Understanding Engineering Materials: Metals

Sep 11, 2024

Notes on Engineering Materials: Metals

Introduction

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  • Engineering materials categorized into four types:
    • Metals
    • Polymers
    • Ceramics
    • Composites
  • Importance of understanding different materials and their properties in engineering.

Focus on Metals

  • Metals are vital for engineering due to their properties and applications.
  • Key metals discussed:
    • Iron: Important for creating steel, a high-strength material.
    • Aluminum: High strength-to-weight ratio, low melting point, cost-effective.
    • Titanium: Excellent strength-to-weight ratio, high melting point, expensive.
    • Other important metals: Magnesium, Copper, and Nickel.

Atomic Structure of Metals

  • Crystalline Structure: Metals have closely packed atoms in a regular grid (crystal lattice).
    • Comparison with amorphous materials like glass.
  • Unit Cell: The repeating unit in a crystal lattice.
  • Common crystal structures:
    • FCC (Face-Centred Cubic): E.g., Copper
    • BCC (Body-Centred Cubic): E.g., Iron
    • HCP (Hexagonal Close-Packed): E.g., Titanium

Packing Factors

  • FCC and HCP = 74% packing factor
  • BCC = 68% packing factor
  • Higher density due to close packing of atoms.

Defects in Lattices

  • Point Defects:
    • Vacancy Defects: Missing atoms
    • Interstitial Defects: Extra atoms in gaps
    • Substitutional Defects: Atoms replaced by impurities
  • Linear Defects (Dislocations):
    • Edge Dislocation: Extra half-plane of atoms
    • Screw Dislocation: Spiral arrangement of atoms
    • Dislocations cause irreversible deformation (plastic deformation).

Deformation Mechanisms

  • Elastic Deformation: Reversible stretching of atomic bonds.
  • Plastic Deformation: Movement of dislocations under stress.
  • Dislocation density affects yield strength: More dislocations = Higher strength.

Grain Structure

  • Metals solidify into polycrystalline structures with various orientations (grains).
  • Grains separated by grain boundaries that impede dislocation movement.
  • Smaller grain size = Stronger material (Hall-Petch equation).
  • Techniques to control grain size:
    • Adding inoculants
    • Controlling cooling rates.

Strengthening Techniques

  1. Grain Boundary Strengthening
  2. Work Hardening: Plastic deformation (cold rolling, forging) increases dislocation density but reduces ductility.
  3. Alloying: Mixing metals for enhanced properties.
  • Ferrous Alloys: Contain iron
  • Non-Ferrous Alloys: Do not contain iron (e.g., Brass = Copper + Zinc).

Important Alloys

  • Steel: Highly important; properties vary with carbon content:
    • Low-carbon steel: < 0.25% carbon (ductile, low-cost)
    • Medium-carbon steel: 0.25%-0.6% carbon (stronger)
    • High-carbon steel: 0.6%-2% carbon (even stronger, more heat treatment options)
    • Cast iron: 2%-4% carbon (brittle, good for casting).
  • Stainless Steel: Contains chromium for corrosion resistance.

Alloy Formation and Phases

  • Alloys formed by melting base metal + alloying elements (substitutional or interstitial).
  • Interstitial alloys distort the lattice, strengthening the material.
  • Phase Diagrams: Understanding phases of iron-carbon alloys:
    • Ferrite (BCC)
    • Austenite (FCC)
    • Cementite: Hard, brittle compound (6.7% carbon by weight).
  • Presence of cementite contributes to steel's strength.

Conclusion

  • Importance of understanding metals, alloying, and heat treatment in engineering.
  • For further learning, check out the extended video on Nebula and CuriosityStream.