Solid Solutions and Fe-C Alloy Structures

Types of Solid Solutions

Metal alloys are solid solutions between two or more elements. Depending on the arrangement of solvent and solute atoms, there are two types:

• Substitution Solid Solutions: Solvent and solute atoms have similar crystalline structures. Solute atoms replace solvent atoms within the crystal lattice.
• Interstitial Solid Solutions: Occur when solute atoms are very small compared to solvent atoms and are placed within the crystal lattice.

Constitution of Fe-C Alloys

Fe-C alloys are composed of constituents whose nature varies with chemical composition and temperature. The most representative constituents are:

• Ferrite: A solid solution of carbon in alpha iron.
• Cementite: The hardest and most brittle constituent of steel.
• Pearlite: A constituent formed by approximately 86.5% ferrite and 13.5% cementite. It presents a laminar structure if cooling is slow (lamellar pearlite) and a more blurred structure when cooling is more abrupt (sorbitic pearlite). If lamellar pearlite is heated to 723°C, the cementite takes the form of globules (spheroidized pearlite).
• Austenite: Consists of an interstitial solid solution of carbon in gamma iron.
• Martensite: After cementite, it is the hardest constituent of steel. Martensite is a supersaturated solid solution of carbon in alpha iron, obtained by rapid cooling of austenite.
• Troostite: A constituent intermediate between martensite and sorbite. It is produced by isothermal transformation of austenite between 500°C and 600°C.
• Sorbite: Also occurs by isothermal transformation of austenite in the temperature range of 600°C to 650°C.
• Bainite: Also produced by isothermal transformation of austenite in the temperature range of 250°C to 550°C.

Properties of Cast Iron

In cast iron, graphite occurs as flakes or sheets. It is brittle and not very resistant to tensile forces. However, its strength and ductility under compressive forces are very high. A characteristic property of cast iron is its effectiveness in damping vibrations, which is why machine bases are often built from it. Cast iron also has high wear resistance. It is easy to mold and is one of the cheapest ferrous materials.