The Hexagonal Close-Packed (HCP) structure is another common crystal structure in metals. HCP metals are densely packed but less ductile than FCC metals, due to fewer slip systems.

Understanding HCP is essential for predicting mechanical behavior, ductility, and deformation characteristics of metals like magnesium, titanium, and zinc.

Definition

Hexagonal Close-Packed (HCP) Structure

HCP is a crystal structure in which atoms are arranged in hexagonal layers with each atom surrounded by 12 nearest neighbors. The structure consists of top, middle, and bottom layers stacked in ABAB sequence.

Structure of HCP

• Unit cell: hexagonal prism

• Atoms per unit cell: 6

• Arrangement:

  • 2 atoms entirely within the unit cell

  • 12 atoms at corners of hexagon (each shared by 6 cells → contributes 2 atoms)

  • 1 atom on the center of top and bottom faces (each shared by 2 cells → contributes 1 atom)

Total atoms per unit cell = 6

• Lattice parameters:

  • a = side of hexagon

  • c = height of hexagonal prism

  • Ideal c/a ratio ≈ 1.633

Coordination Number

• Coordination number = 12

• Each atom has 12 nearest neighbors → dense packing similar to FCC, but less ductile due to limited slip planes.

Atomic Packing Factor (APF)

• APF = 0.74 (74%) → same as FCC

• Atoms are closely packed → high density, strong bonding

Examples of HCP Metals

• Magnesium (Mg)

• Zinc (Zn)

• Titanium (Ti)

• Cadmium (Cd)

Characteristics of HCP Metals

• High density (closely packed)

• Less ductile than FCC metals

• Strong and hard

• Limited slip planes → lower malleability

• Moderate melting point

• Common in structural and aerospace materials (Ti, Mg alloys)

Importance in Engineering

• Titanium (HCP) → used in aerospace and medical implants

• Magnesium (HCP) → lightweight structural applications

• Zinc (HCP) → corrosion-resistant coatings (galvanizing)

HCP structure explains mechanical limitations and guides alloy design and processing methods.

Exam-Focused Points

• HCP unit cell = hexagonal prism

• Atoms per unit cell = 6

• Coordination number = 12

• Atomic Packing Factor = 74%

• c/a ratio ≈ 1.633

• Examples: Mg, Zn, Ti

• Fewer slip systems → less ductile than FCC metals

Common Exam Traps

• Confusing HCP with FCC in terms of ductility

• Forgetting number of atoms per unit cell = 6

• Using wrong c/a ratio

• Assuming all close-packed metals are FCC (HCP is also close-packed)

Example Competitive Exam Questions

How many atoms are present in an HCP unit cell?
Answer — 6 atoms.

What is the coordination number of HCP?
Answer — 12.

Packing efficiency of HCP structure?
Answer — 74%.

Ideal c/a ratio of HCP metals?
Answer — 1.633

Name an HCP metal.
Answer — Magnesium, Titanium, Zinc.

Quick Revision

• HCP = hexagonal prism unit cell

• Atoms per unit cell = 6

• Coordination number = 12

• APF = 74%

• Ideal c/a ratio ≈ 1.633

• Common HCP metals: Mg, Zn, Ti

• Properties: dense, strong, less ductile than FCC