Lecture: 12 Phase diagrams 1

Introduction to Phase Diagrams

  • Lecture series focuses on alloy phase diagrams.
  • Emphasis on binary alloy phase diagrams over three weeks.
  • Topics include:
    • Basic phase diagrams today.
    • Eutectic phase diagrams next week.
    • Iron-carbon diagrams the week after.
    • Linkages to strengthening mechanisms and heat treatments after foundational understanding.

Importance of Phase Diagrams

  • Majority of exam questions will center on phase diagram concepts.
  • Attending lectures and workshops is essential for support and understanding.
  • Recommended reading: Paul Chapter 9, Sections 9.11 to 9.12.

Fundamental Concepts of Phases

  • States of Matter: Solid, Liquid, Gas (sometimes Plasma).
    • Phases are subunits within these states.
  • Differences in phases arise from atomic packing configurations known as unit cells.
  • Unit Cell: The smallest repeating unit that defines crystal structures.

Atomic Arrangements and Structures

  • Metals solidify into specific atomic arrangements:
    • Common structures include body-centered cubic (BCC), face-centered cubic (FCC), and hexagonal close-packed (HCP).
  • Bravais Lattices: Seven types; determine packing efficiency and physical properties.
Types of Cubic Structures

  • Body-Centered Cubic (BCC): 8 corner atoms + 1 center atom.

    • Examples: Low-temperature iron, vanadium, chromium.

  • Face-Centered Cubic (FCC): 8 corner atoms + 6 face-centered atoms.

    • Examples: Aluminum, copper, silver, gold, platinum.

  • Hexagonal Close-Packed (HCP): Another packing form for metals.

    • Examples: Magnesium, titanium, zinc.

Phases and Properties

  • Understanding phase changes is crucial for materials science:

    • Different phases of the same element can exhibit markedly different properties (e.g. graphite vs diamond).
    • Allotropes: Phases of elements that have different atomic arrangements.

  • Example: Diamond (hard) vs Graphite (soft).

  • Phase Diagrams: Visual representations of the states and conditions under which phases exist.

  • Phase Boundaries: Lines in diagrams that indicate changes in phase states.

Solubility and Solid Solutions

  • Solid solutions occur when elements mix and occupy lattice points without separating into distinct phases at high temperatures.
  • Constituents: Visible components seen under microscopes; distinct from phases which may be mixtures of constituents.

Key Elements to Understanding Phase Diagrams

  • Importance of understanding components and constituents.
  • Examples: Gray cast iron has different constituents (graphite and pearlite) and phases (ferrite, cementite).

Binary Phase Diagrams

  • Focus on systems containing two components that produce various phases depending on temperature and composition.
  • Understand liquid and solid phases as well as compositions of each at varying temperatures.
  • Lever Rule: Used to determine the proportions of solid and liquid phases.
    • Amount of phase is determined via the lengths of the tie line.

Practical Applications

  • Application in industrial processes (e.g., production of alloys).
  • Use phase diagrams to identify appropriate cooling rates and compositions for desired mechanical properties.
Solidification and Properties
  • Observations of solidification rates lead to diverse mechanical properties based on cooling rates and compositions.
  • Slow cooling can lead to more uniform structures, while rapid cooling leads to heterogeneity.

Research and Trends in Phase Diagrams

  • Historical interest in understanding solubility rules and developing phase diagrams.
  • Four parameters for alloy formation according to Hume-Rothery rules:
    1. Size factor: Element sizes must be similar (≤ 15% difference).
    2. Crystal structure: Similar patterns are necessary for solid solutions.
    3. Electrochemical reactivity: Must consider if elements react differently.
    4. Valence electron factor: Higher oxidation states can dissolve in lower oxidation states.

Importance of Understanding Phase Diagrams

  • Essential for materials science: informs creation of alloys, understanding of mechanical properties, and alloy composition suitability for applications.
  • Will prepare for more complex material interactions in coming lectures.