Structure of Metals
DOE-HDBK-1017/1-93 GRAIN STRUCTURE AND BOUNDARY
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.
DEFINE the following terms:
If you were to take a small section of a common metal and examine it under a microscope, you
would see a structure similar to that shown in Figure 3(a). Each of the light areas is called a
grain, or crystal, which is the region of space occupied by a continuous crystal lattice. The dark
lines surrounding the grains are grain boundaries. The grain structure refers to the arrangement
of the grains in a metal, with a grain having a particular crystal structure.
The grain boundary refers to the outside area of a grain that separates it from the other grains.
The grain boundary is a region of misfit between the grains and is usually one to three atom
The grain boundaries separate variously-oriented crystal regions
(polycrystalline) in which the crystal structures are identical. Figure 3(b) represents four grains
of different orientation and the grain boundaries that arise at the interfaces between the grains.
A very important feature of a metal is the average size of the grain. The size of the grain
determines the properties of the metal. For example, smaller grain size increases tensile strength
and tends to increase ductility. A larger grain size is preferred for improved high-temperature
creep properties. Creep is the permanent deformation that increases with time under constant
load or stress. Creep becomes progressively easier with increasing temperature. Stress and
strain are covered in Module 2, Properties of Metals, and creep is covered in Module 5, Plant