Iron

Phases of Iron

A phase is a state of matter whose properties vary smoothly (i.e. it is an analytic function of P ; V ; T etc). Connecting phase to smoothness properties allows to shift focus from phases themselves to the transformations between phases called Phase Transitions. Phase transitions are an incredibly important area of physics.

• Alpha Iron (α-Fe):
  • Also known as ferrite, alpha iron has a body-centered cubic (BCC) crystal structure.
  • It is stable below 912 °C (1,674 °F).
  • At high temperatures, it is paramagnetic, but below its Curie temperature (around 771 °C), it becomes ferromagnetic1.
• Gamma Iron (γ-Fe):
  • Austenite, or gamma iron, has a face-centered cubic (FCC) structure.
  • It exists at temperatures above 912 °C.
  • Austenite is important in heat treating of steel1.
• Delta Iron (δ-Fe):
  • Delta iron is less common and forms at very high temperatures.
  • It has a body-centered cubic structure similar to alpha iron but with different properties.
• Epsilon Iron (ε-Fe):
  • Epsilon iron, also known as hexaferrum, exists at very high pressures.
  • It is a fourth form of iron beyond the standard three phases.
  • Epsilon iron is relevant for understanding the solid parts of planetary cores.

For Iron, ferromagnetic is a phase, like a permanent iron magnet. When you heat such a magnet to high enough temperature, it under-goes a phase transition and stops being magnetic. Its the difference between austenite and ferrite.

1. **Austenite**:

  - Austenite represents a high-temperature, **face-centered cubic (FCC)** crystal structure of iron and iron-based alloys.
  - It is formed when iron or iron-based alloys are heated to high temperatures, typically above 912°C (1674°F) for pure iron.
  - In the austenite phase, iron atoms arrange themselves in a face-centered cubic lattice structure, characterized by iron atoms at the corners of a cube and one in the center of each face.
  - Austenite is relatively soft compared to other phases of iron.
  - Alloying iron with elements like nickel, manganese, and chromium stabilizes the FCC structure, allowing austenite to be retained at lower temperatures.
  - **Austenitic stainless steels**, which predominantly consist of austenite, are known for their corrosion resistance, hygiene, and aesthetics. They find use in kitchen appliances, cutlery, medical instruments, and architectural applications.

2. **Ferrite**:

  - Ferrite is a crystalline phase commonly found in metallic materials.
  - It is primarily formed when iron or iron-based alloys are cooled below a certain critical temperature (which varies based on alloy composition).
  - The most common form of ferrite is **alpha ferrite**, which exhibits a **body-centered cubic (BCC)** crystal structure.
  - Unlike austenite, ferrite is generally **magnetic** (ferromagnetic).
  - Ferrite is often low in carbon content.
  - It plays a crucial role in determining the properties and behavior of metals and alloys¹.

so:

• Austenite: FCC structure, non-magnetic or weakly magnetic, high-temperature phase.
• Ferrite: BCC structure, ferromagnetic, forms at lower temperatures.