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Iron Allotropes and Magnetic Properties

Sep 9, 2025

Overview

This lecture covers the allotropic forms of iron, their temperature ranges, lattice structures, and associated magnetic properties.

Allotropic Forms of Iron

  • Iron is an allotropic metal, meaning it exists in more than one crystal (lattice) structure depending on temperature.
  • The three main allotropic forms are: alpha (α), gamma (γ), and delta (δ).
  • Alpha iron (α or ferrite) exists at lower temperatures with a body-centered cubic (BCC) structure.
  • Gamma iron (γ or austenite) exists between certain temperatures with a face-centered cubic (FCC) structure.
  • Delta iron (δ) exists at high temperatures with a BCC structure.

Temperature Ranges & Phase Changes

  • Melting point of iron is 1539°C.
  • Delta iron (δ) exists from 1400°C up to the melting point, having BCC structure.
  • Gamma iron (γ) exists from 910°C to 1400°C and has FCC structure.
  • Alpha iron (α) exists below 910°C with a BCC structure.
  • At 910°C, iron changes from γ (FCC) to α (BCC).
  • At 1400°C, iron changes from δ (BCC) to γ (FCC).

Magnetic Properties & Curie Temperature

  • Curie temperature for iron is 768°C.
  • Below 768°C, alpha iron (BCC) is magnetic.
  • Above 768°C, iron loses its magnetic properties (becomes non-magnetic), even if the structure remains the same.
  • No changes occur in the lattice structure due to magnetism, only in magnetic properties.

Summary of Phase Transitions

  • Above 1400°C: Delta iron (δ), BCC structure.
  • 910–1400°C: Gamma iron (γ, Austenite), FCC structure.
  • Below 910°C: Alpha iron (α, Ferrite), BCC structure.
  • Below 768°C: Alpha iron retains BCC structure and is magnetic.

Key Terms & Definitions

  • Allotropy — The ability of a metal to exist in more than one crystal structure.
  • Alpha iron (α, Ferrite) — BCC structure, exists below 910°C; magnetic below 768°C.
  • Gamma iron (γ, Austenite) — FCC structure, exists between 910°C and 1400°C.
  • Delta iron (δ) — BCC structure, exists above 1400°C up to melting point.
  • Curie Temperature — The temperature (768°C for iron) above which iron loses its magnetic properties.

Action Items / Next Steps

  • Review the phase changes of iron and memorize the temperature ranges and structures.
  • Prepare for possible exam questions on the allotropes and magnetic behavior of iron.

Full transcript