2: Binary Phase Diagrams

Introduction to Binary Phase Diagrams

• The second part of the lecture focuses on real binary phase diagrams, specifically aluminum-silicon and brass.

Binary Phase Diagram of Aluminum and Silicon

• Axes of the Diagram:

  • Horizontal axis: Weight percentage of silicon in aluminum.

  • Left and right sides indicate single phase regions:

    • Single Phase Aluminum: Clearly visible on the left side,

    • Single Phase Silicon: Not visible at this scale, indicating very low solubility in aluminum.

• Solubility Characteristics:

  • Silicon in Aluminum: Max solubility at about 600°C is about a couple of percent.

  • Aluminum in Silicon: Exhibit low solubility.

• Eutectic Point:

  • Located at 12.6% silicon and 577°C.

  • Represents a single melting temperature transitioning from liquid to solid phase.

• Melting Points:

  • Pure Aluminum: Hard to read, approximately 660°C.

  • Pure Silicon: Approximately 1414°C.

• General Trend: Addition of impurities typically lowers the melting point.

• Phases in the Diagram:

  • The liquid phase is indicated, but other regions are understood to represent mixed phases without labels.

  • Two-Phase Regions:

    • Liquid + Silicon Phase (Beta Phase)

    • Liquid + Pure Aluminum Phase (Alpha Phase)

    • Large region for Alpha + Beta phases, denoting mixed properties.

• Lattice Structures:

  • Both aluminum and silicon have FCC lattices, yet are not fully miscible due to differences in electronegativity and valence.

Binary Phase Diagram of Brass (Copper and Zinc)

• Structure Types:

  • Copper: FCC structure.

  • Zinc: Hexagonal Close-Packed (HCP) structure.

• Miscibility Issues: The difference in lattice structures results in a complex phase diagram.

• Phase Regions:

  • Alpha Brasses: Compositions in the large single phase region.

  • Alpha Beta Brasses: Two-phase mixtures that show enhanced hardness and strength.

• Lattice Configurations:

  • Alpha Brass retains FCC structure from copper.

  • Zinc’s single-phase region is HCP.

  • Beta Phase (BCC): Present at low temperatures; transitional structure during heating or cooling.

• Intermediate Regions:

  • The diagram indicates numerous regions including Gamma, Delta, and Epsilon phases, which represent mixtures of the earlier structures.

  • Misalignment of lattice structures complicates the phase diagram considerably.

Introduction to Ternary Phase Diagrams

• Definition: Phase diagrams representing three elements, depicted in three-dimensional volumes instead of two-dimensional graphs.

• Interpretation: Typically analyzed in two-dimensional slices at specific temperatures for clarity.

Reading a Ternary Phase Diagram

• General Method:

  • Align lines parallel to each side of the triangular diagram to establish compositions.

  • Example composition of 20% Zinc, 20% Copper, and 60% Nickel is located using these lines.

• Fully Miscible Phases:

  • For copper and nickel, the composition can vary freely from 100% copper to 100% nickel without phase separation.

• Special Properties:

  • Tailoring magnetic properties with alloying allows for adjustments in functional behavior based on temperature and composition.

Eutectic Structures in Cooling Processes

• Cooling through Eutectic Point: Yields a lamellar structure of alternating Alpha and Beta phases.

  • Plate Characteristics: Faster cooling produces narrower plates formed by rapid segregation of phases.

• Outcome of Non-Eutectic Cooling:

  • Results in a mix of alpha phase grains and separated eutectic structure, with some grains maintaining the eutectic plates' composition, behaving almost like a distinct phase.

Conclusion

• Understanding and interpreting phase diagrams is crucial for predicting material behavior and properties in various alloy systems, impacting applications in material science and engineering.