Metals - Alloys and their properties

Objective: Identify representations of alloys from diagrams of structure

Alloys are mixtures of two or more metals, or a metal with a non-metal. The properties of an alloy are often different from the properties of the pure metals that make it up. This is because the atoms in the alloy disrupt the regular arrangement of atoms in the pure metals.

Types of Alloys and their Structures

Different alloys have different crystal structures, which affect their properties. The most common types of metallic bonding arrangements in alloys are:

• Body-centered cubic (BCC): This structure is found in metals like iron, tungsten, and chromium.
• Face-centered cubic (FCC): This structure is found in metals like aluminum, copper, and gold.
• Hexagonal close-packed (HCP): This structure is found in metals like magnesium, titanium, and zinc.

Identifying Alloys from Structural Diagrams

To identify an alloy from a structural diagram, you need to look at the arrangement of atoms. Here's a breakdown of how to do it:

Body-Centered Cubic (BCC) Structure

In a BCC structure, the atoms are located at each corner of the cube and one atom is located in the center of the cube. The atoms are arranged in a close-packed manner.

Characteristic	Description
Number of atoms per unit cell	8 (corner atoms) + 1 (central atom) = 9
Coordination number	8
Examples	Iron (Fe), Tungsten (W), Chromium (Cr)

Face-Centered Cubic (FCC) Structure

In an FCC structure, the atoms are located at each corner of the cube and one atom is located in the center of each face of the cube. The atoms are also arranged in a close-packed manner.

Characteristic	Description
Number of atoms per unit cell	8 (corner atoms) + 6 (face atoms) = 12
Coordination number	12
Examples	Aluminum (Al), Copper (Cu), Gold (Au)

Hexagonal Close-Packed (HCP) Structure

In an HCP structure, the atoms are arranged in a hexagonal pattern. It has fewer atoms per unit cell compared to BCC and FCC structures.

Characteristic	Description
Number of atoms per unit cell	6
Coordination number	12
Examples	Magnesium (Mg), Titanium (Ti), Zinc (Zn)

Key takeaway: The structure of an alloy can often be determined by the type of metal(s) it is made from. For example, an alloy containing copper and zinc will likely have an FCC structure (if the copper is the dominant component) or a BCC structure (if the zinc is the dominant component).

Example: Consider a diagram showing a crystal structure with 12 atoms per unit cell. This structure is most likely FCC, and the alloy could be aluminum or copper.