Iron crystal structure

Iron has a body-centered cubic (BCC) crystal structure at room temperature, known as α-iron (alpha iron), and it undergoes a transformation to a face-centered cubic (FCC) structure at higher temperatures.

Iron's Crystal Structure:

Alpha Iron (α-iron) - BCC (Body-Centered Cubic):

This is the stable form of iron at room temperature and up to 912°C.

In a BCC structure, each unit cell has one iron atom at each corner of the cube and one in the center.

The BCC structure has a lower atomic packing factor (APF) of 0.68, meaning it is not as densely packed as other structures, like the FCC structure.

Gamma Iron (γ-iron) - FCC (Face-Centered Cubic):

Above 912°C, iron transitions from the BCC structure to an FCC structure, called gamma iron.

In the FCC structure, iron atoms are arranged at each corner of the cube and at the center of each face, leading to a higher atomic packing factor (APF) of 0.74, which means it is more densely packed.

This form of iron is non-magnetic and has better ductility and formability.

Delta Iron (δ-iron) - BCC (Body-Centered Cubic):

Above 1394°C and up to 1538°C (the melting point), iron adopts another BCC structure called delta iron.

This phase is also non-magnetic, similar to gamma iron.

Eutectoid Point:

At 727°C, iron reaches the eutectoid composition where austenite (FCC) transforms into pearlite (a mixture of ferrite and cementite).

Summary of Iron's Crystal Structures:

α-iron (BCC): Stable at room temperature, magnetic, less ductile.

γ-iron (FCC): Forms at higher temperatures, non-magnetic, more ductile.

δ-iron (BCC): High-temperature phase before melting.

This ability to change crystal structure with temperature is important for the physical properties of iron and steel and is key in processes like heat treatment, which can alter the strength, hardness, and other characteristics of iron and its alloys.