ALLOTROPIC FORMS OF IRON: Alpha Iron, Gamma Iron, Delta iron, Beta Iron

ALLOTROPIC FORMS OF IRON

At atmospheric temperature iron has three allotropic forms of crystal at different temperature. 

Beta Iron: Beta iron (β-iron) is a metastable allotrope that forms between 768 degrees Celsius (1,414 degrees Fahrenheit) and 912 degrees Celsius (1,674 degrees Fahrenheit). It has a body-centered cubic (BCC) crystal structure. Beta iron is harder and less ductile compared to alpha iron, and it can transform into alpha iron upon cooling.

Alpha Iron: It occurs from normal temperature to 9100and has bcc structure. Alpha iron (α-iron) is a metastable allotrope that forms below 910 degrees Celsius (1,674 degrees Fahrenheit) and is stable between room temperature and 768 degrees Celsius (1,414 degrees Fahrenheit). It has a body-centered cubic (BCC) crystal structure. Alpha iron is relatively soft and ductile, and it can undergo a phase transformation to ferrite at lower temperatures.

Gamma Iron: It occurs from 9100to 14000and has fcc centred lattice structure. Iron looses its magnetic properties when heated to 7700CGamma iron (γ-iron) is a high-temperature allotrope that exists above 910 degrees Celsius (1,674 degrees Fahrenheit) and up to its melting point at 1,538 degrees Celsius (2,800 degrees Fahrenheit). It has a face-centered cubic (FCC) crystal structure similar to austenite. Gamma iron is non-magnetic and relatively soft and ductile.

Delta iron: This occurs from 14000to 15390(molten state) and has body centred lattice. Its properties can be improved by addition of Co, Ni, C. At very high temperature fourth form exist called Epsilon iron € i.e. hexaferrum. 


These phases of iron at atmospheric pressure are important because of the difference in solubility of carbon, forming different types of steel.

ALLOTROPIC FORMS OF IRON: Alpha Iron, Gamma Iron, Delta iron, Beta Iron


Iron exhibits two major allotropic forms: ferrite and austenite. Let’s explore each form in more detail:

Ferrite:

Ferrite is the stable form of iron at temperatures below 910 degrees Celsius (1,670 degrees Fahrenheit). It is a relatively soft and ductile material with a body-centered cubic (BCC) crystal structure. Ferrite has a maximum solubility of carbon of about 0.022% at 723 degrees Celsius (1,333 degrees Fahrenheit), above which it begins to form a different phase called cementite.

Austenite:


Austenite is the high-temperature form of iron, stable between 910 degrees Celsius (1,670 degrees Fahrenheit) and its melting point of 1,538 degrees Celsius (2,800 degrees Fahrenheit). It has a face-centered cubic (FCC) crystal structure. Austenite is non-magnetic and relatively soft and ductile.

Apart from these two primary allotropes, iron can also undergo phase transformations under specific conditions:

Cementite:

Cementite, also known as iron carbide (Fe3C), is an intermediate compound that forms when the carbon content exceeds the solubility limit of ferrite. Cementite is a hard and brittle compound with an orthorhombic crystal structure. It is commonly found in steels as a constituent phase.

Martensite:


Martensite is a metastable phase of iron that forms when austenite is rapidly cooled, typically through quenching in water or oil. It has a body-centered tetragonal (BCT) crystal structure and is extremely hard and brittle. Martensite formation is responsible for the hardening of steels during heat treatment.

These different allotropes and phases of iron play significant roles in the properties and behavior of iron-based alloys, such as steel, which are widely used in various industries.

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