Chemistry – Metals - Alloys and their properties | e-Consult

Metallic bonding in metals involves delocalised electrons that are free to move throughout the metallic lattice. When metals are mixed to form an alloy, the introduction of different sized atoms disrupts the regular crystal lattice of the pure metal. This disruption affects the metallic bonding in the following ways:

• Increased Lattice Imperfections: The different sized atoms create more imperfections (e.g., dislocations, grain boundaries) in the crystal lattice. These imperfections hinder the movement of dislocations, which are essential for plastic deformation. This leads to increased strength and hardness.
• Altered Electron Distribution: The presence of different atoms alters the electron distribution within the alloy. The delocalised electrons are still able to move, but their movement is influenced by the presence of the foreign atoms. This can affect the electrical conductivity and other properties.
• Changes in Melting Point: Alloys typically have a wider melting range than the pure metals that make them up. This is because the presence of different atoms disrupts the melting process.

Brass Example: In brass, the presence of larger zinc atoms disrupts the regular copper crystal lattice. This creates more dislocations, making brass stronger and harder than pure copper, but also slightly less ductile. The altered electron distribution also affects the electrical conductivity, making it lower than pure copper.

Stainless Steel Example: In stainless steel, the addition of chromium creates a passive layer of chromium oxide on the surface. This layer is formed due to the altered electron distribution and the presence of the chromium atoms. This passive layer protects the underlying iron from corrosion. The presence of nickel and other elements further enhances the stability and integrity of this passive layer, contributing to the overall corrosion resistance of stainless steel.