Hexagonal close-packed

Definition

Hexagonal close-packed (HCP) is a type of crystal structure where atoms are closely packed in a hexagonal arrangement, allowing for the maximum density and efficiency of packing. This structure features two layers of atoms per unit cell that are arranged in a hexagonal pattern, with alternating layers offset to optimize space and minimize voids. HCP is one of the most common arrangements found in metallic solids, influencing their properties and behavior.

5 Must Know Facts For Your Next Test

1. The hexagonal close-packed structure is characterized by an efficient packing arrangement with a packing efficiency of about 74%.
2. In HCP, there are two distinct layers of atoms per unit cell, typically denoted as the A and B layers, creating an alternating pattern.
3. Metals like magnesium, titanium, and zinc often crystallize in the HCP structure, which affects their mechanical properties.
4. The HCP structure results in unique anisotropic properties, meaning that material properties can vary depending on the direction of measurement.
5. The relationship between the hexagonal close-packed structure and its thermal and electrical conductivity can be crucial for applications in materials science.

Review Questions

• Compare and contrast hexagonal close-packed and cubic close-packed structures in terms of their arrangement and packing efficiency.
  • Hexagonal close-packed (HCP) and cubic close-packed (CCP) structures both achieve high packing efficiencies of approximately 74%. However, while HCP features an arrangement where atoms are stacked in alternating layers with a hexagonal symmetry, CCP has a cubic symmetry where atoms are arranged in layers that align perfectly within three dimensions. This difference in arrangement leads to variations in how each structure responds to stress and temperature changes.
• Discuss the significance of the coordination number in determining the properties of materials with hexagonal close-packed structures.
  • The coordination number for hexagonal close-packed structures is 12, meaning each atom is surrounded by 12 other atoms. This high coordination number contributes to the strength and stability of materials formed with this arrangement. As a result, metals with HCP structures tend to exhibit significant ductility and toughness, making them suitable for various applications where mechanical resilience is essential.
• Evaluate how the unique properties associated with hexagonal close-packed structures impact their application in engineering and technology.
  • Materials with hexagonal close-packed structures are critically important in engineering due to their exceptional mechanical properties. The unique atomic arrangement leads to anisotropic behavior, which means they may exhibit different strengths and weaknesses depending on the direction of force applied. This characteristic is particularly valuable in aerospace applications, where lightweight yet strong materials are required. Understanding these properties allows engineers to tailor materials for specific performance requirements, thereby enhancing design efficiency and safety.

Related terms

Cubic close-packed: A crystal structure similar to HCP but with a cubic arrangement of atoms, known for having the same packing efficiency.

Unit cell: The smallest repeating unit in a crystal lattice that shows the full symmetry of the structure.

Coordination number: The number of nearest neighboring atoms surrounding a central atom in a crystal structure, which for HCP is 12.