Materials Technology (ME-303) - Unit 3 Short Notes

I. FUNDAMENTALS OF MATERIALS & ATOMIC STRUCTURE

Definition & Classification of Engineering Materials

• Engineering Materials: Substances used in construction or manufacturing of structures, machines, devices, and products.

• Classification:

  • Based on Properties:

    • Mechanical: Strength, ductility, hardness, toughness.

    • Physical: Density, melting point, thermal/electrical conductivity.

    • Chemical: Corrosion resistance, oxidation stability.

  • Based on Applications:

    • Structural: Load-bearing (steel, concrete).

    • Functional: Specific physical properties (semiconductors, magnets, optics).

Atomic Structure & Bonding

• Metallic Bonding: Non-directional "electron sea" model where valence electrons are delocalized.

  • Properties Explained: High electrical/thermal conductivity, malleability, ductility, luster, moderate strength.

• Ionic Bonding: Electrostatic attraction between oppositely charged ions (e.g., NaCl). Brittle, high melting point, poor conductivity in solid state.

• Covalent Bonding: Directional sharing of electrons (e.g., diamond, Si). Hard, brittle, poor conductivity.

• Secondary Bonds:

  • Dispersion Forces (Van der Waals): Weak, temporary dipoles in non-polar materials.

  • Dipole Bonds: Attraction between permanent molecular dipoles.

Crystal Structures & Imperfections

• Crystal Lattice: 3D periodic array of points representing atomic positions.

• Unit Cell: Smallest repeating building block of the lattice.

• Common Metallic Structures:

  | Structure | Coordination Number | Atoms/Unit Cell | Atomic Packing Factor (APF) | | :--- | :--- | :--- | :--- | | Simple Cubic (SC) | 6 | 1 | 0.52 | | Body-Centered Cubic (BCC) | 8 | 2 | 0.68 | | Face-Centered Cubic (FCC) | 12 | 4 | 0.74 | | Hexagonal Close-Packed (HCP) | 12 | 2 | 0.74 |

  • APF Calculation: $$\displaystyle \text{APF} = \frac{\text{Volume of atoms in unit cell}}{\text{Volume of unit cell}} $$

• Miller Indices (hkl): Notation for crystal planes and directions.

  • Procedure for Planes:

    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). Negative intercepts denoted with bar (e.g., $\bar{1}$).

  [!TIP] Common mistake: Forgetting to take reciprocals or not clearing to smallest integers.

• Crystal Imperfections:

  • Point Defects: Vacancies, interstitials, substitutional impurities.

  • Line Defects: Dislocations (edge, screw) – primary carriers of plastic deformation.

Grain Structure & Size

• Polycrystalline: Aggregate of many small crystals (grains) with different orientations.

• Single Crystal: One continuous crystal lattice throughout.

• Hall-Petch Relationship: Effect of grain size (d) on yield strength ($$\displaystyle \sigma_y $$).

$$\sigma_y = \sigma_0 + \frac{k_y}{\sqrt{d}}$$

where $$\displaystyle \sigma_0 $$ = friction stress, $$\displaystyle k_y $$ = strengthening coefficient. **Finer grains increase strength and toughness.**

II. MECHANICAL PROPERTIES & BEHAVIOR

Stress-Strain Analysis (Polycrystalline Mild Steel)

[!TIP] Must be able to draw and annotate this curve from memory.

• Key Points:

  • Proportional Limit (P): Stress ∝ Strain (Hooke's Law valid).

  • Elastic Limit (E): Maximum stress for full recovery upon unloading.

  • Yield Point (UYP & LYP): Sudden drop in stress after elastic limit; onset of plastic flow.

  • Ultimate Tensile Strength (UTS): Maximum stress on curve.

  • Fracture Point (F): Failure after necking.

• Definitions:

  • Elasticity: Ability to recover original shape after load removal.

  • Plasticity: Ability to undergo permanent deformation.

  • Strain Hardening: Increase in strength & hardness due to plastic deformation (dislocation multiplication).

Key Mechanical Properties

• Creep Curve Stages:

  1. Primary (Transient): Decreasing creep rate (work hardening).

  2. Secondary (Steady-State): Constant creep rate (balance of work hardening & recovery).

  3. Tertiary: Accelerating creep rate leading to rupture (voids, cracks).

• Factors Affecting Fatigue Strength: Surface finish (roughness reduces), size (larger reduces), stress concentration (notches, holes), material properties (inclusions, grain size).

Fracture

• Brittle vs. Ductile Fracture:

  | Feature | Ductile Fracture | Brittle Fracture | | :--- | :--- | :--- | | Appearance | Cup-and-cone, fibrous, shear lips. | Flat, granular, often perpendicular to stress. | | Deformation | Significant plastic deformation. | Minimal. | | Crack Propagation | Slow, requires energy. | Fast, catastrophic. | | Temperature Sensitivity | Ductile-to-brittle transition possible. | More likely at low T. |

• Griffith's Theory (Brittle Fracture): Crack propagates when reduction in elastic strain energy ≥ surface energy required to create new crack surfaces. Flaws/defects critically reduce strength.

III. PHASE DIAGRAMS & SOLIDIFICATION

Binary Phase Diagrams

• Interpretation:

  • Phase: Homogeneous, physically distinct region (e.g., α, L, α+β).

  • Phase Boundaries: Liquidus (L ↔ L+α), Solidus (α ↔ L+α), Solvus (α ↔ α+β).

  • Lever Rule: Calculate phase fractions in two-phase region.

$$\text{Fraction of phase } \alpha = \frac{C_0 - C_\beta}{C_\alpha - C_\beta}$$

(at given T, $$\displaystyle C_0 $$ = overall composition, $$\displaystyle C_\alpha $$, $$\displaystyle C_\beta $$ = phase compositions at tie-line ends).

• Types:

  • Complete Solid Solubility: e.g., Cu-Ni (FCC, similar atom size, electronegativity). Single-phase α over entire range.

  • Limited Solubility with Eutectic: e.g., Pb-Sn. Three single-phase regions (α, β, L). Eutectic reaction: $L \xrightarrow{\text{eutectic T}} \alpha + \beta$.

  • Limited Solubility with Peritectic: e.g., Fe-C (peritectic at 1495°C). Reaction: $L + \delta \xrightarrow{\text{peritectic T}} \gamma$.

• Cooling Curves: Plot temperature vs. time during cooling.

  • Significance: Thermal arrests indicate phase transformations (e.g., liquidus, solidus, eutectic). Different cooling rates affect microstructure.

Iron-Carbon (Fe-Fe₃C) Phase Diagram

[!TIP] Must be able to draw and label this diagram from memory.

• Key Phases:

  • Ferrite (α, δ): BCC, very low C solubility (<0.022% at 727°C), soft, magnetic.

  • Austenite (γ): FCC, high C solubility (2.11% at 1147°C), tough, non-magnetic.

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

  • Pearlite: Lamellar mixture of α + Fe₃C (eutectoid product).

  • Ledeburite: Eutectic mixture of γ + Fe₃C.

• Important Reactions:

  1. Eutectic: $$\displaystyle L_{(4.3\%)} \xrightarrow{1147°C} \gamma_{(2.11\%)} + \text{Fe}_3\text{C} $$ → Ledeburite.

  2. Eutectoid: $$\displaystyle \gamma_{(0.77\%)} \xrightarrow{727°C} \alpha_{(0.022\%)} + \text{Fe}_3\text{C} $$ → Pearlite.

  3. Peritectic: $$\displaystyle L_{(0.53\%)} + \delta_{(0.09\%)} \xrightarrow{1495°C} \gamma_{(0.17\%)} $$.

• Application to Heat Treatment: Basis for understanding transformations during heating/cooling (e.g., austenitizing, formation of martensite, pearlite, bainite).

Other Important Systems

• Eutectoid Solid Solution: Solid solution that undergoes a eutectoid reaction upon cooling (e.g., γ in Fe-C). Significance: Forms pearlite, fundamental to steel heat treatment.

• Partial Al-Cu Diagram (up to 10% Cu):

  DiagramCANVAS: Sketch Al-rich side of Al-Cu phase diagram. Label: L, α (Al-rich solid solution), θ (Al₂Cu). Show eutectic at ~5% Cu, 548°C: L ↔ α + θ. Show solvus line descending from eutectic. Label region for 4-5% Cu alloy.
  • Engineering Significance of 4-5% Cu: Lies in two-phase (α + θ) region after slow cooling. Age-hardenable (precipitation of θ' phase from supersaturated α).

• Solvus Curve: Boundary between single-phase solid solution and two-phase (solid solution + precipitate) region. Role in Age Hardening: Alloy is solution-treated (heated to single-phase), quenched (supersaturated solid solution), then aged (precipitation occurs below solvus).

IV. HEAT TREATMENT OF STEELS

Objectives

• Relieve stresses, refine grain size, modify mechanical properties (hardness, strength, ductility, toughness), improve machinability, enhance corrosion resistance.

Fundamental Processes

Specialized Processes

• Austempering: Austenitize, quench to above Ms (e.g., 300-400°C), hold until bainite forms, cool in air. Result: Ausferrite (lower bainite + retained austenite). Purpose: Reduce distortion/cracking, good toughness.

• Martempering: Austenitize, quench to just above Ms (e.g., 150-300°C), hold for temperature uniformity, cool in air through Ms. Result: Martensite with reduced thermal stresses.

• Case Hardening (Surface Hardening):

  • Carburizing: Add C to surface (pack, gas, liquid). Low-C steel → high-C case. Case depth controlled by time.

  • Cyaniding: Short-time (20-90 min) process in molten cyanide salt (NaCN). Adds C & N. For low-C steels.

  • Nitriding: Introduce N at 500-550°C (gas, plasma). Forms hard nitrides (ε, γ'). Advantages: No quench, minimal distortion, high surface hardness, good fatigue.

  • Carbo-nitriding: Simultaneous C & N addition (gas). For low-C steels.

• Flame Hardening & Induction Hardening: Localized surface heating (flame/induction) followed by rapid quench. Hardened case, tough core.

• Ion Implantation: Bombard surface with high-energy ions (N, B, C). Modifies surface chemistry & properties without thermal cycle.

Hardenability

• Definition: Depth to which a steel can be hardened upon quenching (measure of ease of martensite formation).

• Factors: Carbon content (↑C ↑hardness, ↓hardenability), alloying elements (Cr, Mo, Ni, Mn – delay pearlite/bainite, ↑hardenability), grain size (finer ↑hardenability), quenching severity (medium > water).

• Jominy End Quench Test:

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

  2. One end quenched with water jet.

  3. Rockwell hardness measured at intervals from quenched end.

  4. Hardness vs. distance curve = hardenability curve.

Common Defects & Prevention

• Distortion/Cracking: Due to thermal stresses. Use proper quench media, agitation, martempering/austempering.

• Oxidation/Decarburization: Scale & C loss at surface. Use protective atmosphere (vacuum, inert gas), pack hardening.

• Insufficient Hardness: Incorrect austenitizing T/time, low hardenability, slow quench.

V. STEELS & CAST IRONS

Classification of Steels

Influence of Alloying Elements

Cast Irons

Special Steels

• Hadfield Manganese Steel (Mn 12-14%, C 1%): High impact strength, work-hardens severely. Used in railway switches, crusher jaws.

• Stainless Steels:

  • Austenitic (Cr-Ni): Non-magnetic, corrosion-resistant, weldable (304, 316).

  • Ferritic (Cr): Magnetic, moderate corrosion, cannot harden.

  • Martensitic (Cr): Hardenable, used for cutlery, tools.

  • Duplex (Austenite+Ferrite): High strength, good corrosion.

VI. NON-FERROUS METALS & ALLOYS

Aluminium & Alloys

• Age Hardening (Precipitation Hardening):

  1. Solutionizing: Heat to single-phase region (e.g., α in Al-Cu), hold.

  2. Quenching: Rapid cool → supersaturated solid solution.

  3. Aging: Reheat to intermediate T (below solvus) → fine precipitates (GP zones, θ') form → strength & hardness increase.

  [!TIP] Overaging (long time/high T) → coarse precipitates → strength decreases.

Copper & Alloys

• Pure Copper: High conductivity, ductility, corrosion res. Used in electrical, heat exchangers.

• Brass (Cu-Zn):

  • Alpha (α): Up to ~37% Zn, FCC, single-phase. Ductile, good for cold working.

  • Alpha-Beta (α+β): 37-45% Zn, two-phase. Stronger, used for hot working (cartridge brass).

• Bronze (Cu-Sn):

  • Phosphor Bronze: Cu + Sn + P. High strength, fatigue resistance, corrosion res. Springs, bearings.

  • Gun Metal: Cu + Sn + Zn + Pb. Good casting, pressure tightness.

• Cupronickel (Cu-Ni): 10-30% Ni. High corrosion resistance in seawater. Marine hardware, condensers.

Nickel Alloys

Titanium Alloys

• Alpha Alloys: e.g., Commercially Pure Ti, Ti-6Al-4V (α+β). Properties: High specific strength, excellent corrosion res., biocompatible. Applications: Aerospace, implants.

• Beta Alloys: e.g., Ti-10V-2Fe-3Al. More formable, lower modulus.

• Alpha-Beta Alloys: e.g., Ti-6Al-4V. Most common, good strength & toughness.

Other Alloys

• Babbitt Metals: White Metal (Sn-base): Sn-Pb-Cu (Sb, As). Lead-based: Pb-Sb-Sn. Properties: Low friction, excellent embeddability. Use: Bearing linings.

VII. METALLURGICAL ANALYSIS & TESTING METHODS

Chemical Analysis

• Volumetric Analysis (Titration): React sample with standard solution of known concentration. Measure volume to reach endpoint (indicator/pH meter). Determines % of element (e.g., C by combustion titration).

• Gravimetric Analysis: Convert analyte to insoluble compound, filter, dry, weigh. Direct % calculation. High accuracy (e.g., Si as SiO₂).

Instrumental & Spectroscopic Analysis

• Optical Emission Spectroscopy (OES): Spark excites atoms in sample → emit characteristic wavelengths → spectrometer measures intensity → quantitative elemental analysis. Fast, accurate for bulk composition.

• Colorimetric Methods: Reagent reacts with ion → colored compound → intensity measured by spectrophotometer → concentration. Used for trace elements (e.g., Cu, Ni).

Qualitative Spot Test

• Method: Apply chemical reagent (drop) to metal surface. Observe color change/precipitate.

• Application: Rapid, on-site identification (e.g., Cu → blue with NH₃, Ni → black with DMG).

Non-Destructive Testing (NDT) for Cracks

Destructive Mechanical Testing

• Tensile Testing: Standard specimen pulled to failure → stress-strain curve → UTS, yield strength, ductility.

• Hardness Testing:

  | Test | Indenter | Load/Scale | Principle | | :--- | :--- | :--- | :--- | | Brinell | Hardened steel/ tungsten carbide ball | 500-3000 kgf, 10-10mm | Measure indentation diameter. | | Rockwell | Diamond cone (C-scale) or ball | Minor load 10kgf, major 60-150kgf | Depth of penetration. | | Vickers | Square-based diamond pyramid | 1-100 kgf | Diagonal of square indentation. |

• Impact Testing (Charpy): Notched specimen struck by pendulum. Energy absorbed = toughness. Notch sensitivity: Material's tendency to fail at lower energy with notch. Ductile-Brittle Transition Temperature (DBTT): Temperature where fracture mode changes (seen in BCC metals).

VIII. METALLURGICAL RAW MATERIALS (ORES)

Manganese

• Main Ores:

  • Pyrolusite: MnO₂. Black, granular. Main ore.

  • Rhodochrosite: MnCO₃. Pink, rhombohedral.

  • Braunite: Mn²⁺Mn³⁺₆SiO₁₂. Hard, black.

• Properties & Applications: Mn is a strong deoxidizer and sulfur fixer (MnS). Essential alloying element in steels (↑ strength, hardenability, wear resistance). Used in dry cells, batteries.

Chromium

• Main Ore: Chromite (FeCr₂O₄). Dark brown, octahedral.

• Properties & Significance:

  • Stainless Steel: Cr ≥ 12% forms passive Cr₂O₃ layer → corrosion resistance.

  • Refractory Bricks: High melting point (Cr₂O₃).

  • Plating: Decorative & hard chrome plating.