1: Binary Phase Diagrams

Introduction to Binary Phase Diagrams

• Binary phase diagrams represent the phase behavior of two-component systems.

• Previous discussions focused on pure elements; now we explore solid solutions, mixtures of two different elements.

• Solid Solution: A single phase mixture of two or more elements where the bonding is less coordinated compared to compounds.

Definitions

• Phase: A region with constant composition and atomic structure.

• Two Phase Structure: Contains two distinct phases with different compositions or lattice structures.

• Liquid Phase: Considered a separate phase in binary phase diagrams.

Allotropic Phase Transformation

• Allotropic Phase Transformation: Change in lattice structure at constant composition due to temperature or pressure influences.

• Discussion assumes conditions at ambient pressure, focusing on temperature changes.

• Example: Iron

  • Starts as body-centered cubic (BCC) alpha phase at ambient temperature.

  • Transitions to face-centered cubic (FCC) gamma phase around 910°C.

  • Changes back to BCC delta iron at approximately 1400°C.

  • Melts into the liquid phase at 1539°C.

  • Phase transformations are reversible upon cooling.

  • The historical reference to beta iron is due to its non-magnetic form (Curie temperature), not a distinct microstructure change.

Carbon Allotropes

• Major allotropes of carbon include:

  • Diamond: Tetrahedral lattice, extremely hard, low electrical conductivity, high thermal conductivity through lattice vibrations.

  • Graphite: Hexagonal sheets of carbon, soft, electrically conductive when stacked, forms graphene.

  • Fullerenes: Structures like Buckminster fullerene and carbon nanotubes, exhibiting extreme stiffness and strength combined with electrical conductivity.

  • Amorphous Carbon: Solid carbon without long-range order.

Solubility in Solid Solutions

• Solubility of one element in another depends on:

  • Lattice types of the elements.

  • Relative sizes of the atoms, characterized by lattice parameters (lengths of the unit cell).

• Full miscibility requires:

  • Same lattice type.

  • Size difference < 15%.

• Exceeding critical size difference leads to significantly reduced solubility.

Binary Phase Diagram Features

• Schematic Representation: Displaying full miscibility.

  • Vertical axis for composition of elements (A and B).

  • The left side = 100% A, the right = 100% B.

  • Presence of a single phase (solid) throughout the range of composition at high temperatures.

• Liquid Phase Behavior: Fully miscible in the liquid phase.

  • Transition to solid occurs during cooling, leading to a two-phase mushy region (liquid and solid).

Important Lines in Binary Phase Diagrams

• Liquidus Line: Above this line, the composition is completely liquid.

• Solidus Line: Below this line, the composition is completely solid.

• Eutectic Point: Where direct transition from liquid to two-phase region occurs at a single temperature.

• Eutectic Line: Connects eutectic point, indicating maximum solid solubility of one component in another at eutectic temperature.

Types of Alloys in Binary Systems

• Hypoeutectic Alloys: Compositions below eutectic composition.

• Hypereutectic Alloys: Compositions above eutectic composition.

Conclusion

• The lecture concludes with foundational concepts in binary phase diagrams, setting the stage for further exploration in part two.