All metals used in a reactor have crystalline structures. Crystalline microstructures are arranged in three-dimensional arrays called lattices. This chapter will discuss the three most common lattice structures and their characteristics.

EO 1.2DEFINE the following terms:

a. Crystal structure

b. Body-centered cubic structure

c. Face-centered cubic structure

d. Hexagonal close-packed structure

EO 1.3STATE the three lattice-type structures in metals.

EO 1.4Given a description or drawing, DISTINGUISH between the three most common types of crystalline structures.

EO 1.5IDENTIFY the crystalline structure possessed by a metal.

Common Crystal Structures

In metals, and in many other solids, the atoms are arranged in regular arrays called crystals. A crystal structure consists of atoms arranged in a pattern that repeats periodically in a three-dimensional geometric lattice. The forces of chemical bonding causes this repetition. It is this repeated pattern which control properties like strength, ductility, density (described in Module 2, Properties of Metals), conductivity (property of conducting or transmitting heat, electricity, etc.), and shape.

In general, the three most common basic crystal patterns associated with metals are: (a) the body-centered cubic, (b) the face-centered cubic, and (c) the hexagonal close-packed. Figure 2 shows these three patterns.

Body-centered Cubic

In a body-centered cubic (BCC) arrangement of atoms, the unit cell consists of eight atoms at the corners of a cube and one atom at the body center of the cube.

Face-centered Cubic

In a face-centered cubic (FCC) arrangement of atoms, the unit cell consists of eight atoms at the corners of a cube and one atom at the center of each of the faces of the cube.

Hexagonal Close-packed

In a hexagonal close-packed (HCP) arrangement of atoms, the unit cell consists of three layers of atoms. The top and bottom layers contain six atoms at the corners of a hexagon and one atom at the center of each hexagon. The middle layer contains three atoms nestled between the atoms of the top and bottom layers, hence, the name close-packed.

Most diagrams of the structural cells for the BCC and FCC forms of iron are drawn as though they are of the same size, as shown in Figure 2, but they are not. In the BCC arrangement, the structural cell, which uses only nine atoms, is much smaller.

Figure 2 Common Lattice Types

Metals such as (x-iron (Fe) (ferrite), chromium (Cr), vanadium (V), molybdenum (Mo), and tungsten (W) possess BCC structures. These BCC metals have two properties in common, high strength and low ductility (which permits permanent deformation). FCC metals such as y--iron (Fe) (austenite), aluminum (Al), copper (Cu), lead (Pb), silver (Ag), gold (Au), nickel (Ni), platinum (Pt), and thorium (Th) are, in general, of lower strength and higher ductility than BCC metals. HCP structures are found in beryllium (Be), magnesium (Mg), zinc (Zn), cadmium (Cd), cobalt (Co), thallium (TI), and zirconium (Zr).

Summary

The important information in this chapter is summarized below.

Common Lattice Types Summary

A crystal structure consists of atoms arranged in a pattern that repeats periodically in a three-dimensional geometric lattice.

Body-centered cubic structure is an arrangement of atoms in which the unit cell consists of eight atoms at the corners of a cube and one atom at the body center of the cube.

Face-centered cubic structure is an arrangement of atoms in which the unit cell consists of eight atoms at the corners of a cube and one atom at the center of each of the six faces of the cube.

Hexagonal close-packed structure is an arrangement of atoms in which the unit cell consists of three layers of atoms. The top and bottom layers contain six atoms at the corners of a hexagon and one atom at the center of each hexagon. The middle layer contains three atoms nestled between the atoms of the top and bottom layers.

Metals containing BCC structures include ferrite, chromium, vanadium, molybdenum, and tungsten. These metals possess high strength and low ductility.

Metals containing FCC structures include austenite, aluminum, copper, lead, silver, gold, nickel, platinum, and thorium. These metals possess low strength and high ductility.

Metals containing HCP structures include beryllium, magnesium, zinc, cadmium, cobalt, thallium, and zirconium. HCP metals are not as ductile as FCC metals.

GRAIN STRUCTURE AND BOUNDARY

Metals contain grains and crystal structures. The individual needs a microscope to see the grains and crystal structures. Grains and grain boundaries help determine the properties of a material.

EO 1.6DEFINE the following terms:

a. Grain

b. Grain structure c. Grain boundary d. Creep