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Understanding Metal Alloys and Phase Diagrams

Apr 27, 2025

Lecture Notes: Metal Alloys, Phase Diagrams, and Lever Rule

Introduction to Metal Alloys

  • Presenter: Dr. Billy Wu
  • Importance: Used in everyday items like coins.
  • Material Requirements for Coins:
    • Distinguishability
    • Security against counterfeiting
    • Ease of manufacture
    • Wear and corrosion resistance
    • Anti-bacterial properties
  • Examples:
    • 2 Euro Coin: Outer ring (75% copper, 25% nickel) and inner ring (75% copper, 20% zinc, 5% nickel)
    • 1 Euro Coin: Opposite composition of the 2 Euro coin
    • Nordic Gold: 89% copper, 5% aluminum, 5% zinc, 1% tin
    • Lower denomination coins made of copper-plated steel

Material Selection

  • Periodic Table: Basis for selecting elements, but limited by scarcity and suitability.
  • Alloying: Mixing elements to enhance properties and expand possibilities.

Examples of Alloys

  • Bronze: Copper and tin, hard and corrosion-resistant.
  • Brass: Copper and zinc, softer and malleable.

Basic Concepts

  • Alloy: Mixture of metals or metals with non-metals.
  • Binary, Ternary Alloys: Refers to the number of elements mixed.
  • Component: An element in the alloy.
  • Phase: A region with uniform physical and chemical properties.
  • Optical Micrograph Example: Steel (iron and carbon) showing different phases.
  • Constitution: Sum of phases, mass of each phase, and composition.
  • Phase Diagram: Equilibrium constitution of temperature and composition.

Understanding Phases

  • Example: H2O phase diagram (solid, liquid, gas changes with temperature).
  • Solid Solution: Homogeneous mixture forming a single phase.
  • Complete Solid Solubility: Solute atoms are entirely soluble in solvent atoms.

Phase Diagrams

  • Copper-Nickel Alloy Example: Illustrates complete solid solubility.
  • Single Phase Regions: Liquid and solid solution phases.
  • Cooling Process: Shows transition from liquid to solid and grain formation.

Limited Solid Solubility

  • Example: Aluminum and copper.
  • Two-phase Systems: Solute atoms not completely soluble; a second phase forms.
  • Sugar-Water Analogy: Shows limited solubility and phase separation.

Silver-Copper Alloy Phase Diagram

  • Single-phase Regions: Alpha, beta, and liquid.
  • Two-phase Regions: Alpha + liquid, beta + liquid, alpha + beta.
  • Phase Boundaries: Liquidus, solidus, solvus lines.
  • Microstructure Impact: Affects mechanical properties.

Lever Rule

  • Purpose: Quantifies phase amounts in two-phase regions.
  • Application: Example with nickel-copper alloy.
  • Equations:
    • Mass fraction of phases using tie line.
    • Explanation through mass balance equations.
  • Example in Limited Solid Solubility: Silver-copper alloy at 600°C.

Summary

  • Alloys: Change in composition affects properties.
  • Phase Diagrams: Aid in understanding structures at various compositions.
  • Lever Rule: Helps quantify phase changes.

This lecture provides a comprehensive understanding of metal alloys, phase diagrams, and the application of the lever rule to predict and quantify phase changes in different alloy systems.

Full transcript