 Hardenability

• Definition: Ability of steel to be hardened (form martensite) to a given depth under specified conditions.

• Factors:

  1. Carbon content: ↑C → ↑hardness but ↓hardenability (Mₛ lowered).

  2. Alloying elements (Cr, Mo, Ni, Mn, Si): Slow cooling → shift TTT nose right → increase hardenability.

  3. Grain size: Finer grain → ↑hardenability (more nucleation sites for martensite).

  4. Quenching medium: Agitation, severity (water > oil > air).

• Jominy End-Quench Test:

  1. Standard specimen (25 mm dia, 100 mm long) austenitized.

  2. One end quenched with water jet.

  3. Rockwell hardness measured along length (1.5 mm from surface at intervals).

  4. Hardenability = depth to given hardness (e.g., HRC 50).

• Quench Severity: Cooling intensity at center of quenched part → determined by Grossmann's H-value (medium-dependent).

5.6 Common Heat Treatment Defects

6.0 Steels and Cast Irons

6.1 Classification of Steels

• Role of Key Alloying Elements:

  | Element | Primary Effect | Secondary Effects | |-------------|-----------------------------------------|-------------------------------------| | Cr | Increases hardenability, wear resistance, corrosion (stainless) | Forms carbides | | Ni | Increases toughness, strength, corrosion resistance | Stabilizes austenite | | Mo | Increases hardenability, tempering resistance | Reduces temper brittleness | | Mn | Deoxidizes, combines with S (MnS), increases hardenability | Austenite stabilizer | | Si | Deoxidizes, increases strength (solid solution) | Reduces magnetism in electrical steels | | V | Refines grain, forms stable carbides (VC) → secondary hardening | Increases strength/toughness | | Cu | Precipitation hardening, atmospheric corrosion resistance | Improves weather resistance |

6.2 Special Steels

• Hadfield Manganese Steel (12-14% Mn, 1% C):

  • Properties: High impact strength, work-hardening (surface hardens under impact).

  • Applications: Railway switches, crusher jaws, high-wear applications.

6.3 Cast Irons

7.0 Non-Ferrous Alloys and Engineering Materials

7.1 Aluminum Alloys

• Series (AA Designation):

  | Series | Main Alloying Element | Heat Treatable? | Key Alloys & Uses | |------------|---------------------------|---------------------|-----------------------------------------------| | 1xxx | Pure Al (99%+) | No | Electrical conductors, chemical equipment | | 2xxx | Cu | Yes | 2024 (Cu-Mg): aircraft structures (high strength) | | 3xxx | Mn | No | 3003 (Mn): general purpose, cookware | | 4xxx | Si | No | Welding rods, brazing alloys | | 5xxx | Mg | No | 5052, 5083: marine, pressure vessels (weldable, corrosion-resistant) | | 6xxx | Mg + Si | Yes | 6061 (Mg₂Si): structural, architectural (good extrusion, weldable) | | 7xxx | Zn + Mg/Cu | Yes | 7075 (Zn-Mg-Cu): aerospace (highest strength) | | 8xxx | Others (Li, etc.) | Varies | Advanced aerospace (lightweight) |

• Age-hardening in Al-Cu (2xxx, 7xxx):

  1. Solution Treat: Heat above solvus → homogeneous α (supersaturated).

  2. Quench: Retain supersaturated solid solution.

  3. Age: Hold at moderate T (100-200°C) → precipitate θ' (Al₂Cu) → hardening.

  • Overaging: Coarse precipitates → softening.

7.2 Copper Alloys

7.3 Nickel Alloys

• Monel Metal (Ni-Cu, ~67% Ni, 30% Cu, Fe, Mn):

  • Properties: Excellent corrosion resistance (acids, seawater), high strength.

  • Applications: Chemical plants, marine hardware, heat exchangers.

• Inconel (Ni-Cr-Fe): Oxidation/corrosion-resistant at high T → gas turbines, nuclear reactors.

• Hastelloy (Ni-Mo-Cr): Superior chemical resistance → chemical processing.

7.4 Titanium Alloys

• Alpha Alloys (Al, Sn, Zr): HCP, good creep resistance, weldable.

  • Example: Ti-5Al-2.5Sn → aircraft structures, jet engines.

• Alpha-Beta (Al, V, Mo, Cr): BCC + HCP, high strength → airframes, landing gear.

• Beta (V, Cr, Mo): Metastable BCC, formable, heat-treatable.

8.0 Metallurgical Raw Materials and Chemical Analysis

8.1 Main Ores

• Manganese Ores:

  | Ore | Composition | Properties/Uses | |------------------|-----------------------|-----------------------------------------| | Pyrolusite | MnO₂ | Main ore, used in batteries, steel deoxidizer | | Rhodochrosite| MnCO₃ | Carbonate, source of Mn | | Hausmannite | Mn₃O₄ | Mixed oxide |

  • Industrial Use: Steelmaking (deoxidizer, sulfur fixer, alloying).

• Chromium Ores:

  | Ore | Composition | Properties/Uses | |------------------|-----------------------|-----------------------------------------| | Chromite | (Fe,Mg)Cr₂O₄ | Main ore, refractory, stainless steel production | | Uvarovite | CaCr₂(SiO₄) | Garnet, minor |

  • Industrial Use: Stainless steel (Cr for corrosion resistance), refractories, pigments.

8.2 Chemical Reagents in Metallurgy

8.3 Chemical Analysis Methods

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Exam Focus: Distinguish volumetric (titration, volume measured) vs. gravimetric (mass measured). Spot test is qualitative, colorimetric is quantitative.

9.0 Non-Destructive Testing (NDT) Techniques

Importance: Ensure structural integrity, safety-critical components (aerospace, pressure vessels), in-service inspection, quality control without destroying part.

END OF UNIT 1 NOTES
Aligned with RGPV past papers (2022-2025). Focus on definitions, diagrams (stress-strain, phase diagrams, TTT), comparative tables, and formula applications.