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Hexagonal Close Packed (HCP)

Reviewed by Raghvendra GopalCheckmark
Last updated: March 21, 2023

What Does Hexagonal Close Packed (HCP) Mean?

Hexagonal close packed (HCP) is a crystal structure consisting of closely packed layers of atoms or ions arranged in a hexagonal pattern. In this structure, each atom or ion is surrounded by six nearest neighbors arranged in a hexagonal ring, and twelve next-nearest neighbors arranged in a slightly distorted octahedral shape.


Corrosionpedia Explains Hexagonal Close Packed (HCP)

The hexagonal close packed structure has numerous real-world industry applications in materials science, metallurgy and manufacturing. The HCP structure is found in a variety of materials, including metals, ceramics and semiconductors. One of the most significant applications of HCP materials is in the production of structural alloys, such as titanium alloys, which are widely used in aerospace, automotive and medical industries.

In aerospace, HCP materials are used for the construction of turbine blades and other high-stress components that require high strength and excellent corrosion resistance. The unique properties of HCP materials, such as their high melting point, low density and high strength-to-weight ratio, make them ideal for these applications. For example, HCP titanium alloys are used extensively in the construction of jet engine components, such as compressor blades and fan discs, due to their excellent high-temperature strength and fatigue resistance.

In the automotive industry, HCP materials are used for the production of lightweight and high-strength components, such as engine blocks, suspension components and brake discs. HCP magnesium alloys, for example, are used to produce lightweight components that can improve fuel efficiency and reduce emissions. (For more on lightweight components in the automotive industry, read: Corrosion Prevention and Lightweighting Vehicles.)

In the medical industry, HCP materials are used for the production of orthopedic implants, such as hip and knee replacements. In these circumstances, enhanced biocompatibility is required due to the relatively aggressive corrosion environment present in bodily fluids. HCP metals, such as titanium and zirconium, are biocompatible and have excellent corrosion resistance, which makes them ideal for these applications.

The HCP structure also plays a crucial role in the development of new materials with improved properties. For example, researchers are investigating the use of HCP materials in the production of new high-entropy alloys, which are a class of materials that have multiple metallic elements in equal or near-equal proportions. These materials have unique properties, such as high strength, ductility and corrosion resistance, that make them promising candidates for a wide range of industrial applications.

Overall, the HCP structure is an essential component of many industrial materials, and its unique properties make it ideal for a wide range of applications, from aerospace to medical implants to automotive components.

HCP metals show anisotropy in their mechanical behavior due to drastic differences between various slip systems' critical resolved shear stress (CRSS) values. Prismatic slip systems, the main one in titanium and zirconium, display the lowest CRSS. Basal slip occurs in magnesium and zinc.

Metals and alloys with a HCP structure display mechanical behaviors which are highly sensitive to microstructure and texture. Processing and thermomechanical treatment strongly affect these characteristics too. Thus, it's crucial to understand the relationships between processing conditions and material microstructure to predict a material's behavior.

A proper surface finish is important for an HCP material's lifespan, so developing suitable surface treatments for HCP metals can further enhance their application potential.


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