Alloys and Their Properties

Definition of Alloys

• Alloy: A mixture of a metal with one or more other elements (which may be metals or non-metals), combined to improve or alter certain physical and/or chemical properties.
• In alloys, the atoms of the added elements are dispersed throughout the metallic lattice, disrupting its regularity.
• The added elements can be metals (e.g., zinc added to copper) or non-metals (e.g., carbon added to iron).
• Purpose of making alloys: To obtain properties not present in the pure metal, such as increased hardness, corrosion resistance, or strength.

Examples of Alloys

• Brass:
  • Composition: Copper + Zinc.
  • Properties:
    • Stronger than pure copper.
    • Resistant to corrosion.
    • Has a golden colour and is relatively easy to shape.
  • Uses:
    • Musical instruments.
    • Decorative items.
    • Plumbing fittings.
• Stainless Steel:
  • Composition: Iron + Chromium + Nickel + Carbon.
  • Properties:
    • Very hard and strong.
    • Resistant to rusting and corrosion due to chromium forming a protective oxide layer.
    • Maintains shiny appearance over time.
  • Uses:
    • Cutlery.
    • Surgical instruments.
    • Kitchen equipment.
• Other examples (not required in syllabus but useful for understanding):
  • Bronze: Copper + Tin → corrosion-resistant and used for statues, medals.
  • Duralumin: Aluminium + Copper + Magnesium → lightweight and strong, used in aircraft manufacturing.

Structure of Alloys and Why They Are Stronger

• Structure of a Pure Metal:
  • Metal atoms are the same size and arranged in regular layers.
  • These layers can slide over each other easily → metal is soft and malleable.
• Structure of an Alloy:
  • Atoms of added elements are different sizes compared to the main metal atoms.
  • This distorts the regular arrangement of atoms.
  • Layers cannot slide over each other easily → alloy becomes harder and stronger than pure metal.
• Diagram understanding:
  • Pure metal: uniform layers of equal-sized spheres.
  • Alloy: irregular arrangement with larger or smaller atoms fitted in between the main metal atoms, preventing easy layer movement.

Why Alloys Are Made

• To increase hardness (e.g., stainless steel for cutlery).
• To increase strength without making it too heavy (e.g., aircraft alloys).
• To improve resistance to corrosion (e.g., stainless steel, brass).
• To change colour and appearance for decorative purposes.
• To alter melting point for specific industrial uses.

Uses of Alloys in Terms of Their Physical Properties

• Stainless Steel in Cutlery:
  • Reason: Hardness allows sharp edges; corrosion resistance prevents rusting in contact with food and water.
• Brass in Plumbing Fittings:
  • Reason: Does not rust, easy to machine, and withstands water exposure.
• Aircraft Alloys (e.g., Duralumin):
  • Reason: Combination of lightness (from aluminium) and strength (from copper and magnesium).
• Bronze in Sculptures:
  • Reason: Corrosion resistance and attractive appearance.

Identification of Alloys in Structural Diagrams

• Diagrams will show atoms in a metallic lattice:
  • Pure Metal: Regular, repeating arrangement of same-sized atoms.
  • Alloy: Disrupted pattern with differently sized atoms randomly scattered within the structure.
• Recognising an alloy involves spotting irregular spacing in the lattice.