Materials for engineering, 3rd Edition - (Malestrom)

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Structure of engineering materials
1.1 Crystal structure
Crystal structure refers to the ordering of atoms into different crystalline
arrangements. It is the arrangement of these atoms – the strength and
directionality of the interatomic bonds – which determines the ultimate strength
of the solid. Techniques involving X-ray or electron diffraction are employed
to determine crystal structures, and four types of interatomic bonding are
recognized: van der Waals, covalent, ionic and metallic. The latter three
‘primary’ bonds are limiting cases, however, and a whole range of intermediate
bonding situations also exist in solids.
The van der Waals force is a weak ‘secondary’ bond and it arises as a
result of fluctuating charges in an atom. There will be additional forces if
atoms or molecules have permanent dipoles as a result of the arrangement of
charge inside them. In spite of their low strength, these forces can still be
important in some solids; for example it is an important factor in determining
the structure of many polymeric solids.
Many common polymers consist of long molecular carbon chains with
strong bonds joining the atoms in the chain, but with the relatively weak van
der Waals bonds joining the chains to each other. Polymers with this structure
are thermoplastic, i.e. they soften with increasing temperatures and are readily
deformed, but on cooling they assume their original low-temperature properties
and retain the shape into which they were formed.
Covalent bonding is most simply exemplified by the molecules of the
non-metallic elements hydrogen, carbon, nitrogen, oxygen and fluorine. The
essential feature of a covalent bond is the sharing of electrons between
atoms, enabling them to attain the stable configuration corresponding to a
filled outermost electron shell. Thus, an atom with n electrons in that shell
can bond with only 8 – n neighbours by sharing electrons with them.
For example, when n = 4, as in carbon in the form of diamond, one of the
hardest materials known, each atom is bonded equally to four neighbours at
the corners of a regular tetrahedron and the crystal consists of a covalent
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